MedWorm.com provides a medical RSS filtering service. Over 6000 RSS medical sources are combined and output via different filters. This feed contains the latest items from the 'Acta Materialia' source. Wed, 02 Mar 2011 15:11:41 +0100 http://www.actamat.org/article/PIIS1359645410007627/abstract?rss=yes (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167770 Tue, 16 Nov 2010 02:22:12 +0100 4167770 http://www.actamat.org/article/PIIS1359645410006956/abstract?rss=yes Abstract: ZnO nanorod arrays (ZNAs) with various degrees of density and vertical alignment were grown on ZnO/Si substrates by a hydrothermal approach containing ZnO seed layers with designed microstructural characteristics. The nucleation mechanisms of ZNAs and the effect of the seed layers on the density and vertical alignment of ZNAs were systematically studied. Grain size and surface roughness were demonstrated to compete as the dominating characteristics for nucleation resulting in ZNAs with variable density and vertical alignment. Experimental results reveal that the nucleation mechanism of ZNAs switches from grain-boundary nucleation to surface nucleation when the characteristics of the seed layer change from small grains and low surface roughness to large grains and high surface rou... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167814 Thu, 28 Oct 2010 00:00:00 +0100 4167814 http://www.actamat.org/article/PIIS1359645410006920/abstract?rss=yes Abstract: The mechanical and electrical properties of nanocomposites created by gold and titanium implantation into polydimethysiloxane (PDMS) are reported for doses from 1015 to 5×1016at.cm−2, and for ion energies of 2.5, 5 and 10keV. Transmission electron microscopy (TEM) cross-section micrographs allowed detailed microstructural analysis of the implanted layers. Gold ions penetrate up to 30nm and form crystalline nanoparticles whose size increases with ion dose and energy. Titanium forms a nearly homogeneous amorphous composite with the PDMS up to 18nm thick. Using TEM micrographs, the metal volume fraction of the composite was accurately determined, allowing both electrical conductivity and Young’s modulus to be plotted vs. the volume fraction, enabling quantitative use of percola... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167813 Thu, 28 Oct 2010 00:00:00 +0100 4167813 http://www.actamat.org/article/PIIS135964541000683X/abstract?rss=yes Abstract: Macroscopic quantities such as strain rate and viscosity during sintering can be obtained from the microscopic motion of particles normal and tangential to grain boundaries. When grain boundary diffusion is the dominant transport mechanism of sintering, the viscous shear modulus and viscous Poisson’s ratios are dependent on the grain boundary diffusion coefficient and the microscopic viscosity of grain boundary sliding. The microscopic viscosity is not constant, varying with microstructural evolution during sintering. The viscous Poisson’s ratio decreases with increasing microscopic viscosity, and increases with relative density. The viscous Poisson’s ratio for a given relative density is also dependent on the local configuration and the ratio of grain boundary energy to su... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167807 Thu, 28 Oct 2010 00:00:00 +0100 4167807 http://www.actamat.org/article/PIIS1359645410006750/abstract?rss=yes Abstract: The traditional methods for predicting the occurrence of deleterious topologically close-packed (TCP) phases in Ni-based superalloys have been based on the PHACOMP and newPHACOMP methodologies. These schemes use the average number of holes or the centre of gravity of the elemental d-bands to predict whether or not a given multicomponent alloy will be prone to TCP formation. However, as both these one-dimensional methodologies are well-known to fail with respect to new generations of alloys, a novel two-dimensional structure map is introduced where is the average electron concentration and is a composition-dependent size-factor difference. This map is found to separate the experimental data on the TCP phases of binary A–B transition metal alloys into well-defined but sometimes o... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167805 Thu, 28 Oct 2010 00:00:00 +0100 4167805 http://www.actamat.org/article/PIIS1359645410006919/abstract?rss=yes Abstract: Cu–Cu2O core–shell nanodendrites and Cu2O hollow dendrites were prepared for the first time by distinct heat treatments of single crystalline Cu nanodendrites synthesized by the proposed low toxicity electrodeposition route. The crystallographic directions of the nanodendrites were studied by high resolution transmission electron microscopy and the orientation relations of Cu2O and Cu from the Cu–Cu2O core–shell nanodendrites were found to be Cu(111)[01]||Cu2O(001)[10]. Hollow Cu2O nanodendrites were characterized as quasi-single crystalline structures, which are proposed to form via the evaporation and escape of Cu in the core part of nanodendrites during specific heat treatments. Raman spectra show reverse surface-enhanced Raman scattering (SERS) effects for Cu and Cu�... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167812 Mon, 25 Oct 2010 00:00:00 +0100 4167812 http://www.actamat.org/article/PIIS1359645410006907/abstract?rss=yes Abstract: Dislocation interaction with and accumulation at twin boundaries have been reported to significantly improve the strength and ductility of nanostructured face-centered cubic (fcc) metals and alloys. Here we systematically describe plausible dislocation interactions at twin boundaries. Depending on the characteristics of the dislocations and the driving stress, possible dislocation reactions at twin boundaries include cross-slip into the twinning plane to cause twin growth or de-twinning, formation of a sessile stair-rod dislocation at the twin boundary, and transmission across the twin boundary. The energy barriers for these dislocation reactions are described and compared. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167811 Mon, 25 Oct 2010 00:00:00 +0100 4167811 http://www.actamat.org/article/PIIS1359645410006890/abstract?rss=yes Abstract: The correlation between experimentally observed plasticity and various materials properties that have been mentioned as being related to the mechanical properties of bulk metallic glasses (BMGs) has been investigated for Cu–Zr binary BMG alloys using a molecular dynamics simulation. Most of the properties examined show a roughly comparable correlation with plasticity, but the accuracy is not sufficient to be used for alloy design. The pentagon fraction obtained from a Voronoi polygon analysis and atomic displacement during plastic deformation directly computed using the atomistic approach can be selected as quantities that have an acceptable correlation with plasticity, even though the clearness and the statistical stability are unsatisfactory. The fractional volume change of m... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167810 Mon, 25 Oct 2010 00:00:00 +0100 4167810 http://www.actamat.org/article/PIIS1359645410006865/abstract?rss=yes Abstract: We investigate in detail the transient response induced by an edge dislocation near a circular elastic inclusion with simultaneous interface slip and diffusion. A rigorous solution to the interaction problem is derived in series form. As the time approaches infinity, our solution just recovers the classical one derived by Srolovitz et al. (Acta Metall 1984;32:1079) for fully relaxed boundary conditions. In addition, we observe that the edge dislocation will induce a uniform rigid-body rotation in the inclusion as the time approaches infinity. When the dislocation is far away from the inclusion, simple asymptotic expressions of the glide and climb forces on the dislocation are also obtained. Furthermore, five extreme cases for the imperfect interface are discussed; in particular, ... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167809 Mon, 25 Oct 2010 00:00:00 +0100 4167809 http://www.actamat.org/article/PIIS1359645410006841/abstract?rss=yes Abstract: The shock-compression response of magnetic nano-Fe3O4 powders of ∼12nm and ∼6nm, and their bimodal mixture (∼12nm+∼6nm in a 4:1 ratio), was investigated to determine the densification behavior of nanoparticles and properties of the consolidated magnets. Compacts recovered following shock compaction using a three-capsule, plate-impact, gas-gun setup at impact velocities of ∼800ms−1 and ∼1100ms−1 showed maximal densification of ∼80% theoretical maximum density (TMD) for ∼12nm particles, ∼70% TMD for ∼6nm particles, and ∼75% TMD for the bimodal mixture (∼12nm+∼6nm in a 4:1 ratio) nanoparticles. Pressure–volume compressibility (Hugoniot) plots of nano-Fe3O4 powder compacts were constructed to assess their dynamic densification behavior. Scanning electr... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167808 Mon, 25 Oct 2010 00:00:00 +0100 4167808 http://www.actamat.org/article/PIIS1359645410006671/abstract?rss=yes Abstract: A special micromechanism for the formation of elongated nanoscale voids at grain boundaries (GBs) in deformed nanocrystalline materials is suggested and theoretically described. Within our description, the formation of nanoscale voids represents a slow (diffusion-controlled) process driven by release of the elastic energy of GB disclination configurations formed due to GB sliding. It is demonstrated that the nucleation of elongated nanoscale voids at GB disclination dipoles occurs as an energetically favorable process in deformed nanocrystalline Ni and Al2O3 (sapphire) in wide ranges of their parameters. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167797 Mon, 25 Oct 2010 00:00:00 +0100 4167797 http://www.actamat.org/article/PIIS1359645410006762/abstract?rss=yes We report on the role of selected silicon nanocrystal (Si-nc) synthesis techniques on the photoelectric properties of Si-ncs and conjugated poly(hexylthiophene) (P3HT)-based blends. Nanosecond laser processing – both ablation and fragmentation – in liquids and electrochemical etching are employed to fabricate surfactant-free Si-ncs. We demonstrate that the transport properties of the blend can be tuned by the processing conditions. The blends containing Si-ncs produced by laser ablation clearly show superior photovoltaic properties due to the enhanced bulk heterojunction surface area and improved charge transport. Finally, we argue that the nanosecond laser processing in liquid medium could be a flexible and efficient tool to tailor the photoelectric properties of the Si-ncs/P3HT nanoc... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167806 Fri, 22 Oct 2010 00:00:00 +0100 4167806 http://www.actamat.org/article/PIIS1359645410006749/abstract?rss=yes Abstract: A generalized phase transformation kinetics model is used to understand the martensite to austenite transformation in a cold-rolled and annealed metastable AISI 301LN ultrafine-grained austenitic stainless steel. The model shows that the presence of interstitial nitrogen and heavy cold-rolling is important to promote fast transformation kinetics, through rapid nitrogen-diffusion and austenite nucleation at austenite/martensite phase boundaries. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167804 Fri, 22 Oct 2010 00:00:00 +0100 4167804 http://www.actamat.org/article/PIIS1359645410006737/abstract?rss=yes This study investigates the behavior of electrical contact resistance for copper–copper and aluminum–aluminum sphere-on-flat contact as a function of current through the interface. It is observed that the contact resistance may either increase or decrease with increasing current, depending on the current level as well as the current history. At low current levels the voltage drop across the interface increases initially with increasing current until it saturates. The voltage level remains nearly constant even if the current is increased beyond the value corresponding to saturation. Hereafter any subsequent decrease in current yields a corresponding decrease in voltage, so that the associated current cycle shows substantial hysteresis. However, subsequent cycles of current are reversibl... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167803 Fri, 22 Oct 2010 00:00:00 +0100 4167803 http://www.actamat.org/article/PIIS1359645410006725/abstract?rss=yes Abstract: By choosing the right conditions it is possible to produce thick open porous titanium coatings by means of vacuum plasma spraying. In this work such coatings were produced using both pure and surface alloyed Ti precursors. The effect of silicon, as the alloying element, on the final coating in terms of the particle–particle interface, coating appearance and mechanical strength of the coating was of particular interest. The coatings produced were ∼1mm thick and exhibited a porosity of 45–60%. Silicon was still found to be located primarily at the surface of the particles and at the interfaces. It formed mixed phases with Ti of varying Si content up to ∼7wt.%. The local hardness measured on a coating cross-section increased with increasing Si content. The mechanical propert... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167802 Fri, 22 Oct 2010 00:00:00 +0100 4167802 http://www.actamat.org/article/PIIS1359645410006713/abstract?rss=yes Abstract: In situ vitrification in a synchrotron beam during cooling of a Pd42.5Cu30Ni7.5P20 melt allowed continuous acquisition of X-ray diffraction spectra in the supercooled liquid region between the liquidus temperature Tl and the glass transition temperature Tg for the first time in a metallic liquid alloy. These and subsequent acquisitions on reheating allowed determination of the structural changes in the supercooled and glassy states using the real space atomic pair distribution function PDF(R) and radial distribution function RDF(R). The nearest neighbor (NN) distance in the first coordination shell has a low R peak (pre-peak) that appears and increases on cooling from the liquidus temperature to the glass transition temperature in the supercooled liquid region. In this temperatur... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167801 Fri, 22 Oct 2010 00:00:00 +0100 4167801 http://www.actamat.org/article/PIIS1359645410006701/abstract?rss=yes Abstract: A novel mathematical framework called materials knowledge systems (MKS) was recently formulated to extract, store and recall computationally efficient hierarchical linkages that are at the core of multi-scale modeling of materials phenomena. A salient feature of this new framework is that it facilitates flow of high-fidelity information in both directions between the constituent length scales, and thereby offers a new strategy for concurrent multi-scale modeling. The viability of this new framework has thus far been largely explored for capturing the mechanical response of composite material systems. This paper extends the MKS framework to applications involving microstructure evolution, where the local states are typically defined in a continuous local state space. In particular... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167800 Fri, 22 Oct 2010 00:00:00 +0100 4167800 http://www.actamat.org/article/PIIS1359645410006695/abstract?rss=yes Abstract: The effect of boundary faceting on densification has been studied in a 5mol.% TiO2–excess BaTiO3 model system. It was possible to inhibit grain growth during sintering in moderately reducing atmospheres after a hydrogen presintering treatment and, thus, to exclude the effect of grain growth on densification. For a given oxygen partial pressure () there was a saturated relative density which decreased with increasing and boundary faceting. The fraction of small pores was also higher in the samples sintered under high than in those sintered under low . These experimental observations demonstrate the presence of a critical driving force for densification in systems with faceted boundaries and its dependence on boundary faceting, i.e. the critical driving force is larger for system... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167799 Fri, 22 Oct 2010 00:00:00 +0100 4167799 http://www.actamat.org/article/PIIS1359645410006683/abstract?rss=yes We describe the manufacturing and mechanical properties of a novel class of auxetic (negative Poisson’s ratio) foams, which can assume arbitrary and complex shapes, and can be produced in large bulk quantities. Samples of sheets produced following the new manufacturing technique show a more uniform distribution of the Poisson’s ratio behaviour under tensile loading compared to classical negative Poisson’s ratio foams, and up to one order of magnitude higher energy dissipated per unit volume under cyclic tensile–tensile loading. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167798 Fri, 22 Oct 2010 00:00:00 +0100 4167798 http://www.actamat.org/article/PIIS1359645410006488/abstract?rss=yes Abstract: Three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2μm) but with the same martensite content (∼30vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing. Their mechanical properties were compared, and the response of the ultrafine-grained steel (1.2μm) to aging at 170°C was investigated. The deformation and fracture mechanisms were studied based on microstructure observations using scanning electron microscopy and electron backscatter diffraction. Grain refinement leads to an increase in both yield strength and tensile strength, whereas uniform elongation and total elongation are less affected. This can be partly explained by the increase in the initial strain-... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167795 Fri, 22 Oct 2010 00:00:00 +0100 4167795 http://www.actamat.org/article/PIIS1359645410006440/abstract?rss=yes Abstract: The migration of a grain boundary system with ridges of the first order is studied. A theory of steady-state motion of a grain boundary half-loop with facets and ridges is elaborated. The shape and properties of a moving grain boundary which contains additional structural elements (curved segments, facets and ridges of the first order) are addressed. The major kinetic parameters (mobility and its activation enthalpy) of a “rough-to-rough” grain boundary ridge of the first order were measured. It is shown that the steady-state motion of a boundary half-loop with facets and ridges complies with the principle of maximum rate of free energy dissipation. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167791 Thu, 21 Oct 2010 00:00:00 +0100 4167791 http://www.actamat.org/article/PIIS135964541000621X/abstract?rss=yes Abstract: The structure of T1 precipitates which formed under ageing in Al–Li–Cu alloys has been studied using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging. Complementary information on the precipitate composition has been obtained on the same system by small-angle X-ray scattering (SAXS). According to HAADF-STEM imaging, the currently accepted structures based on the stacking of {111} type planes are ruled out. It is shown that the T1 precipitate structure is consistent with the one derived from X-ray diffraction study of the bulk T1–Al2LiCu phase. However, for this very thin plate shape precipitates (thickness less than one cell), deviations to the Al2LiCu stoichiometry may occur. A structural analysis based on local environment and p... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167775 Thu, 21 Oct 2010 00:00:00 +0100 4167775 http://www.actamat.org/article/PIIS1359645410006543/abstract?rss=yes (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077105 Mon, 18 Oct 2010 17:09:37 +0100 4077105 http://www.actamat.org/article/PIIS135964541000649X/abstract?rss=yes Abstract: Growth of grain boundary voids during high-temperature power-law creep of metals is usually approximated as the growth of a hole in a nonlinearly viscous solid. Using synchrotron tomography, we show that the functional form of the continuum law is valid but that the real growth rates in copper are higher than the prediction of the viscous model by a factor of about 40. Submicrometer resolution tomography showing faceted void shapes as well as the large scatter of individual growth rates suggest that local dislocation glide has significative contribution to void growth. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167796 Mon, 18 Oct 2010 00:00:00 +0100 4167796 http://www.actamat.org/article/PIIS1359645410006476/abstract?rss=yes Abstract: ZnO nanowire was bent in a high-resolution transmission electron microscope (HRTEM). The growth process of tensile and compressive stress-induced asymmetrical ZnO quantum dots (QDs) on bent ZnO nanowire (NW) surface was observed in situ at the atomic scale. The positionally resolved atomic-level strain distribution along the radial directions was mapped directly from the atomic-level strained HRTEM images of the bent ZnO NW. The size, growth rate and density of the QDs can be significantly affected by the strain type and magnitude. These results are helpful in controlling the fabrication of ZnO QDs. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167794 Mon, 18 Oct 2010 00:00:00 +0100 4167794 http://www.actamat.org/article/PIIS1359645410006464/abstract?rss=yes Abstract: Ti–6Al–4V, with a network of elongated, open pores aligned along two perpendicular directions, is produced by a two-step replication process: (i) Ti–6Al–4V powder or foil preforms containing low-carbon steel wire meshes are densified by hot pressing under transformation superplasticity conditions; (ii) porosity is created by electrochemical dissolution of the low-carbon steel wires and the adjacent Fe-containing Ti–6Al–4V matrix. If high-carbon steel wires are used, Fe diffusion into Ti–6Al–4V is inhibited by a carbide layer forming at the wire/matrix interface, and pores exactly replicate the shape of the wires. Ti–6Al–4V with ∼19% and 34% porosity, without and with Fe–Ti interdiffusion respectively, shows low oxygen contamination and good compressive duc... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167793 Mon, 18 Oct 2010 00:00:00 +0100 4167793 http://www.actamat.org/article/PIIS1359645410006452/abstract?rss=yes Abstract: By expanding our level of understanding of structure–processing–property relationships through a data-mining methodology, this study demonstrates how to remove obstructions in complex high-throughput (HT) data analyses for developing new transparent conducting oxides. The demonstration is performed with principal component analysis (PCA) as an exploratory data analysis tool in the context of Co-doped ZnO (Co:ZnO) thin films generated from combinatorial HT syntheses. With the use of minimal available information, X-ray diffraction (XRD) patterns and their corresponding processing conditions, PCA enabled effective detection of pervasive changes in intensity and peak shifts as a function of composition, processing or a combination of both. These identifications are nearly imposs... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167792 Mon, 18 Oct 2010 00:00:00 +0100 4167792 http://www.actamat.org/article/PIIS1359645410006439/abstract?rss=yes Abstract: Alloys based on Mg–rare earth (RE) systems are of increasing technical interest in automotive powertrain applications due to their superior elevated temperature creep resistance. However, there is a deficiency in the literature of phase diagrams of multi-component RE systems that could assist alloy development and composition refinement for enhanced property optimization. The phase relationships in the Mg-rich corner of the Mg–Ce–Nd system have been investigated through the evaluation of selected compositions in the as-cast and heat-treated condition. Consistent thermodynamic CALPHAD-type assessments have also been generated for the Mg–Ce–Nd system. It is shown that this system reveals a significant degree of metastability under technologically significant solidificatio... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167790 Mon, 18 Oct 2010 00:00:00 +0100 4167790 http://www.actamat.org/article/PIIS1359645410006427/abstract?rss=yes Abstract: A constitutive relation based on crystal plasticity was derived by equating the energy of dislocations required to generate the imposed incremental strain with that which was stored as determined from the flow stress. The dynamic annihilation of created dislocations was accounted for by using a factor to balance the equation. The specific case of Taylor’s parabolic relation was reproduced and microstructure-based parameters were explicitly formulated in the proportionality constant usually attributed as empirical in the Hollomon relation. The nearly precise replication of the stress–strain relation using at least two curve-fits for aluminum and its alloys validates the quantitative determination of the mean slip distance. The intersection of the two fits appears to be analogo... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167789 Thu, 14 Oct 2010 00:00:00 +0100 4167789 http://www.actamat.org/article/PIIS1359645410006403/abstract?rss=yes Abstract: X-ray diffraction contrast tomography is a recently developed, non-destructive synchrotron imaging technique which characterizes microstructure and grain orientation in polycrystalline materials in three dimensions. By combining it with propagation-based phase-contrast tomography it is possible to get a full picture description for the analysis of local crack growth rate of short fatigue cracks in three dimensions: the three-dimensional crack morphology at different propagation stages, and the shape and orientation of the grains around the crack. An approach has been developed on the metastable beta titanium alloy Ti 21S that allows for visualization and analysis of the growth rate and crystallographic orientation of the fracture surface. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167788 Thu, 14 Oct 2010 00:00:00 +0100 4167788 http://www.actamat.org/article/PIIS1359645410006397/abstract?rss=yes Abstract: At elevated temperatures, single crystal thin films sometimes undergo solid-state dewetting to form holes and islands with anisotropic morphologies and regular features. We have measured the rates of edge retraction for single crystal Ni(100) and Ni(110) films with edges patterned along different in-plane crystallographic directions. The polar plots of the edge retraction velocities have anisotropic shapes with prominent cusps in particular directions. The characteristic anisotropy in the edge retraction velocity can be understood in terms of in-plane orientation-dependent edge faceting, with single faceting of specific edges and multiple micro-faceting in other edges. Faceting occurs due to surface energy minimization, and is modified by changes in the annealing ambient. Changes... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167787 Thu, 14 Oct 2010 00:00:00 +0100 4167787 http://www.actamat.org/article/PIIS1359645410006385/abstract?rss=yes Abstract: The flow and failure behaviour of replicated pure aluminium foams in the presence of holes and notches is explored, varying the pore size and the notch depth or hole radius. Flat dog-bone tensile specimens containing a cylindrical hole, and cylindrical V-notched samples of 400μm and 75μm pore size microcellular aluminium were tested. Both pore size foams exhibit a notch strengthening effect, i.e. the peak failure stress increases as the depth of notches in cylindrical samples increases. In dog-bone samples, the presence of a hole ranging from 0 to 4mm in diameter (in a sample 9mm wide) does not affect the net section peak failure stress of the 75μm foam while the 400μm pore size foam exhibits a slight increase in net section failure stress as the hole diameter is increased to... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167786 Thu, 14 Oct 2010 00:00:00 +0100 4167786 http://www.actamat.org/article/PIIS1359645410006373/abstract?rss=yes In this study, we report the fabrication of high-aspect-ratio single-crystal flakes with a submicron thickness and textured polynanocrystalline flakes with a submicron or nanosize thickness of SmCo5 by one-step surfactant-assisted HEBM in heptane with 15wt.% oleic acid. Single-crystal micron flakes were first formed via a basal cleavage along the easy glide (001) planes of the initial irregular and large single-crystal particles during the first stage of HEBM. Subsequently, single-crystal submicron flakes were formed by continuous cleavage. With further ball milling, polycrystalline submicron flakes with small-angle grain boundaries were formed. Finally, crystallographically anisotropic polycrystalline SmCo5 nanoflakes were formed with [001]-out-of-plane texture, thicknesses of 6–80nm, a... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167785 Thu, 14 Oct 2010 00:00:00 +0100 4167785 http://www.actamat.org/article/PIIS1359645410006348/abstract?rss=yes Abstract: In shape memory alloys, the reversible phase transformations between austenite and martensite give rise to superelasticity and shape memory properties. Here we systematically study the sample size dependence of these properties and the associated transformations in polycrystalline shape memory alloy microwires with a bamboo grain structure, i.e. where the wire diameter is completely spanned by individual grains. Cu–Al–Ni wires with diameters ranging from ∼500 down to ∼20μm are fabricated by the Taylor liquid processing technique, and are characterized by both isothermal uniaxial tensile testing and mechanically constrained thermal cycling. We observe size effects in both the transformation stresses and temperatures. What is more, we find that the stress hysteresis in a m... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167783 Thu, 14 Oct 2010 00:00:00 +0100 4167783 http://www.actamat.org/article/PIIS1359645410006336/abstract?rss=yes Abstract: In this paper, a detailed magnetic force microscopy study has been made of the domain structure of isotropic nanocomposite Nd2Fe14B/Fe3B magnet ribbons prepared by melt-spinning. The domain structure is composed of interaction domains a few hundred nanometers in size, which indicates the presence of a strong exchange coupling between the grains. Within the interaction domains, a fine substructure on the same scale as the grain size is observed with high-contrast and clarity, which is an improvement over earlier results. As a consequence, the interaction domains are clusters of many single-domain grains whose magnetization directions are not precisely the same but differ slightly. In general, the interaction domains have both perpendicular and in-plane magnetization components. Th... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167782 Thu, 14 Oct 2010 00:00:00 +0100 4167782 http://www.actamat.org/article/PIIS1359645410006294/abstract?rss=yes Abstract: A microstructural investigation of epitaxial CoFe2O4 films grown by pulsed laser deposition on single crystal SrTiO3 substrates was performed. We quantitatively analysed the microscopic mechanisms for structural mismatch compensation and the mode of the film growth. X-ray diffraction revealed very low microstrain in all three crystallographic directions of the film, which together with the lattice strain is not large enough to compensate effectively for the mismatch. Quantitative high-resolution transmission electron microscopy showed that most of the compensation occurs by formation of (400) edge dislocations in the film. These dislocations are concentrated within a critical layer above which a change from layer-by-layer growth to island growth was observed. These observations c... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167780 Wed, 13 Oct 2010 00:00:00 +0100 4167780 http://www.actamat.org/article/PIIS135964541000635X/abstract?rss=yes Abstract: Grain boundaries and their networks have profound influence over properties and structure evolution in every class of polycrystalline materials. Despite recent advances in characterization techniques, there remain fundamental problems in representing grain boundary network information; existing methods neglect the full complexity of misorientation information and often rely on boundary classification schemes of dubious physical significance. This situation has arisen in part because grain boundary misorientations have no known mapping to a simple Euclidean space; conventional wisdom suggests that the misorientation space is equivalent to the rotation space, which is known to require five variables for a continuous one-to-one mapping. In this paper, we show that, contrary to this ... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167784 Mon, 11 Oct 2010 00:00:00 +0100 4167784 http://www.actamat.org/article/PIIS1359645410006324/abstract?rss=yes Abstract: Magnetic phase diagrams of body-centered cubic and face-centered cubic Fe–Ni alloys were constructed using available experimental data and ab initio calculations. The results show that significant improvements in the “standard” diagrams (handbooks and CALPHAD databases) are required. The present work demonstrates that the CALPHAD magnetic model is not sophisticated enough to describe the Fe–Ni system. In addition, a new thermodynamic description of the lattice stability for pure Ni is urgently needed, since the recommended magnetic properties for CALPHAD modeling are distinct from the experimental and ab initio results. This work indicates that the construction of magnetic phase diagrams is indispensable during the phase transformation study of magnetic systems. (Source: ... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167781 Mon, 11 Oct 2010 00:00:00 +0100 4167781 http://www.actamat.org/article/PIIS1359645410006270/abstract?rss=yes Abstract: High-energy X-ray diffraction experiments were performed using a synchrotron beam source to investigate the effect of strontium on the liquid atomic structure of Al–Si hypoeutectic alloys. The high-temperature liquid diffraction experiments were carried out on Al alloys with 3, 7, 10 and 12.5 (eutectic) wt.% Si, respectively, with 0 and 0.04wt.% addition of Sr to each of the alloys. Further, the diffraction data for all the alloys were obtained at various melt temperatures (5–220K) above the respective liquidus temperature. It was observed that the addition of 0.04wt.% Sr results in significant change in the liquid structure parameters, such as structure factor, pair distribution function, radial distribution function, coordination number and packing density, at any given mel... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167779 Mon, 11 Oct 2010 00:00:00 +0100 4167779 http://www.actamat.org/article/PIIS1359645410006257/abstract?rss=yes Abstract: The combined effects of overload-induced enlarged compressive residual stresses and crack tip blunting with secondary cracks are suggested to be responsible for the observed changes in the crack opening load and resultant post-overload transient crack growth behavior [Lee SY, Liaw PK, Choo H, Rogge RB, Acta Mater 2010;59:485–94]. In this article, in situ neutron diffraction experiments were performed to quantify the influence of the combined effects by investigating the internal-stress evolution at various locations away from the crack tip. In the overload-retardation period, stress concentration occurs in the crack blunting region (an overload point) until a maximum crack arrest load is reached. The stress concentration is then transferred from the blunting region to the propa... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167778 Mon, 11 Oct 2010 00:00:00 +0100 4167778 http://www.actamat.org/article/PIIS1359645410006269/abstract?rss=yes Abstract: Neutron diffraction and electric potential experiments were carried out to investigate the growth behavior of a fatigue crack subjected to a single tensile overload. The specific objectives were to (i) probe the crack tip deformation and fracture behaviors under applied loads; (ii) examine the overload-induced transient crack growth micromechanism; (iii) validate the effective stress intensity factor range based on the crack closure approach as the fatigue crack tip driving force; and (iv) establish a quantitative relationship between the crack tip driving force and crack growth behavior. Immediately after a single tensile overload was introduced and then unloaded, the crack tip became blunt and enlarged compressive residual stresses in both magnitude and zone size were observed ... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167777 Mon, 11 Oct 2010 00:00:00 +0100 4167777 http://www.actamat.org/article/PIIS1359645410006245/abstract?rss=yes The objective of this work is to study the subsurface microstructure and mechanical response of hydrothermal degraded yttria stabilized tetragonal zirconia polycrystals (3Y-TZP). Samples of 3Y-TZP have been subjected to accelerated degradation for different times and several techniques are used to analyse the surface degraded layer: Raman spectroscopy, X-ray diffraction, nanoindentation, atomic force microscopy and transmission electron microscopy (TEM). Results show that there is a non-lineal gradient in the in-depth distribution of monoclinic phase determined by Raman spectroscopy, while the drop in elastic modulus and hardness recorded by nanoindentation is attributed to the extensive microcracking produced by the large shear strain of the first martensite plate formed during the transf... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167776 Mon, 11 Oct 2010 00:00:00 +0100 4167776 http://www.actamat.org/article/PIIS1359645410006142/abstract?rss=yes Abstract: Open-pore replicated microcellular 99.99% pure aluminium is tested in tensile creep, varying the temperature from 150 to 450°C, the applied stress from 0.15 to 0.5MPa, and the relative density from 0.14 to 0.28. Tensile creep curves are of classical shape, with a well-defined secondary stage of steady-state creep, for all except a few samples that were tested at higher temperature and lower stress; these display signs of extensive oxidation along the pore surface. All other samples crept at 250°C or above exhibit a steady-state creep rate with an activation energy of 141kJmol−1, a stress exponent of 7.2±0.4, and a dependence on relative density to a power near −21. The substructure of microcellular aluminium crept in this regime consists in subgrains that straddle, as in a... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167773 Mon, 11 Oct 2010 00:00:00 +0100 4167773 http://www.actamat.org/article/PIIS1359645410006361/abstract?rss=yes We report the magnetic and magnetocaloric properties in Co-doped Ni–Mn–Ga Heusler alloys around the Mn-rich composition Ni50Mn30Ga20. The presence of Co affects profoundly the critical temperatures and alters the exchange interactions of martensite and austenite to different extents; by varying the composition it is possible to tune the critical temperatures and to induce a paramagnetic gap between the magnetically ordered martensite and magnetic austenite, thus giving rise to a reverse magnetostructural transformation. Contrary to the Co-free alloys, the saturation magnetization moment of austenite is strongly enhanced by Co with respect to the martensitic one: thus the magnetocaloric effect turns from direct into inverse. Remarkable values of the magnetic properties related to the ma... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124863 Mon, 11 Oct 2010 00:00:00 +0100 4124863 http://www.actamat.org/article/PIIS1359645410006312/abstract?rss=yes Abstract: The influence of the starting powders’ morphology on the thermoelectric anisotropy of Bi–Te-based thermoelectric materials that are fabricated by spark plasma sintering has been investigated. Starting powders with three types of morphologies are prepared through a chemical reaction method (nano-sized particles with a spherical shape), a conventional pulverization method (flake-like shape) and a gas atomizing method (spherical shape). The thermoelectric anisotropy of each sintered body is determined by measuring the electrical resistivity and thermal conductivity in both the parallel and perpendicular directions to the spark plasma sintering pressing direction. The p-type Bi0.5Sb1.5Te3.0-sintered body that is composed of the spherically shaped powder has isotropic thermoelectr... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124862 Mon, 11 Oct 2010 00:00:00 +0100 4124862 http://www.actamat.org/article/PIIS1359645410006300/abstract?rss=yes Abstract: Films of Cu–Ru–Zr and Cu–Zr were deposited on SiO2/Si substrates by magnetron sputtering. Samples were subsequently annealed at temperatures of up to 500°C for 1h and analyzed by four-point probe measurement, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy. The XRD data suggest that the Cu film has a preferential (111) crystal orientation. According to the TEM results the grain size of the alloy Cu film is smaller than that of a pure Cu film. XPS indicates that a ZrOx layer has formed at the Cu alloy/SiO2 interface and that its thickness in the annealed Cu–Ru–Zr/SiO2/Si sample becomes larger due to Ru incorporation. After annealing the resistivity values of the annealed Cu alloy fil... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124861 Mon, 11 Oct 2010 00:00:00 +0100 4124861 http://www.actamat.org/article/PIIS1359645410006221/abstract?rss=yes Abstract: Ni–W alloys are promising materials for applications where mechanical stability at relatively high temperature is required. Several Ni–W thin film gradients were synthesized by magnetron sputtering spanning the entire binary composition range. Microstructure and phase composition were characterized by means of X-ray diffraction and transmission electron microscopy, while the mechanical properties were assessed using nanoindentation. As-deposited films exhibit non-equilibrium phases such as amorphous phases in the center of the phase diagram and β-tungsten in the tungsten-rich regime. After annealing, a new X-phase is observed in the center of the phase diagram and non-equilibrium phase compositions are observed for most compositions, such as the presence of α-tungsten at a ... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124860 Mon, 11 Oct 2010 00:00:00 +0100 4124860 http://www.actamat.org/article/PIIS1359645410006191/abstract?rss=yes We present atom probe tomography results across martensite/austenite interfaces in a precipitation-hardened maraging-TRIP steel (12.2 Mn, 1.9 Ni, 0.6 Mo, 1.2 Ti, 0.3 Al; at.%). The system reveals compositional changes at the phase boundaries: Mn and Ni are enriched while Ti, Al, Mo and Fe are depleted. More specific, we observe up to 27 at.% Mn in a 20nm layer at the phase boundary. This is explained by the large difference in diffusivity between martensite and austenite. The high diffusivity in martensite leads to a Mn flux towards the retained austenite. The low diffusivity in the austenite does not allow accommodation of this flux. Consequently, the austenite grows with a Mn composition given by local equilibrium. The interpretation is based on DICTRA and mixed-mode diffusion calculatio... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124858 Fri, 08 Oct 2010 00:00:00 +0100 4124858 http://www.actamat.org/article/PIIS1359645410006166/abstract?rss=yes Abstract: In this paper, the first investigation of the Orowan strengthening mechanism in the thermal plastic regime of body-centered cubic (bcc) materials is made with dislocation dynamics simulations. In bcc crystals, the mobility of dislocations strongly depends on temperature and dislocation line characters. Unlike the classical picture of the Orowan mechanism, simulations show that the difference in mobility between screw and non-screw characters is the key parameter. Simulations at different temperatures and with different precipitate microstructures illustrate the contribution of two mechanisms to Orowan strengthening: a thermally activated mechanism induced by the length dependency of the screw dislocation mobility and an athermal mechanism associated with dislocation line tension.... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167774 Thu, 07 Oct 2010 00:00:00 +0100 4167774 http://www.actamat.org/article/PIIS1359645410006208/abstract?rss=yes Abstract: The paper demonstrates the feasibility of upscaling of severe plastic deformation (SPD) techniques using an integrated process that combines conventional extrusion and equal channel angular pressing in a single processing step. The process was applied for production of magnesium ZK60 bars 16mm in diameter and up to 4m in length. The processed material exhibited an excellent balance of strength and tensile ductility. It had a yield strength of 310MPa and the ultimate tensile strength of 351MPa. The total elongation of 17.1% was about 2.5 times and the reduction in area of 42.5% was more than 10 times larger than the corresponding values in the as-received condition. A transition from brittle to ductile fracture suggesting a change in the deformation mechanisms was also found in th... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124859 Thu, 07 Oct 2010 00:00:00 +0100 4124859 http://www.actamat.org/article/PIIS135964541000618X/abstract?rss=yes Abstract: Hydroxyapatite (HAp) coatings with and without octacalcium phosphate (OCP) were uniformly formed on pure magnesium by a hydrothermal treatment using a Ca-EDTA solution. The crystal structure, crystallographic orientation and lattice images were investigated using transmission electron microscopy (TEM) and high-resolution TEM. It was demonstrated that the crystal phase and microstructure of the calcium phosphate-coatings can vary with the pH of the treatment solution. In a weak acid treatment solution, a dual-layer structure was formed: an outer coarse layer consisting of plate-like OCP crystals and an inner dense layer consisting primarily of HAp crystals. One piece of the OCP plate corresponded to a single OCP crystal growing parallel to the (100)OCP. In a weak alkali treatment ... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124857 Thu, 07 Oct 2010 00:00:00 +0100 4124857 http://www.actamat.org/article/PIIS1359645410006178/abstract?rss=yes Abstract: CuInS2 (CIS) thin films were deposited on Mo-coated glass substrates using in situ sulfurization of thermally evaporated precursor Cu–In alloys. Samples were removed before, during and after the sulfurization for X-ray diffractometry (XRD) and Raman spectroscopy analysis. While various phases, including the desired CIS, can be identified by XRD, we show that only Raman spectroscopy can reveal the existence of the two structure orders of the CIS phase, namely the CuInS2–chalcopyrite and the CuInS2–CuAu metastable structure orders. Furthermore, this study also provides direct evidence to show that the CIS phase grows at the expense of an intermediate Cu11In9 phase. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124856 Thu, 07 Oct 2010 00:00:00 +0100 4124856 http://www.actamat.org/article/PIIS1359645410006130/abstract?rss=yes Abstract: In many engineering applications, fatigue is the dominant failure mechanism governing the life of a component. Thus, many studies have focused on this phenomenon, although there is a need for a model that addresses fatigue based on the material’s microstructure, specifically the energetics of the grain boundaries (GBs) and persistent slip bands (PSBs). Our approach is to model the energy of a PSB structure and use its stability with respect to dislocation motion as our failure criterion for fatigue crack initiation. The components that contribute to the energy of the PSB are identified, namely the stress field resulting from the applied external forces, dislocation pile-ups and work-hardening of the material is calculated at the continuum scale. Further, energies for dislocatio... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124854 Thu, 07 Oct 2010 00:00:00 +0100 4124854 http://www.actamat.org/article/PIIS1359645410006117/abstract?rss=yes Abstract: Experiments were conducted on high-purity aluminum to determine the degree of hardness homogeneity attained during processing by high-pressure torsion (HPT). The HPT processing was conducted at room temperature using several disks of identical initial thickness with an applied pressure of 6.0GPa and torsional straining from 1/4 to 20 turns. Following HPT, the flow patterns were observed by optical microscopy and values of the Vickers microhardness were recorded on different planes. The results show that, for any selected processing conditions, the distributions of microhardness values and the appearance of the etched surfaces in optical microscopy are independent of the plane of sectioning within the HPT disks. There is also a gradual evolution in homogeneity with increasing numb... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124852 Thu, 07 Oct 2010 00:00:00 +0100 4124852 http://www.actamat.org/article/PIIS1359645410006105/abstract?rss=yes Abstract: The total energy, lattice constant, bulk modulus and its first derivative of stable binary face-centered cubic (fcc)-ZrN, hexagonal close-packed (hcp)(β)-Si3N4, and unstable hcp-Zr3N4 and fcc-SiN, as well as the total energy and lattice constant of the ternary fcc(NaCl)- and hcp(β)-Zr1−xSixNy solid solution phases were calculated by ab initio density functional theory (DFT) at 0K. A combination of ab initio DFT and thermodynamic calculations was then used to study the phase stabilities and possible mechanisms of the decomposition of the Zr1−xSixNy solid solution at elevated temperatures accounting for the mixing entropy and for the temperature dependence of the interaction parameter that describes the mixing enthalpy. It is shown that the exponential dependence of the inter... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124851 Thu, 07 Oct 2010 00:00:00 +0100 4124851 http://www.actamat.org/article/PIIS1359645410006099/abstract?rss=yes Abstract: Grain boundaries (GBs) provide a strengthening mechanism in engineering materials by impeding dislocation motion. In a polycrystalline material, there is a wide distribution of GB types with characteristic slip transmission and nucleation behaviors. Slip–GB reactions are not easy to establish analytically or from experiments; furthermore, there is a strong need to quantify the energy barriers of the individual GBs. We introduce a methodology to calculate the energy barriers during slip–GB interaction, in concurrence with the generalized stacking fault energy curve for slip in a perfect face-centered cubic material. By doing so, the energy barriers are obtained at various classifications of GBs for dislocation transmission through the GB and dislocation nucleation from the GB.... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124850 Thu, 07 Oct 2010 00:00:00 +0100 4124850 http://www.actamat.org/article/PIIS1359645410005793/abstract?rss=yes Abstract: The laser floating zone technique (LFZ) was used to produce CaCu3Ti4O12 (CCTO) samples with different pulling rates (Rp). Polycrystalline samples were obtained for higher Rp, and single crystals for low Rp. Morphologic characterization was done by scanning electron microscopy (SEM), while the structural analysis was accomplished using X-ray diffraction and Raman spectroscopy. The best crystallinity was obtained for the fibre grown at the lowest pulling rate, resulting in a CCTO single crystal. Dielectric spectroscopy was performed between 40Hz and 50MHz in the temperature range of 90–350K. The dielectric strength varies from 3×104 for the fibres grown at higher Rp (80mmh−1), to 5×104 for the ones grown at lower Rp (5mmh−1). The Cole–Cole model was used to fit the dielec... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124833 Thu, 07 Oct 2010 00:00:00 +0100 4124833 http://www.actamat.org/article/PIIS1359645410006154/abstract?rss=yes Abstract: Friction and wear of copper rubbed in the presence of lubricants over a wide range of loads and sliding velocities were studied. The results of friction and wear experiments in the boundary lubrication (BL) and elastohydrodynamic lubrication (EHL) regimes are briefly considered. The structural state of subsurface layers in different lubricant regions is studied by optical, transmission and scanning microscopy analysis. Dislocation density and Burgers vector of individual dislocations in EHL and BL lubricant regimes were determined. A laminated structure with thin and elongated grains is formed in the EHL region. A range of nanocrystalline structures with grain sizes of 20–100nm is formed in the surface layers in the BL region. Strong plastic deformation (SPD) of thin surface la... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124855 Wed, 06 Oct 2010 00:00:00 +0100 4124855 http://www.actamat.org/article/PIIS1359645410006129/abstract?rss=yes Abstract: A new mesoscale model was developed to understand the precipitation sequence of topologically close-packed (TCP) phases in single-crystal nickel-base superalloys. Nucleation and growth phenomena are described in detail. The model allows the multicomponent, multiphase and multiparticle calculation of TCP phase volume fractions and size distributions from supersaturated solid solution to thermodynamic equilibrium. It is based on the numerical Kampmann–Wagner method and applies CALPHAD (ThermoCalc) and DICTRA methodology for thermodynamics and diffusion. The capabilities of the simulations are demonstrated by the prediction of the size distributions of TCP phases and comparison with experimental data. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124853 Wed, 06 Oct 2010 00:00:00 +0100 4124853 http://www.actamat.org/article/PIIS135964541000604X/abstract?rss=yes Abstract: The microstructure and mechanical properties of hot extruded Mg–7Al–1Zn–1Ca powder alloys with an addition of 1.5% La or 3.3% La were investigated. Both rapidly solidified powders, produced via spinning water atomization process, and cast billets were extruded at 573, 623 and 673K to optimize the processing conditions for obtaining better mechanical response. Powders were consolidated using both cold compaction and spark plasma sintering. The tensile properties of the extruded alloys were then evaluated and correlated to their microstructures. The results showed that the use of rapidly solidified Mg–7Al–1Zn–1Ca alloy powders with La additions could lead to effective grain refinement and super saturation of alloying elements, which in turn resulted in the improved mech... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124849 Mon, 04 Oct 2010 00:00:00 +0100 4124849 http://www.actamat.org/article/PIIS1359645410006038/abstract?rss=yes Abstract: Cross-correlation-based analysis of electron back-scatter diffraction (EBSD) patterns has been used to obtain high angular resolution maps of lattice rotations and elastic strains near carbides in a directionally solidified superalloy MAR-M-002. Lattice curvatures were determined from the EBSD measurements and used to estimate the distribution of geometrically necessary dislocations (GNDs) induced by the deformation. Significant strains were induced by thermal treatment due to the lower thermal expansion coefficient of the carbide inclusions compared to that of the matrix. In addition to elastic strains the mismatch was sufficient to have induced localized plastic deformation in the matrix leading to a GND density of 3×1013m–2 in regions around the carbide. Three-point bending... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124848 Mon, 04 Oct 2010 00:00:00 +0100 4124848 http://www.actamat.org/article/PIIS1359645410005938/abstract?rss=yes Abstract: The partitioning of rotations is considered for two-phase interfaces. In the isotropic, linear elastic approximation, the rotations associated with tilt components of interface dislocations and disconnections partition equally to the two phases. With anisotropic elasticity and with nonlinearities, the partitioning is unequal. Results for linear anisotropic elasticity, average anisotropic elasticity and nonlinear effects as incorporated in an atomistic simulation are compared. The results also apply to most cases of single phase grain boundaries. The results of the atomistic simulations are as predicted in the topological theory of phase transformation. The results have implications for habit plane determination in phase transformations, for the relief of coherency stresses at int... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124846 Mon, 04 Oct 2010 00:00:00 +0100 4124846 http://www.actamat.org/article/PIIS1359645410005902/abstract?rss=yes Abstract: We have developed new bulk metallic glasses (BMGs) with optimized glass-forming ability (GFA), in ternary Zr–Cu–Al and quaternary Zr–Ti–Cu–Al systems. The relatively large dimensions of these BMGs permitted fracture toughness tests, leading to the discovery of Zr61Ti2Cu25Al12 (ZT1) that has a toughness among the highest for monolithic BMGs (fatigue pre-cracked fracture toughness, Kmax, in excess of ∼100MPa√m, with a nonlinear-elastic fracture behavior as seen in the resistance curve). We have also measured the toughness for Zr–TM–Al (TM=Co, Ni) BMGs, which have previously been optimized for GFA. Comparing the BMGs studied, in conjunction with literature data on Zr-based BMGs, we have identified trends in chemistry effects on BMG fracture toughness, in particular... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124843 Mon, 04 Oct 2010 00:00:00 +0100 4124843 http://www.actamat.org/article/PIIS1359645410005896/abstract?rss=yes Abstract: This paper proposes a method to engineer the effects of mesa aspect ratio on polarization switching for single-crystal and polycrystalline PZT nanostructures. The out-of-plane polarization switching of single-crystal and polycrystalline structures as a function of crystallographic orientation, epitaxial strain and mesa aspect ratio are explored. The results are summarized in terms of the mesa geometrical parameters, crystallographic orientation and expitaxial strain. The results demonstrate a strong correlation of single-crystal properties to the polarization hysteresis behavior of a central representative grain in a polycrystalline film. The average remnant polarization and its reliability are controlled through the aspect ratio of the mesa. Calculations demonstrate that the str... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124842 Mon, 04 Oct 2010 00:00:00 +0100 4124842 http://www.actamat.org/article/PIIS1359645410005458/abstract?rss=yes Abstract: Mg–Y alloys show significantly enhanced room temperature ductility compared to pure Mg and other classical Mg wrought alloys. The presented study focuses on understanding the mechanisms for this ductility improvement by microstructure analysis, texture analysis and slip trace analysis based on electron backscatter diffraction and transmission electron microscopy. As expected, pure Mg mainly deforms by 〈a〉 basal slip and tensile twinning. In contrast, Mg–Y shows a high activity of compression twinning, secondary twinning and pyramidal 〈c+a〉 slip. These additional deformation modes cause a homogeneous deformation with a weaker basal texture, more balanced work hardening and enhanced ductility. Additionally, in Mg–Y shear bands are much more frequent and carry less str... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167772 Fri, 01 Oct 2010 00:00:00 +0100 4167772 http://www.actamat.org/article/PIIS1359645410006026/abstract?rss=yes Abstract: Grain boundary engineering (GBE) aims at optimizing the properties of face centred cubic materials with low stacking fault energy by creating a high content of special twin boundaries. Quantifying twinning and its parameters, such as the twin related domains (TRDs) and the related ∑3n special grain boundaries, is thus of prime importance for GBE. A method is presented in detail to treat the electron backscatter diffraction (EBSD) maps. The TRDs are automatically reconstructed, the twinning chain trees of the TRDs are determined and represented with fractal graphs, and the ∑3n grain boundaries are identified up to order n=12. New parameters, such as the numbers of grains (Ng), the lengths of the longest chain (LLC), the twinning polysynthesism (p) and twinning anisotropy (a) f... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124847 Fri, 01 Oct 2010 00:00:00 +0100 4124847 http://www.actamat.org/article/PIIS1359645410005914/abstract?rss=yes Abstract: Acicular structures on NiTi are often interpreted as martensite or martensitic surface relief. These structures are identified as artifacts and form during etching using hydrofluoric acid based etching solutions. Their origin is clarified in the present work. After standard metallographic preparation, the acicular structures cannot be distinguished from bulk material by standard analysis, e.g. energy dispersive X-ray analysis and Auger electron spectroscopy. By investigating samples after etching, but previous to cleaning with water, rod-like crystals on the NiTi surface were detected. The crystals are identified as NiTiF6⋅6H2O by X-ray diffraction. During etching, the crystals develop on the surface and locally prevent contact of etching solution and NiTi base material. Furthe... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124844 Fri, 01 Oct 2010 00:00:00 +0100 4124844 http://www.actamat.org/article/PIIS1359645410005884/abstract?rss=yes Abstract: The decomposition of an equiatomic AlCoCrCuFeNi high-entropy alloy produced by splat quenching and casting was investigated by the analytical high resolution methods: transmission electron microscopy and three-dimensional atom probe. It could be shown that splat-quenched alloy consisted of an imperfectly ordered body-centred cubic phase with a domain-like structure, whereas normally cast alloy formed several phases of cubic crystal structure. The cast alloy decomposed into both dendrites and interdendrites. A detailed local compositional analysis carried out by atom probe within the dendrites revealed that the alloying elements in the Ni–Al-rich plates and Cr–Fe-rich interplates are not randomly distributed, but segregate and form areas with pronounced compositional fluctuati... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124841 Fri, 01 Oct 2010 00:00:00 +0100 4124841 http://www.actamat.org/article/PIIS1359645410005860/abstract?rss=yes Abstract: We examine the propensity for the formation of thin films of cubic TiC, VC, CrC, ZrC, NbC, MoC, HfC, TaC and WC in the metal–ceramic interface between simple hexagonal WC and Co using density functional theory. Films of TiC, VC, CrC and MoC are predicted to be stable, of which VC and MoC show the strongest propensity for formation. The results provide an explanation for the experimentally observed WC grain growth inhibition effect and alteration of WC grain shape attained in VC-doped WC–Co. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124840 Fri, 01 Oct 2010 00:00:00 +0100 4124840 http://www.actamat.org/article/PIIS135964541000580X/abstract?rss=yes Abstract: β Phase elasticity, stress-induced α″ transformation and superelasticity in hot-forged Ti–24Nb–4Zr–8Sn–(0.08–0.40)O (wt.%) has been investigated by in situ synchrotron X-ray diffraction, Rietveld refinement and texture analysis. The β elastic constants were determined by Eshelby–Kröner–Kneer self-consistent modelling. A micromechanical model based on texture, martensite crystallography and Schmid law nucleation criteria was used to analyse the generation of non-linear strain. Oxygen increased the C′ cubic shear constant. The β phase showed increased compliance along 〈110〉β as precursor to transformation, which occurs above 0.15wt.% O by growth of nanosized nuclei, and is nearly suppressed at 0.40wt.% O by structural convergence of parent/martensite pha... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124834 Fri, 01 Oct 2010 00:00:00 +0100 4124834 http://www.actamat.org/article/PIIS1359645410005756/abstract?rss=yes Abstract: In this article, we examine the unusual plastic deformation under uniaxial compression of an Al2O3–15mol.% Y2O3 (A15Y) glass synthesized by a wet chemical route. At a low temperature of 650–725°C, plastic deformation of this glass is largely non-viscous through shear instabilities. In contrast, deformation near the crystallization temperature (850°C) occurs homogeneously with work hardening and with a monotonic increase in the true density of the glass by 10–12%, accompanied by an increase in hardness (H) and elastic modulus (E) of up to 100%. We hypothesize a phenomenon of molecular densification of the amorphous structure through a hierarchy of multiple phases, analogous to density- or entropy-driven amorphous-to-amorphous phase transitions (polyamorphism). These result... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124831 Fri, 01 Oct 2010 00:00:00 +0100 4124831 http://www.actamat.org/article/PIIS1359645410005719/abstract?rss=yes Abstract: In order to obtain modulated martensite in our epitaxial Ni–Mn–Ga films, we tuned the composition by using a co-sputtering process. Here we present how the composition affects the variant distribution of the 14-modulated martensite at room temperature. The nature of such modulated martensites is still strongly debated for magnetic shape memory alloys. It has been very recently demonstrated that the modulated martensites in Ni–Mn–Ga are adaptive phases. The results presented here corroborate this theory for the first time, for three different compositions. Moreover, we demonstrate with the help of the adaptive modulations theory that b-variants of the 14-modulated martensite form close to the free surface of the film to release the stress induced by branching of macro-twin... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124830 Fri, 01 Oct 2010 00:00:00 +0100 4124830 http://www.actamat.org/article/PIIS1359645410005987/abstract?rss=yes (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012915 Wed, 29 Sep 2010 23:00:27 +0100 4012915 http://www.actamat.org/article/PIIS1359645410005926/abstract?rss=yes Abstract: The oxidation resistance of three prototype single-crystal nickel-based superalloys for industrial (electricity-generating) gas turbine applications is studied. All contain greater quantities of Cr than in most existing single-crystal superalloys; two are alloyed with Si. All alloys are found to be marginal Al2O3-formers, with the performance being better at 1000°C rather than 900°C, and when Si is added. Microstructural analysis indicates that the ability to form an Al2O3 layer is better in the interdendritic regions; the dendritic regions are prone to internal oxidation. In all cases, an outer scale of Cr2O3 is formed which is in contact with either Ta2O5 (at 1000°C) or NiTa2O6 (at 900°C). To explain the results, the factors known to influence the rate of Al2O3 scale format... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124845 Tue, 28 Sep 2010 00:00:00 +0100 4124845 http://www.actamat.org/article/PIIS1359645410005859/abstract?rss=yes In this study, we use molecular dynamics (MD) simulations of Cu–Zr binary metallic glasses to examine the effects of the next level of structural hierarchy: the interpenetrating connection of icosahedra (ICOI) and the linkage of the medium-range ICOI patches to form networks of icosahedra over extended range. The mechanical properties of these metallic glasses, especially the shear transformations that mediate plasticity, are found to be dependent on the degree of ICOI and development of the ICOI network. The evolution of the ICOI network during shear deformation, as well as the composition dependence, has been monitored and discussed. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124839 Tue, 28 Sep 2010 00:00:00 +0100 4124839 http://www.actamat.org/article/PIIS1359645410005847/abstract?rss=yes Abstract: Titanium foams with aligned, elongated pores were created by directional freeze-casting of aqueous slurries of titanium powders, followed by ice sublimation and powder sintering. Increasing sintering times from 8 to 24h and decreasing powder size from 20 to 10μm resulted in improved densification within cell walls and decreased overall foam porosity, with a concomitant increase in compressive stiffness, yield strength and energy absorption. A simple model for foam stiffness and strength is in general agreement with experimental measurements of strength but overpredicts stiffness, probably because it does not take into account micro-plasticity occurring during measurements. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124838 Tue, 28 Sep 2010 00:00:00 +0100 4124838 http://www.actamat.org/article/PIIS1359645410005835/abstract?rss=yes Abstract: The dynamic behavior of nanoscale prismatic dislocation loops can significantly affect the microstructural variation in crystalline materials upon processes such as plastic deformation and high-energy particle irradiation. Using in situ transmission electron microscopy, this study experimentally demonstrates a reaction which follows the collision between two loops with different Burgers vectors in α-iron. Even after the formation of the junction, the reaction progresses further, unlike conventional reactions between dislocations of macroscopic length, and the larger loop finally absorbs the smaller one. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124837 Tue, 28 Sep 2010 00:00:00 +0100 4124837 http://www.actamat.org/article/PIIS1359645410005823/abstract?rss=yes Abstract: An antiphase boundary (APB)-like structure of the B19′ martensite in Ti–Ni alloy was investigated by transmission electron microscopy. The APB-like structure shows atomic shifts on both the (010)B19′ plane along the c-axis and the (001)B19′ plane along the b-axis; a kind of ledge-and-step structure on the b–c plane, in addition to a displacement along the a-axis. The displacement vector can be expressed as R=〈−0.16481/20.4328〉 in terms of the conventional atomic coordinates of Ti and Ni in the B19′ martensite. The APB-like structure is not inherited from APB in the B2 parent phase. We conclude that the APB-like structure is developed by accidental impingement of differently nucleated martensitic domains during the transformation. This structure is defined as a k... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124836 Tue, 28 Sep 2010 00:00:00 +0100 4124836 http://www.actamat.org/article/PIIS1359645410005811/abstract?rss=yes Abstract: The photoluminescence properties of ZnO were investigated experimentally and by a first-principles method. Thermal treatment of ZnO in various atmospheres provides insight into the ultraviolet (UV) and green emission mechanisms. These mechanisms were investigated in detail from the thermal quenching behavior of the emission spectra of ZnO. The negative thermal quenching of the green emission could be fitted exactly with a model equation based on the assumption that UV emission loss by thermal quenching affects green emission. The electronic states of ZnO clusters with and without an oxygen vacancy were calculated by first-principles calculations using the discrete variational (DV) Xα molecular orbital method. This method gave a reasonably accurate calculation of band-gap energy.... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124835 Tue, 28 Sep 2010 00:00:00 +0100 4124835 http://www.actamat.org/article/PIIS1359645410005768/abstract?rss=yes Abstract: Epitaxial silver films with (111) and (110) orientations were deposited by magnetron sputtering onto silicon (111) and (110) substrates, respectively. High density growth twins with an average spacing of a few nanometers are observed in the (111) oriented films, with twin boundaries oriented normal to the growth direction. Twins are observed in the (110) oriented films with a much lower twin density, and the twin boundaries are oriented 60° from the growth direction. The epitaxial nanotwinned Ag (111) and (110) films have respective indentation hardness values of ∼2 and 1GPa. The influences of deposition parameters, stacking fault energy, and orientation of {111} planes on the formation of nanotwins are discussed. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124832 Tue, 28 Sep 2010 00:00:00 +0100 4124832 http://www.actamat.org/article/PIIS1359645410005707/abstract?rss=yes Abstract: Stainless steel is a good example of a metal that is not easily machined. To explain such behavior an understanding of the fundamental adhesion between the workpiece and the tool is invaluable. It is a well-known fact that build-up layers form in the interface, but little attention has been given to the very first layer that adheres to the tool surface. Although this layer rapidly becomes covered by successive material transfer, this layer and its ability to stick to the tool surface control the successive material transfer and influence the cutting properties. In this work, a quick stop test is employed to interrupt the cutting of a 316L stainless steel using a TiN-coated cemented carbide cutting insert. Different analytical techniques, such as transmission electron microscopy, ... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124829 Tue, 28 Sep 2010 00:00:00 +0100 4124829 http://www.actamat.org/article/PIIS135964541000577X/abstract?rss=yes In this study, the effect of the presence of Al2O3 nanoparticles during mechanical alloying on the extension of Cr solid solubility in Cu was investigated. The lattice parameter, dislocation density and crystallite size were evaluated by the X-ray diffraction technique. Also, the microstructure was characterized by scanning electron microscopy and transmission electron microscopy. The final product was a nanocrystalline and supersaturated Cu–Cr solid solution with a mean crystallite size ranging from 8 to 19nm depending on the composition. The Gibbs free energy changes in these systems due to the dislocation density and crystallite size variations during milling were calculated. It was found that the presence of Al2O3 nanoparticles was beneficial to the process, and this was particularly... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077133 Mon, 27 Sep 2010 23:00:00 +0100 4077133 http://www.actamat.org/article/PIIS1359645410005744/abstract?rss=yes This study suggests that a strain rate in the order of 102s−1 should be adopted in severe plastic deformation techniques to produce nanometer-sized grains. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077132 Mon, 27 Sep 2010 23:00:00 +0100 4077132 http://www.actamat.org/article/PIIS1359645410005732/abstract?rss=yes Abstract: The addition of small amounts of B to Ti–6Al–4V alloy reduces the as-cast grain size by an order of magnitude and introduces TiB phase into the microstructure. The effects of these microstructural modifications on both the high cycle fatigue and cyclic stress–strain response were investigated. Experimental results show that B addition markedly enhances the fatigue strength of the alloy; however, the influence of prior-β grain size was found to be only marginal. The presence of TiB particles in the matrix appears to be beneficial with the addition of 0.55wt.% B to Ti–6Al–4V enhancing the fatigue strength by more than 50%. Strain-controlled fatigue experiments reveal softening in the cyclic stress–strain response, which increases with the B content in the alloy. Transm... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077131 Mon, 27 Sep 2010 23:00:00 +0100 4077131 http://www.actamat.org/article/PIIS1359645410005720/abstract?rss=yes Abstract: The composition and temperature dependence of the elastic properties and phase stability of quaternary Ti–Nb–Ta–Zr β-phase alloys with a body-centered cubic structure, developed for biomedical applications, were investigated using their single crystals, in order to clarify the origin of the low Young’s modulus in polycrystals. Transmission electron microscopy observations clarified that α″ martensitic transformation occurred in a temperature range that depended on the β-phase stability below room temperature. Electromagnetic acoustic resonance measurements clarified that the shear moduli c′ and c44 of single crystals softened upon cooling from room temperature and became rather low near the martensitic transformation start temperature, i.e. the lower limit of β-ph... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077130 Mon, 27 Sep 2010 23:00:00 +0100 4077130 http://www.actamat.org/article/PIIS1359645410005690/abstract?rss=yes Abstract: Three-dimensional spatially resolved strains were mapped in a model NiAl/Mo composite after nanoindentation. The depth-dependent strain distributed in the two phases and partitioned across the composite interfaces is directly measured at submicron length scale using X-ray microdiffraction and compared with a detailed micromechanical stress analysis. It is shown that indentation-induced deformation in the composite material is distinct from deformation expected in a single-phase material. This difference arises in part from residual thermal strains in both phases of the composite in the as-grown state. Interplay between residual thermal strains and external mechanical strain results in a complex distribution of dilatational strain in the Mo fibers and NiAl matrix and is distinct i... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077129 Mon, 27 Sep 2010 23:00:00 +0100 4077129 http://www.actamat.org/article/PIIS1359645410005677/abstract?rss=yes Abstract: In this work orientation imaging microscopy (OIM), based on electron backscatter diffraction in scanning electron microscopy, was employed to examine in detail the relationship between laser cladding processing parameters and he properties and the microstructure of single and overlapping laser tracks. The study was performed on thick (∼1mm) Co-based coatings prepared by 2kW Nd:YAG laser cladding a 42CrMo4 steel substrate using different laser beam scanning speeds (1.0–15mms−1). It was found that the directional growth of individual primary grains led to the formation of a typical solidification fiber texture. The dependence of this texture on the processing speed and the shape of the solidification front were investigated in detail. Strong epitaxial growth of Co grains on a... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077127 Mon, 27 Sep 2010 23:00:00 +0100 4077127 http://www.actamat.org/article/PIIS135964541000563X/abstract?rss=yes Abstract: The mechanical behavior and the kinetics of shear banding in bulk metallic glasses was investigated at room and liquid nitrogen temperature using the acoustic emission (AE) technique. It was demonstrated that the intensive AE reflecting the activity of strongly localized shear bands at room temperature vanishes at the transition of plastic flow from serrated to non-serrated with a reduction in temperature. The disappearance of AE clearly suggests that the shear band propagation velocity significantly decreases at low temperature, and sliding along the principle shear band is observed at the machine-driven rate. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077124 Mon, 27 Sep 2010 23:00:00 +0100 4077124 http://www.actamat.org/article/PIIS1359645410005628/abstract?rss=yes Abstract: Yellowish-orange-emitting ZnGeN2 and orange-red-emitting ZnGeN2:Mn were synthesized by a facile and mild gas-reduction–nitridation reaction at 1153K under NH3 flow with air-stable raw materials ZnO, GeO2 and MnCO3. The structure, composition, morphology, photoluminescence and cathodoluminescence properties of ZnGeN2 doped with or without Mn2+ were systematically investigated. Rietveld refinements show that the as-synthesized samples are obtained as single-phase compounds and crystallize as an orthorhombic structure with a space group of Pna21. The actual chemical composition of the as-prepared ZnGeN2 determined by energy dispersive X-ray spectroscopy suggests that the Ge vacancy defects probably exist in the host. The SEM image reveals that the Zn0.99Mn0.01GeN2 particles form a... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077123 Mon, 27 Sep 2010 23:00:00 +0100 4077123 http://www.actamat.org/article/PIIS1359645410005665/abstract?rss=yes Abstract: The formation of stoichiometric MpXq hollow nanospheres by reaction of metallic M nanospheres with the gaseous X phase must be preceded by the formation of a sufficiently thick MpXq nanoshell on the metallic core of phase M. During this stage, high supersaturation of vacancies in the M core and/or hydrostatic stress in the M core, due to the misfit between the core and the nanoshell, are developed and provide favourable conditions for the hollow nucleation. The misfit is caused by the Kirkendall effect.Based on the application of the thermodynamic extremal principle, a kinetic model of MpXq nanoshell formation is derived. The kinetics is driven by the change of the chemical energy due to reaction of the M and X components, the interface and surface energies, and the elastic strai... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124828 Mon, 20 Sep 2010 00:00:00 +0100 4124828 http://www.actamat.org/article/PIIS1359645410005781/abstract?rss=yes Abstract: Thermal barrier coatings (TBCs) used in gas-turbine engines afford higher operating temperatures, resulting in enhanced efficiencies and performance. However, at these high operating temperatures, environmentally ingested airborne sand/ash particles melt on the hot TBC surfaces and form calcium–magnesium–aluminosilicate (CMAS) glass deposits. The molten CMAS glass penetrates the TBCs, leading to loss of strain tolerance and TBC failure. Here we demonstrate the use of the commercial manufacturing method of air-plasma-spray (APS) to fabricate CMAS-resistant yttria-stabilized zirconia (YSZ)-based TBCs containing Al and Ti in solid solution. Results from thermal stability studies of these new TBCs and CMAS/TBC interaction experiments are presented, together with a discussion of t... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077134 Sun, 19 Sep 2010 23:00:00 +0100 4077134 http://www.actamat.org/article/PIIS1359645410005689/abstract?rss=yes Abstract: A series of binary magnesium-based alloys have been prepared with Gd concentrations between 0.22 and 4.65wt.% Gd. These alloys were processed by hot rolling in order to refine the microstructure and examine the recrystallisation behaviour. The addition of small amounts of Gd was found to significantly decrease the recrystallised grain size and at higher alloy concentrations nucleation of recrystallisation became more strongly inhibited, but the growth rate remained largely unchanged. The effect of Gd concentration on solute strengthening was quantified and it was found that strengthening of the prismatic slip system above 100MPa could be achieved through alloying with Gd. A fivefold increase in ductility also resulted from Gd addition, and this was attributed to changes in the re... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077128 Sun, 19 Sep 2010 23:00:00 +0100 4077128 http://www.actamat.org/article/PIIS1359645410005653/abstract?rss=yes Abstract: Fe–Mn–Si is a well-characterized ternary shape memory alloy. Research on this alloy has consistently shown that the addition of 5–6wt.% Si is desirable to enhance the reversibility of stress-induced martensite vis-à-vis shape memory. This paper examines the effect of Si on the morphology and the crystallography of the martensite in the Fe–Mn–Si system. It is concluded that the addition of Si increases the c/a ratio of the martensite, reduces the transformation volume change and decreases the atomic spacing difference between the parallel close-packed directions in the austenite–martensite interface (habit) plane. It is proposed that, in addition to austenite strengthening, Si enhances reversibility by reducing the volume change and the interfacial atomic mismatch bet... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077126 Sun, 19 Sep 2010 23:00:00 +0100 4077126 http://www.actamat.org/article/PIIS1359645410005616/abstract?rss=yes Abstract: A thermodynamically consistent framework is developed to combine the convection and dissipative contributions with the mass and energy fluxes in multicomponent materials. The method, which is known as bi-velocity or Darken method, is based on the postulate of the unique (independent on the frame of reference) transport of the mass due to diffusion. In this work we merge: (i) the combined statements of the first and second laws of thermodynamics (linear irreversible thermodynamics) and (ii) the Euler and Liouville theorems. We derive an expression for the rate of the entropy production that does not depend on the frame of reference and is always positive. We show that the derived set of partial differential equations describing the mechanochemical mass transport is thermodynamical... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077122 Sun, 19 Sep 2010 23:00:00 +0100 4077122 http://www.actamat.org/article/PIIS1359645410005641/abstract?rss=yes Abstract: A crystallographic model is used to predict the nucleation potencies of TiB2 particles during solidification of TiAl-based alloys. Two nucleation scenarios are investigated. In scenario 1, primary TiB2 grows in the melt before formation of the body-centred cubic β phase. In scenario 2, secondary TiB2 precipitates after the first β phase but before formation of the hexagonal close-packed α phase. The model predicts high α and β nucleation potencies of TiB2 in both scenarios. However, pre-existing β grains in scenario 2 are predicted to be preferred α nucleation sites. The experimentally observed β refinement by primary TiB2 agrees with the model predictions. Grain refinement in scenario 2 is attributed to α nucleation on β, which is interleaved with secondary TiB2. (Sour... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077125 Thu, 16 Sep 2010 23:00:00 +0100 4077125 http://www.actamat.org/article/PIIS1359645410005501/abstract?rss=yes Abstract: Next-neighbor misorientation distributions (NNMD) in severely deformed polycrystalline materials are commonly measured by orientation imaging. A procedure is proposed which enables the separation of NNMD of ultrafine-grained materials into two parts: the distribution of misorientations between newly emerged grains within the original (“parent”) grain interior (“internal daughter grains”) and the distribution of misorientations between grains adjacent to an original grain boundary on its opposite sides (“grain boundary daughter grains”). The procedure is based on electron backscatter diffraction orientation map analyses carried out on different planes of deformed samples considering the evolution of the grain size and shape during severe plastic deformation. It was app... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077121 Mon, 13 Sep 2010 23:00:00 +0100 4077121 http://www.actamat.org/article/PIIS1359645410005550/abstract?rss=yes (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=3957766 Sat, 11 Sep 2010 09:19:37 +0100 3957766 http://www.actamat.org/article/PIIS1359645410005471/abstract?rss=yes In this study, the equilibrium shape of a coherent Ni4Ti3 precipitate and the concentration and stress fields around it are determined quantitatively using the phase field method. Most recent experimental data on lattice parameters, elastic constants, precipitate–matrix orientation relationship and thermodynamic database are used as model inputs. The effects of the concentration and stress fields on subsequent martensitic transformations are analyzed through interaction energy between a nucleating martensitic particle and the existing microstructure. Results indicate that R-phase formation prior to B19′ phase could be attributed to both direct elastic interaction and stress-induced spatial variation in concentration near Ni4Ti3 precipitates. The preferred nucleation sites for the R-pha... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077119 Thu, 09 Sep 2010 23:00:00 +0100 4077119 http://www.actamat.org/article/PIIS135964541000546X/abstract?rss=yes Abstract: The vacancy model of diffusion is applied to magnetite and hematite, and mathematical expressions for the iron flux in the lattice-fixed frame of reference, as a function of the defect structure, are presented. The defect structures, i.e. the vacancy content on the different type of sites, and the thermodynamic factors are calculated from the available Calphad type of thermodynamic descriptions for the oxides. Expressions for Fe tracer diffusion coefficients are derived and the relations between mobility and tracer diffusivity are given. The mobilities are fitted by a least-squares optimization to experimental data on tracer diffusion from the literature. For magnetite, an excellent representation of the experimental tracer data is achieved together with a satisfactory descriptio... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124827 Thu, 09 Sep 2010 00:00:00 +0100 4124827 http://www.actamat.org/article/PIIS1359645410005483/abstract?rss=yes Abstract: An aged Al–5Zn–1.6Mg alloy with fine η′ precipitates was grain refined to ∼100nm grain size by means of confined channel die pressing. Microstructure observations and mechanical tests were carried out to characterize the materials before and after various degrees of severe plastic deformation. Deformation processing enhanced the strength of the alloy, but limited its work hardenability. An analysis of deformation mechanisms revealed that plasticity proceeded by dislocation slip through ultrafine-grained cellular and subgrain arrangements. η′ precipitates strengthened the alloy by dispersion hardening, but retarded an increase in the strain rate sensitivity during grain refinement. The influence of η′ precipitates is discussed with respect to their effect on disloca... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077120 Wed, 08 Sep 2010 23:00:00 +0100 4077120 http://www.actamat.org/article/PIIS1359645410005446/abstract?rss=yes Abstract: Local lattice rotations and in-grain orientation gradients at κ precipitates in matrix grains with orientations near the 45° rotated cube {001}〈110〉 (RC) and the γ-fiber components {111}〈112〉 were investigated in a Fe3Al alloy warm-rolled to reductions of between 10% and 60%. Near-RC grains showed larger local lattice rotations at precipitates than γ-fiber grains. In RC-oriented grains the local lattice rotations about the transverse direction (TD) were dominant at low reductions, but rotations about the rolling direction (RD) also occurred at higher strains. In the γ-fiber grains the axes of the in-grain lattice rotations were scattered between TD and RD. The rotations around the particles and their orientation dependence were analyzed using 3-D crystal plasticity f... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077118 Wed, 08 Sep 2010 23:00:00 +0100 4077118 http://www.actamat.org/article/PIIS1359645410005434/abstract?rss=yes Abstract: The crystal structure and complex twinning microstructure of epitaxial Ni–Mn–Ga films on (100) MgO substrates was studied by X-ray diffraction using 2θ scans, pole figure measurements and reciprocal space mapping (RSM). The orientation distribution of all variants is visualized by RSM, which forms the basis for a better understanding of the crystallographic relation between variants and substrate. Above the martensitic transformation temperature the film consists of single austenite phase with lattice constant a=5.81Å at 419K. At room temperature some epitaxially grown residual austenite with a=5.79Å remains at the interface with the substrate, followed by an intermediate layer exhibiting orthorhombic distortion, atrans=6.05Å, btrans=5.87Å, ctrans=5.73Å and a major frac... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077117 Wed, 08 Sep 2010 23:00:00 +0100 4077117 http://www.actamat.org/article/PIIS1359645410005380/abstract?rss=yes Abstract: Thin film components of conventional and flexible solid-state devices experience mechanical strain during fabrication and operation. At the bulk scale, small values of strain do not affect thermal conductivity, but this may not true for grain sizes comparable with the electron and phonon mean free paths and for higher volume fraction of grain boundaries. To investigate this hypothesis, thermal and electrical conductivity of nominally 125-nm-thick aluminum films (average grain size 50nm) were measured as functions of tensile thermo-mechanical strain, using a modified version of the 3-ω technique. Experimental results show pronounced strain–thermal conductivity coupling, with ∼50% reduction in thermal conductivity at ∼0.25% strain. The analysis shows that mechanical strain d... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077112 Thu, 02 Sep 2010 23:00:00 +0100 4077112 http://www.actamat.org/article/PIIS1359645410005355/abstract?rss=yes Abstract: Unmodified and Sr-modified (after 5 and 120min of melt holding) Al–15Si alloys were investigated by transmission electron microscopy and focused ion beam tomography using energy-selective backscattered electrons for imaging. The three-dimensional visualization of the microstructure provided not only the true morphology of the Al–Si eutectic and Fe-rich intermetallics, but also allowed us to estimate their volume fractions. The evolution of Fe-rich α-phase morphology in the unmodified alloy proceeded during eutectic growth according to a model proposed. In the unmodified alloy, only Fe-rich α-phase was found, whereas in the Sr-modified alloy after 5min of melt holding, two morphologies of Fe-rich phases were observed, namely a Fe-rich α-phase and a Fe-rich δ-phase. Both ph... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077110 Thu, 02 Sep 2010 23:00:00 +0100 4077110 http://www.actamat.org/article/PIIS1359645410005422/abstract?rss=yes Abstract: The orientation characteristics of the elastic properties of sub-microcrystalline copper produced by equal channel angular pressing (ECAP) were studied by measuring the velocities of the longitudinal and transverse ultrasonic waves in different spatial directions. It was shown that the effect of an “anomalous” reduction in the elastic moduli observed after ECAP treatment reflects a strong spatial anisotropy of the material. Internal stresses arising in copper as a result of ECAP processing were determined by means of X-ray diffraction line broadening analysis. An appreciable anisotropy of the internal stresses was also found. Possible mechanisms responsible for the anomalies of the elastic properties are discussed. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077116 Wed, 01 Sep 2010 23:00:00 +0100 4077116 http://www.actamat.org/article/PIIS1359645410005410/abstract?rss=yes Abstract: Microcracking of porous stabilized aluminum titanate (AT) was investigated by a combination of models and experiments. The strong hypothesis that the b-axis is completely disconnected from the surrounding material is needed in order for the integrity factor model to reproduce the macroscopic thermal expansion of AT. Electron backscattering and in situ neutron diffraction compressive tests validated this hypothesis. While the model predicts that microscopic-scale residual stresses are locked into the material after firing (compressive along the negative expansion crystal axis and tensile along the other two positive expansion axes), neutron diffraction provides evidence of this stress. Indeed, some deformation mechanisms are proposed to explain the apparent anomalous behavior of t... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077115 Wed, 01 Sep 2010 23:00:00 +0100 4077115 http://www.actamat.org/article/PIIS1359645410005409/abstract?rss=yes Abstract: An amorphous structure of NiTi was obtained by repeated cold rolling. Relaxation and crystallization were studied applying a combination of sensitive calorimetric methods, X-ray diffraction and transmission electron microscopy. Upon deformation, a nanostructured mixture of crystalline and amorphous phase occurs. With increasing degree of RCR, the fraction of the crystalline phase decreases; at an equivalent strain of 16.8, an almost completely amorphous phase arises. Isothermal calorimetry yields a monotonously decreasing signal followed by a peak occurring by relaxation of the amorphous phase and nanocrystallization, respectively. The relaxation signal is well described by a Kohlrausch–Williams–Watts function. Crystallization kinetics agree with those expected from the Johns... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077114 Wed, 01 Sep 2010 23:00:00 +0100 4077114 http://www.actamat.org/article/PIIS1359645410005367/abstract?rss=yes Abstract: The grain stresses within hydrides precipitated in rolled zircaloy-4 plates were determined by synchrotron X-ray diffraction experiments using an 80keV photon beam and a high-speed area detector placed in transmission geometry. Results showed large compressive stresses (360±20MPa) in the hydrides along the plate rolling direction. The origin of these stresses was investigated by performing hydride dissolution/precipitation in situ for thermal cycles between room temperature and 400°C. A large stress hysteresis was observed, with a steady decrease on heating and an abrupt change on cooling. The observed stresses are explained by the constraint imposed by grain boundaries on the growth of hydride platelets on the rolling–transverse plane of the rolled plates. (Source: Acta Mate... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077111 Wed, 01 Sep 2010 23:00:00 +0100 4077111 http://www.actamat.org/article/PIIS1359645410005343/abstract?rss=yes Abstract: Twinning in certain metals or under certain conditions is a major plastic deformation mode. Here we present a phase field model to describe twin formation and evolution in a polycrystalline fcc metal under loading and unloading. The model assumes that twin nucleation, growth and de-twinning is a process of partial dislocation nucleation and slip on successive habit planes. Stacking fault energies, energy pathways (γ surfaces), critical shear stresses for the formation of stacking faults and dislocation core energies are used to construct the thermodynamic model. The simulation results demonstrate that the model is able to predict the nucleation of twins and partial dislocations, as well as the morphology of the twin nuclei, and to reasonably describe twin growth and interaction.... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012950 Wed, 01 Sep 2010 23:00:00 +0100 4012950 http://www.actamat.org/article/PIIS1359645410005379/abstract?rss=yes Abstract: The AlN incorporation probability in single crystal Hf1−xAlxN(001) layers is controllably adjusted between ∼0% and 100% by varying the ion energy (Ei) incident at the growing film over a narrow range, 10–40eV. The layers are grown on MgO(001) at 450°C using ultrahigh vacuum magnetically unbalanced reactive magnetron sputtering from a Hf0.7Al0.3 alloy target in a 5%-N2/Ar atmosphere at a total pressure of 20mTorr (2.67Pa). The ion to metal flux ratio incident at the growing film is constant at 8. Epitaxial film compositions vary from x=0.30 with Ei=10eV, to 0.27 with Ei=20eV, 0.17 with Ei=30eV, and ⩽0.002 with Ei⩾40eV. Thus, the AlN incorporation probability decreases by greater than two orders of magnitude. This extraordinary range in real-time manipulation of film che... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4167771 Wed, 01 Sep 2010 00:00:00 +0100 4167771 http://www.actamat.org/article/PIIS1359645410005264/abstract?rss=yes Abstract: A Monte Carlo methodology has been developed as a means for three-dimensional (3-D) reconstruction of the microstructure of a three phase anode used in solid oxide fuel cells, based on two-point statistical functions. The salient feature of the presented reconstruction methodology is the ability to realize the 3-D microstructure from two-dimensional (2-D) scanning electron micrographs for a three phase medium extendable to n phase media. In the realization procedure different phases of the heterogeneous medium are represented by different cells, which are allowed to grow. The growth of cells, however, is controlled via several optimization parameters related to rotation, shrinkage, translation, distribution and growth rates of the cells. Indeed, the proposed realization algorithm... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124825 Wed, 01 Sep 2010 00:00:00 +0100 4124825 http://www.actamat.org/article/PIIS1359645410005392/abstract?rss=yes Abstract: Al/SiC nanolaminates possess an excellent combination of mechanical strength and flexibility. While nanoindentation provides a reasonable estimate of the mechanical properties such as Young’s modulus and hardness of these materials, the stress state under nanoindentation is extremely complex. Micropillar compression has become an attractive method of studying the mechanical properties of materials at small length scales in a nominally homogeneous stress state. In this work, micropillars of Al/SiC nanolaminate were fabricated using focused ion beam milling. Compression testing was carried out using a flat-end nanoindenter head. The actual displacement of the pillar during micropillar compression was deconvoluted by subtracting the “extraneous” displacements of the system. Fr... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077113 Tue, 31 Aug 2010 23:00:00 +0100 4077113 http://www.actamat.org/article/PIIS135964541000532X/abstract?rss=yes Abstract: Nanocomposite coatings consisting of Ag and TiCx (x (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4077109 Tue, 31 Aug 2010 23:00:00 +0100 4077109 http://www.actamat.org/article/PIIS1359645410005331/abstract?rss=yes Abstract: The crystallization kinetics of amorphous Zr50Al10Ni40, as measured by means of both isothermal and isochronal differential scanning calorimetry, were evaluated using a new procedure involving application of a modular analytical model to provide a complete description of the phase-transformation kinetics, in combination with a preceding analysis of the transformation-rate maximum. The power of detailed analysis of the position of the transformation-rate maximum, as a function of the transformed fraction, was demonstrated by identification of the operating impingement mode. On this basis, the kinetic parameters governing the crystallization kinetics could then be determined quantitatively using the modular analytical model. The crystallization governing mechanisms could be varied ... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012949 Tue, 31 Aug 2010 23:00:00 +0100 4012949 http://www.actamat.org/article/PIIS1359645410005318/abstract?rss=yes Abstract: The mobility of dislocations is shown to be a size-dependent phenomenon. When dislocations intersect free surfaces, the mobility decreases as the dislocation length decreases, suggesting that dislocation motion in small structures may be more difficult. This increased drag may be related to surface forces acting where the dislocation intersects the free surface or from altered dislocation–phonon interactions. Mobility, however, is not as dependent on the film thickness and converges rapidly to bulk values. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012948 Tue, 31 Aug 2010 23:00:00 +0100 4012948 http://www.actamat.org/article/PIIS1359645410005306/abstract?rss=yes Abstract: It has been demonstrated recently that intensive melt shearing can be an effective approach to the grain refinement of both shape casting and continuous casting of Mg alloys. In the present study, the mechanisms of grain refinement by intensive melt shearing were investigated through a combination of both modelling and experimental approaches. The measurement of the cooling curves during solidification, quantification of grain size of the solidified samples, and image analysis of the MgO particle size and size distribution in the pressurized filtration samples were performed for the AZ91 alloy with and without intensive melt shearing. The experimental results were then used as input parameters for the free growth model to investigate the mechanisms of grain refinement by intensiv... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012947 Tue, 31 Aug 2010 23:00:00 +0100 4012947 http://www.actamat.org/article/PIIS1359645410005288/abstract?rss=yes Abstract: Common failure mechanisms in microelectronics, such as electromigration, creep and fatigue, can be positively influenced by microstructure optimization. In this paper a combination of post-deposition heat treatment and self-ion bombardment is proposed as a valid candidate to gain control over the microstructure of (111) fiber textured thin silver films. Irradiation can induce a strong in-plane texture and hence lead to biaxially textured films through a process of selective grain growth. Moreover, we report microstructural stability of the irradiated regions over a wide range of temperatures (up to 600°C), in contrast to non-irradiated portions of the film, which underwent abnormal growth of the (100) out-of-plane oriented grains, and a consequent texture change, at temperatures... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012946 Sun, 29 Aug 2010 23:00:00 +0100 4012946 http://www.actamat.org/article/PIIS1359645410005276/abstract?rss=yes Abstract: The thermal stability of nanocomposite metals (a nanostructured copper matrix embedding niobium nanotubes) is investigated via time-resolved in situ annealing under synchrotron high-energy X-rays. The diffraction peak profile analysis demonstrates that internal-stress relaxation begins in the Nb nanotubes at a temperature far below the bulk recrystallization temperature and follows size-specific regimes originating from a proximity effect with the nanostructured Cu matrix: the increased Cu–Nb interface surface disrupts internal-stress relaxation processes, confirming the larger thermal resistance of nanostructured materials. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012945 Sun, 29 Aug 2010 23:00:00 +0100 4012945 http://www.actamat.org/article/PIIS135964541000529X/abstract?rss=yes Abstract: Atomistic simulations show that organosilicates, used as low permittivity dielectric materials in advanced integrated circuits, can be made substantially stiffer than amorphous silica, while maintaining a lower mass density. The enhanced stiffness is achieved by incorporating organic cross-links to replace bridging oxygen atoms in the silica network. To elucidate the mechanism responsible for the enhanced stiffness, the conformational changes in the network upon hydrostatic and shear loading are examined. The structural and mechanical impact of terminal methyl groups is also assessed quantitatively and compared with continuous random network theory. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124826 Fri, 27 Aug 2010 00:00:00 +0100 4124826 http://www.actamat.org/article/PIIS1359645410005252/abstract?rss=yes Abstract: Different types of inorganic oxide films composed of a chemical composition gradient single layer were designed, fabricated and characterized. Compositionally graded thin films were created by power-controlled co-sputtering of alumina (Al2O3) and silica (SiO2) at room temperature, allowing the structural design of the film to be tailored at the nanometer scale. Two distinct graded thin films were fabricated: one with a compositionally asymmetric structure consisting of a SiO2-rich bottom interface and a Al2O3-rich top surface, and the other with a compositionally balanced sandwich structure consisting of both the top surface and bottom interface rich in SiO2 and a core rich in Al2O3 (referred to as SGS for ‘sandwich graded structure’). Smoothly graded thin films without inter... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012944 Thu, 26 Aug 2010 23:00:00 +0100 4012944 http://www.actamat.org/article/PIIS1359645410005240/abstract?rss=yes Abstract: The definition of representative plastic strain induced by a Vickers indent has received considerable attention in recent years. Previous reports have attempted to define a universal value that was independent of a material’s plastic response. However, the work presented here will show that a material-dependent representative plastic strain is valid in the conversion of flow stress to indentation hardness. This representative plastic strain is the volume average plastic strain within the plastic zone of Vickers indentation. The increase in indentation hardness within the plastic zones of macro-indents was experimentally determined by micro-Vickers indentation and then compared with that predicted by finite element modeling, which utilizes the proposed representative plastic str... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012943 Wed, 25 Aug 2010 23:00:00 +0100 4012943 http://www.actamat.org/article/PIIS1359645410005239/abstract?rss=yes Abstract: This paper reports on the determination of elastic and hardness properties of Co–B composite nanoparticles (CNP). Co boride materials is usually known for their functional properties (hydrogen catalysis, magnetism, corrosion, biomedics), but nanoscale dimensions also bring significant mechanical properties. In situ compression tests of 70–150nm core–shell silica-coated Co2B CNP (Composite nanoparticles) were performed for the first time with a nanoindenter in the load range 30–300μN. The CNP modulus is comparable with the bulk material (ECNP=159–166GPa), but the hardness is as much as 5 times higher (∼4.5±1.0GPA). Both modulus and hardness (to a lesser extent) are found to increase with the applied pressure. The paper first addresses the limitations of ordinary cont... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012942 Wed, 25 Aug 2010 23:00:00 +0100 4012942 http://www.actamat.org/article/PIIS1359645410004489/abstract?rss=yes (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=3895712 Tue, 24 Aug 2010 09:53:59 +0100 3895712 http://www.actamat.org/article/PIIS1359645410005173/abstract?rss=yes (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=3895711 Tue, 24 Aug 2010 09:53:59 +0100 3895711 http://www.actamat.org/article/PIIS1359645410005124/abstract?rss=yes Abstract: Grain boundary evolution in copper bicrystals is investigated during uniaxial tension at 10K. Grain boundary structures are generated using molecular statics employing an embedded atom method potential, followed by molecular dynamics simulation at a constant 1×109s−1 strain rate. Interfacial free volume is continuously measured during boundary deformation, and its evolution is investigated both prior to and during grain boundary dislocation nucleation. Free volume provides valuable insight into atomic-scale processes associated with stress-induced grain boundary deformation. Different boundary structures are investigated in this work to analyze the role of interface structure, stress state and initial free volume on dislocation nucleation. The results indicate that the free vo... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012941 Sun, 22 Aug 2010 23:00:00 +0100 4012941 http://www.actamat.org/article/PIIS1359645410005112/abstract?rss=yes Abstract: The microstructures and crystallographic textures produced during friction stir welding of the near-α Ti-5111 titanium alloy were characterized as a function of welding speed. The textures produced were compared with ideal hexagonal close packed (hcp) shear textures and with predicted textures of hcp Burgers variants of ideal body-centered cubic (bcc) shear textures, showing that the deposited welds are dominated by the hcp P1 and bcc D1 textures. The hcp P1 shear texture was dominant at slow weld speeds, while the bcc D1 shear texture was dominant at the fast weld speed. This variation appears to result from a poor transmission of the shear deformation from the rotating tool to the deposited weld that develops at faster welding speeds. These observations are compared to other s... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012940 Sun, 22 Aug 2010 23:00:00 +0100 4012940 http://www.actamat.org/article/PIIS1359645410005100/abstract?rss=yes Abstract: The relationship between elastic anisotropy and extreme Poisson’s ratio behaviour (either positive or negative) in single-crystalline materials has been investigated using experimentally determined single-crystal elastic constants for a wide range of solid materials. This makes use of a recently proposed elastic anisotropy index that is applicable to all crystal symmetries. For many real materials we find a striking correlation between the value of the elastic anisotropy index and the magnitudes of maximum and minimum Poisson’s ratios this is independent of crystal symmetry. This structure–property relationship provides new examples of auxetics and shows that negative Poisson’s ratios are actually not uncommon among many classes of inorganic (and organic) materials, inclu... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012939 Sun, 22 Aug 2010 23:00:00 +0100 4012939 http://www.actamat.org/article/PIIS1359645410005082/abstract?rss=yes Abstract: Aluminum reinforced by 60vol.% diamond particles has been investigated as a potential heat sink material for high power electronics. Diamond (CD) is used as reinforcement contributing its high thermal conductivity (TC≈1000WmK−1) and low coefficient thermal expansion (CTE≈1ppmK−1). An Al matrix enables shaping and joining of the composite components. Interface bonding is improved by limited carbide formation induced by heat treatment and even more by SiC coating of diamond particles. An AlSi7 matrix forms an interpenetrating composite three-dimensional (3D) network of diamond particles linked by Si bridges percolated by a ductile α-Al matrix. Internal stresses are generated during temperature changes due to the CTE mismatch of the constituents. The stress evolution was de... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012937 Sun, 22 Aug 2010 23:00:00 +0100 4012937 http://www.actamat.org/article/PIIS1359645410005033/abstract?rss=yes Abstract: The influence of microalloying additions of Au to the binary Al–Cu system is investigated utilizing hardness measurements, positron annihilation spectroscopy and X-ray absorption spectroscopy. No room temperature ageing could be observed in the case of the ternary Al–Cu–Au alloy. Instead, quenched-in vacancies were trapped by Au atoms and thus could not catalyse the decomposition of the alloy. However, artificial ageing dissolves the complexes of Au and vacancies. A comparison between experimental and calculated absorption spectra shows that Au atoms did not agglomerate significantly during either natural or artificial ageing. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012932 Sun, 22 Aug 2010 23:00:00 +0100 4012932 http://www.actamat.org/article/PIIS1359645410004933/abstract?rss=yes Abstract: At long tempering times in steels when both cementite (Fe3C) and ferrite (body-centered cubic (bcc) Fe-rich solid solution) phases are present, alloying elements tend to segregate to either of the two phases. The elements V, Cr, Mn, Mo and W are found to partition to the cementite phase, while Al, Si, P, Co, Ni and Cu partition to ferrite. We show that partitioning of alloying elements and cementite (de)stabilization by alloying in mixtures of bcc Fe and cementite are intimately related through the introduction of a partitioning enthalpy. The formation enthalpy of alloying-element-substituted cementite is shown not to be a proper gauge for addressing these questions. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012922 Sun, 22 Aug 2010 23:00:00 +0100 4012922 http://www.actamat.org/article/PIIS1359645410005094/abstract?rss=yes Abstract: The present work characterizes the microstructure of a hot-extruded Ti–45Al–5Nb–0.2B–0.2C (at.%) alloy with a fine-grained duplex microstructure after shear creep deformation (temperature 1023K; shear stress 175MPa; shear deformation 20%). Diffraction contrast transmission electron microscopy (TEM) was performed to identify ordinary dislocations, superdislocations and twins. The microstructure observed in TEM is interpreted taking into account the contribution of the applied stress and coherency stresses to the overall local stress state. Two specific locations in the lamellar part of the microstructure were analyzed, where either twins or superdislocations provided c-component deformation in the L10 lattice of the γ phase. Lamellar γ grains can be in soft and hard orie... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012938 Thu, 19 Aug 2010 23:00:00 +0100 4012938 http://www.actamat.org/article/PIIS1359645410005070/abstract?rss=yes Abstract: The present work focuses on the effect of microstructural refinement on the thermo-mechanical cyclic stability of a Ti50.3Ni33.7Pd16 high temperature shape memory alloy (HTSMA) which was severe plastically deformed using equal channel angular extrusion (ECAE). The grain/subgrain size of the high temperature austenite phase was refined down to about 100nm, the lowest reported to date in HTSMAs. The increase in strength differential between the onset of transformation and the macroscopic plastic yielding after ECAE led to a notable enhancement in the cyclic stability during isobaric cooling–heating experiments. The reduction in irrecoverable strain levels was attributed to the increase in critical stress for dislocation slip due to the microstructural refinement during the ECAE p... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012936 Thu, 19 Aug 2010 23:00:00 +0100 4012936 http://www.actamat.org/article/PIIS1359645410005069/abstract?rss=yes Abstract: A first principles continuum analytical model for cationic segregation to the grain boundaries in complex ceramic oxides is presented. The model permits one to determine the electric charge density and the segregation-induced electric potential profiles through the grain and can be extrapolated to the range of nanostructured grain sizes. The theoretical predictions are compared with existing data for yttria-stabilized tetragonal zirconia polycrystals. The implications for physical properties (mainly high temperature plasticity and hardening behaviour) are then discussed. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012935 Thu, 19 Aug 2010 23:00:00 +0100 4012935 http://www.actamat.org/article/PIIS1359645410005057/abstract?rss=yes Abstract: A new method is developed for reconstruction of the local orientation of the parent austenite based on the orientation of lath martensite measured by electron backscattered diffraction. The local orientation of austenite was obtained by least squares fitting as the difference between the experimental data and the predicted martensite orientation was minimal, assuming the specific orientation relationship (OR) between martensite and the parent austenite. First, the average OR between austenite and lath martensite was precisely determined and it was shown that both close packed planes and directions between martensite and the parent austenite deviated by more than 1° in low carbon martensite. The quality of the reconstructed austenite orientation map depended strongly on the OR us... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012934 Thu, 19 Aug 2010 23:00:00 +0100 4012934 http://www.actamat.org/article/PIIS1359645410005045/abstract?rss=yes Abstract: Frictional normal contact probing methods involving instrumented, depth-sensing indentation can be used to estimate the mechanical properties of small-volume structures and materials such as thin films and components of micro-electro-mechanical systems. This paper describes a new method for estimating the plastic properties, i.e. the yield strength and strain hardening exponent, of ductile materials from the topography of scratches formed by a conical tip during an instrumented, depth-sensing frictional sliding test. The proposed reverse analysis (or inverse analysis) uses dimensionless functions derived from computational simulations to extract plastic properties from an instrumented scratch response performed on a standard, commercially available instrument. Sensitivity analysi... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012933 Tue, 17 Aug 2010 23:00:00 +0100 4012933 http://www.actamat.org/article/PIIS1359645410005021/abstract?rss=yes Abstract: By means of brief pauses in radiofrequency (RF) sputter deposition between individual layers, ultrathin copper oxide layers were formed through adsorption in the Cu/Cu multilayers. Their mechanical properties were compared with the Cu/Cu(O) multilayers whose oxide layers were deliberately deposited between copper layers. The mechanical hardness value of the Cu/Cu(O) multilayers approached that of nanostructured copper thin films. The Young’s modulus of the multilayers was tunable, in accordance with the elasticity theories of composites. In addition, the Hall–Petch slope of the RF sputter-deposited Cu monolayers indicated that their theoretical strength approached the shear modulus of copper. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012931 Tue, 17 Aug 2010 23:00:00 +0100 4012931 http://www.actamat.org/article/PIIS135964541000501X/abstract?rss=yes Abstract: Expansion and contraction phenomena during solidification of liquid metal foams were studied. Such foams were processed by mixing metal powders with TiH2 powder and compacting the resulting blends, after which the compacted powders were melted. The subsequent foaming process was monitored in situ by X-ray radioscopy. An intermediate expansion stage during solidification was observed. This solidification expansion (SE) could be linked to phase transformations in the alloy. SE was found to depend mainly on the time spent at the foaming temperature before cooling (holding time), the cooling rate and the alloy composition. The interplay between gas shrinkage, solidification shrinkage, gas production by the blowing agent and gas losses due to out-diffusion was identified as the main r... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012930 Tue, 17 Aug 2010 23:00:00 +0100 4012930 http://www.actamat.org/article/PIIS1359645410004891/abstract?rss=yes Abstract: Ledge expansion and the concomitant growth of a stacking fault tetrahedron (SFT) are investigated in Al and Cu by molecular dynamics (MD) simulations by addition of vacancy rods with selected lengths. Ledge expansion is largely governed by the site preference of vacancies on the SFT edges resulting in distinct stable ledged SFTs. Both edge- and corner-facing ledge configurations may be adopted. The growth of SFTs, especially large ones, is controlled by thermal agitation. The mobile part of the ledges consists of a dipole of Shockley partials generally oriented in the 60° mixed orientation that move in a thermally activated manner, reflecting a certain core reorganization of the Shockley dipole. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124824 Mon, 16 Aug 2010 00:00:00 +0100 4124824 http://www.actamat.org/article/PIIS135964541000488X/abstract?rss=yes Abstract: The growth of stacking fault tetrahedra (SFTs) resulting from dipole annihilation is investigated by molecular dynamics (MD) simulations. The atomistic processes involved during growth immediately after nucleation are studied. Analyzed for up to three vacancies, the site preference of vacancies on a perfect SFT favors vacancy segregation at edge centers and corners in Cu and Al, respectively. The formation of small sized SFTs does not require a prior triangular Frank loop. Instead, SFT growth involves vacancies and their clusters agglomerating as complex faceted configurations which, assisted by accelerated vacancy migration along SFT edges, rearrange into near-perfect and perfect SFTs. SFT growth by the ledge mechanism is investigated in Part III. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124823 Mon, 16 Aug 2010 00:00:00 +0100 4124823 http://www.actamat.org/article/PIIS1359645410004908/abstract?rss=yes Abstract: The present work focuses on stacking fault tetrahedron (SFT) nucleation upon annihilation of edge dislocation dipoles in Al and Cu. SFT nucleation is promoted by relatively low vacancy migration barriers along defected channels, typical of pipe diffusion-like processes, forming small vacancy clusters and elemental SFTs. Within about 1ns, depending on the material under investigation, the small initial vacancy clusters aggregate into larger clusters, including complete, incomplete and truncated SFTs. Cluster aggregation occurs by diffusion of vacancy complexes whose mutual transformation and energetics are analyzed and compared. It is concluded that the unique presence of deformation-induced SFTs, such as those found experimentally in Al, originates from a local mechanism of force... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4124822 Mon, 16 Aug 2010 00:00:00 +0100 4124822 http://www.actamat.org/article/PIIS1359645410005008/abstract?rss=yes Abstract: Owing to its significant influence in heat and mass transfer, liquid metal flow during stationary laser welding has been studied for different concentrations of oxygen in the surrounding environment. Two predominant types of surface flow motion are observed: an inward flow, corresponding to a positive surface tension temperature gradient, and an outward flow, corresponding to a negative gradient. Time-dependent changes in oxygen concentration at the surface are observed to flip the surface tension temperature gradient from negative to positive under appropriate shielding conditions. The oxygen concentration affects not only the flow motion, but also the laser absorption coefficient, which increases with increasing environmental oxygen concentration. It is shown here that the evol... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012929 Sun, 15 Aug 2010 23:00:00 +0100 4012929 http://www.actamat.org/article/PIIS1359645410004994/abstract?rss=yes Abstract: The near-eutectic permeability of aluminum–copper alloys has been determined through physical and numerical modeling. The physical models are large-scale analogues of interdendritic structures produced by a rapid prototyping technique from three-dimensional (3-D) geometries obtained by X-ray microtomography. A glycerin-based solution was passed through the physical models and the permeability was calculated from measurements of the discharge flow rate and pressure drop. Mathematical models, considering the continuity and momentum equations, were developed for the corresponding unstructured meshes of the 3-D geometries used for the physical models. The numerically determined values of permeability are in good agreement with those measured. With the aid of this 3-D characterizati... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012928 Sun, 15 Aug 2010 23:00:00 +0100 4012928 http://www.actamat.org/article/PIIS1359645410004982/abstract?rss=yes Abstract: An analytical model is developed to determine the length scales over which load is transferred from outer to inner walls of multiwall carbon nanotubes (MWCNTs) as a function of the amount of bonding between walls. The model predicts that the characteristic length for load transfer scales as , where t is the CNT wall spacing, E is the effective wall Young’s modulus, and is the average interwall shear modulus due to interwall coupling. Molecular dynamics simulations for MWCNTs with up to six walls, and with interwall coupling achieved by interwall sp3 bonding at various densities, provide data against which the model is tested. For interwall bonding having a uniform axial distribution, the analytic and simulation models agree well, showing that continuum mechanics concepts apply ... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012927 Sun, 15 Aug 2010 23:00:00 +0100 4012927 http://www.actamat.org/article/PIIS1359645410004970/abstract?rss=yes We report for the first time on mechanical properties of anodically formed, self-organized TiO2 nanotube layers on titanium. We compare their behavior in their virgin state and after conversion to their semi-metallic form by an acetylene treatment. We show that the acetylene treatment leads to significant enhanced hardness, tensile strength and friction behavior. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012926 Thu, 12 Aug 2010 23:00:00 +0100 4012926 http://www.actamat.org/article/PIIS1359645410004969/abstract?rss=yes Abstract: The thermal transport properties of a series of electron-doped CaMnO3 perovskites have been investigated. Throughout the temperature range 5–300K, phonon thermal conductivity is dominant, and both electron and spin wave contributions are negligible. The short phonon mean free paths in this system result in the relatively low thermal conductivities. The strong phonon scatterings stem from the A-site mismatch and bond-length fluctuations induced by local distortions of MnO6 octahedra. The thermal conductivity in the magnetically ordered state is enhanced as a result of the decrease in spin–phonon scattering. The results also indicate that above the magnetic ordering temperature, observable thermal excitation of optical phonons occurs. The contribution of optical phonons to ther... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012925 Wed, 11 Aug 2010 23:00:00 +0100 4012925 http://www.actamat.org/article/PIIS1359645410004957/abstract?rss=yes Abstract: Fatigue tests were conducted on smooth specimens of ultrafine-grained copper produced by 4 and 12 passes of equal channel angular pressing (henceforth referred to as UFG4 and UFG12, respectively). A major crack was initiated from shear bands at an early stage of stressing. The UFG4 and UFG12 samples exhibited different growth behavior tendencies at a low crack growth rate (CGR). For UFG12, the CGR initially increased as the crack was extended with continued fatigue cycling, but then abruptly decreased before CGR reaching 10−6mm/c. This drop was temporary and was gradually recovered with subsequent cycling. The drop and recovery in CGR corresponded to the transitions from planar to granular fracture surface and from granular to striated fracture surface, respectively. For UFG4, ... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012924 Wed, 11 Aug 2010 23:00:00 +0100 4012924 http://www.actamat.org/article/PIIS1359645410004945/abstract?rss=yes Abstract: The mechanical properties of Mg97Zn1Y2 extruded alloy, composed of Mg matrix phase and a long-period stacking ordered phase, the so-called LPSO phase, with a volume fraction of approximately 24%, were investigated using compression tests at room temperature. The microstructure was varied to a large degree by various heat treatments at high temperatures above 400°C, and the relationship between the microstructure and mechanical properties was clarified. The plastic behavior of the Mg/LPSO two-phase alloy was compared with that of Mg99.2Zn0.2Y0.6 alloy, composed almost Mg-solid-solution phase, and the strengthening mechanisms at work in the Mg97Zn1Y2 extruded alloy are discussed. The existence of the LPSO phase strongly enhanced the refinement of Mg matrix grains during extrusion,... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012923 Wed, 11 Aug 2010 23:00:00 +0100 4012923 http://www.actamat.org/article/PIIS1359645410004921/abstract?rss=yes Abstract: We successfully fabricated nanocrystalline (NC) indium (In) particles embedded in Al-based amorphous matrix. Systematic investigations indicate that thermal interaction between the NC In and the amorphous matrix significantly influences their respective thermal behaviors. The melting temperature of NC In was found to be depressed by 10–30K, owing to the specific interfacial structure of NC In/amorphous system. The simultaneous appearance of the liquid/amorphous interface destabilizes the amorphous matrix, leading to face-centered cubic-Al precipitation at the interface of In sphere/amorphous matrix at a relatively lower temperature. This effect is attributed to the diffusion of La from the matrix to the liquid In particles. (Source: Acta Materialia) Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012921 Wed, 11 Aug 2010 23:00:00 +0100 4012921 http://www.actamat.org/article/PIIS135964541000491X/abstract?rss=yes Abstract: Fe36Co36B19.2Si4.8Nb4 bulk glassy rods were synthesized by copper mould casting. The effects of annealing treatments on the microstructure, elastic and mechanical properties of this alloy are investigated. Annealing below the glass transition temperature induces the formation of atomic clusters with pseudo-tenfold symmetry with a close relationship to the Fe23B6 phase. Annealing at sufficiently high temperatures promotes the formation of stable Fe2B and FeB phases and Fe(Co) solid solution. The as-cast alloy exhibits ultra-high hardness (H>14GPa), high reduced Young’s modulus (Er>200GPa) and good wear resistance. These properties are further enhanced after thermal treatments (H>18GPa and Er>260GPa are achieved in the fully crystallized sample). The mechanical hardening is accom... Acta Materialia journals http://www.medworm.com/rss/comments.php?id=4012920 Wed, 11 Aug 2010 23:00:00 +0100 4012920