Acta Materialia RSS feed: Articles in Press. Acta Materialia provides a forum for publishing full-length, original papers and commissioned overviews which advance the in-depth understanding of the relationship between the processing, the structure and the properties of inorganic materials. Papers which have a high impact potential are sought. The structure encompasses atomic and molecular arrangements, chemical and electronic structures, and microstructure. Emphasis is on either the mechanical or functional behavior of inorganic solids at all length scales down to nanostructures. The following aspects of the science and engineering of inorganic materials are of particular interest: (i) Cutting-edge experiments and theory as they relate to the understanding of the properties, (ii) Simulation and modeling preferably combined with experimentation specifically as they relate to the understanding of the properties, (iii) Elucidation of the mechanisms involved in the synthesis and processing of materials specifically as they relate to the understanding of the properties, and (iv) Characterization of the structure and chemistry of materials specifically as it relates to the understanding of the properties. Short communications and comments to papers published in Acta Materialia may be submitted to Scripta Materialia.http://www.actamat.org//inpress?rss=yesElsevier Inc.en © 2010 Acta Materialia Inc. Published by Elsevier Inc All rights reserved. Acta Materialia1359-64542010-03-08 © 2010 Acta Materialia Inc. Published by Elsevier Inc All rights reserved. healthpermissions@elsevier.comhttp://www.actamat.org/article/PIIS1359645410000947/abstract?rss=yesAbstract: Epoxy resin nanocomposites, based on the diglycidyl ether of bisphenol-A (DGEBA) and tetraglycidyl diamino diphenyl methane (TGDDM), are prepared via in situ co-polymerization with 4,4′-diaminodiphenylsulfone (DDS) in the presence of octa-aminophenyl silsesquioxane (OAPS) at levels of up to 20wt.% of the latter. The curing reaction involving epoxy, DDS and OAPS is investigated using Fourier transform infrared (FTIR) spectroscopy. Differential scanning calorimetry and dynamic mechanical analysis show that the glass transition temperatures of the polyhedral oligomeric silsesquioxane (POSS) containing nanocomposites are higher than the corresponding neat epoxy systems at lower concentrations of POSS (⩽3wt.%). Thermogravimetric analysis indicates that the POSS–epoxy nanocomposites display high ceramic yields, suggesting improved flame retardancy. The increasing concentration of OAPS into epoxy–amine networks exhibits a decreasing trend in the values of dielectric constant compared with those values obtained from neat epoxy systems. The higher epoxy functionality present in TGDDM leads to nanocomposites which possess enhanced thermal stability and higher dielectric constants than the DGEBA-based nanocomposites. X-ray diffraction analysis reveals that the molecular level reinforcement of POSS cages occurs in both the cases of DGEBA- and TGDDM-based hybrid epoxy nanocomposites. Furthermore, homogeneous dispersion of POSS cages in the epoxy matrices is evidenced by scanning electron microscopy, which further confirms that the POSS molecule has become an integral part of the organic–inorganic inter-cross-linked network systems.Octasilsesquioxane-reinforced DGEBA and TGDDM epoxy nanocomposites: Characterization of thermal, dielectric and morphological properties - Uncorrected ProofShanmugam Nagendiran, Muthukaruppan Alagar, Ian Hamerton10.1016/j.actamat.2010.02.008Acta Materialia (2010)2010-03-08Acta Materialia2010-03-08http://www.actamat.org/article/PIIS1359645410001291/abstract?rss=yesAbstract: Atomic scale characterization of the white etching layer (WEL) in a rail track surface made of pearlitic steel was performed with respect to elemental distribution using atom probe tomography. The atomic distributions of the alloying elements at various depths below the surface were analyzed inside and outside the WEL. Manganese-enriched and silicon-depleted zones corresponding to pre-existing cementite lamellae were observed in the WEL, although the cementite lamellae of the matrix pearlite had completely decomposed. The manganese-enriched and silicon-depleted zones were more distinct in the bottom region than in the topmost surface region of the WEL. The width and spacing of the zones indicated that the WEL region did not undergo heavy deformation. A temperature rise in the rail-to-wheel contact areas was predicted by friction calculation, which was confirmed through shape analysis of the manganese-enriched zones. These results support the hypothesis that martensitic transformation after rapid austenization was the origin of the WEL.Atom probe tomography analysis of the white etching layer in a rail track surface - Uncorrected ProofJ. Takahashi, K. Kawakami, M. Ueda10.1016/j.actamat.2010.02.030Acta Materialia (2010)2010-03-04Acta Materialia2010-03-04http://www.actamat.org/article/PIIS1359645410001308/abstract?rss=yesAbstract: This paper concerns optimization of procedures and algorithms for extraction of stress–strain relationships from quasi-static nanoindentation experiments, using finite element method modelling. Several issues are highlighted, including the usefulness of incorporating residual indent shape in the comparisons, as well as load–displacement–time data, and the significance of creep and interfacial friction. The study is focused on extruded copper bar, using a spherical indenter and assuming transverse isotropy throughout. It is shown that, using the methodology presented here, experimental nanoindentation data could be used to estimate the yield stress and work-hardening rate, with good accuracy, i.e. the yield stress could have been obtained to a precision of about ±10%, and the work-hardening rate to about ±25%. Such inferred constitutive relations are more likely to be reliable if the comparisons are made in regimes within which creep does not significantly influence the behaviour, and in general the timescale of measurement is important.Use of quasi-static nanoindentation data to obtain stress–strain characteristics for metallic materials - Uncorrected ProofJ. Dean, J.M. Wheeler, T.W. Clyne10.1016/j.actamat.2010.02.031Acta Materialia (2010)2010-03-04Acta Materialia2010-03-04http://www.actamat.org/article/PIIS1359645410001060/abstract?rss=yesAbstract: Neutron Larmor diffraction (LD) is a high-resolution diffraction technique based on the Larmor precession of polarized neutrons. In contrast to conventional diffraction, LD does not depend on the accurate measurement of Bragg angles, and thus the resolution is independent of the beam collimation and monochromaticity. At present, a relative resolution for the determination of the crystal lattice spacing d of is achieved, i.e. at least one order of magnitude superior to conventional neutron or X-ray techniques. This work is a first step to explore the application of LD to high-resolution problems in the analysis of residual stresses, where both the accurate measurement of absolute d values and the possibility of measuring type II and III stresses may provide additional information beyond those accessible by conventional diffraction techniques. Data obtained from Inconel 718 samples are presented.Neutron Larmor diffraction measurements for materials science - Uncorrected ProofJ. Repper, T. Keller, M. Hofmann, C. Krempaszky, W. Petry, E. Werner10.1016/j.actamat.2010.02.020Acta Materialia (2010)2010-03-02Acta Materialia2010-03-02http://www.actamat.org/article/PIIS1359645410001084/abstract?rss=yesAbstract: This study investigates the interphase boundary structures of lenticular martensites in Fe–33Ni and Fe–31Ni, and discusses the accommodation mechanism for transformation strain. Despite the large difference in the macroscopic morphology of the interphase boundary, the nature of interfacial dislocations in both alloys is almost the same; there are two sets of screw dislocations with a/2 [1]M and a/2 [11]M Burgers vectors on the interphase boundary. On the basis of the array of interfacial dislocations, the shear planes of dislocations with a/2 [1]M and a/2 [11]M Burgers vectors were determined to be (112)M (=(101)A) and (12)M (=(011)A). Two sets of interfacial dislocations accommodate the atomistic misfit strain due to the change in the stacking sequence and the shape of parallel close packed planes. The macroscopic transformation strain and elastic strain energy of lenticular martensite are also reduced by the presence of the observed lattice invariant deformations.Interphase boundary structure and accommodation mechanism of lenticular martensite in Fe–Ni alloys - Uncorrected ProofA. Shibata, T. Furuhara, T. Maki10.1016/j.actamat.2010.02.022Acta Materialia (2010)2010-03-02Acta Materialia2010-03-02http://www.actamat.org/article/PIIS1359645410001096/abstract?rss=yesAbstract: In situ straining experiments have been carried out in pure Fe, in order to determine the geometry and the kinetics of dislocation glide at room temperature. Straight screw dislocations glide slowly in {110} elemental slip planes, at a velocity proportional to their length, whereas curved non-screw parts are highly mobile. The exact loop shape can yield the local stress as well as the difference of core energy between pure screw and near-screw orientations. The velocity–stress dependence of screws has been measured at the scale of a single dislocation source, and compared with macroscopic activation areas. The results are discussed in terms of the kink-pair mechanism.Kinetics of dislocations in pure Fe. Part I. In situ straining experiments at room temperature - Uncorrected ProofD. Caillard10.1016/j.actamat.2010.02.023Acta Materialia (2010)2010-03-02Acta Materialia2010-03-02http://www.actamat.org/article/PIIS1359645410001102/abstract?rss=yesAbstract: In situ straining experiments have been carried out at low temperature in pure Fe, in order to study the change of mechanism occurring at around 250K. The local stress necessary to move individual screw dislocations is in good agreement with the macroscopic yield stress at various temperatures. In the lower temperature range, straight screw segments have a jerky motion in {110} planes, at variance from the steady motion observed near room temperature. The distributions of waiting times in locked positions, and jump distances, the temperature variation of the average jump distance, and the stress/temperature variation of the macroscopic activation areas, are inconsistent with the kink-pair mechanism observed above 250K. They have been interpreted in terms of a locking–unlocking mechanism, already proposed in hexagonal-closed-packed metals. Under such conditions, the change of mechanism at 250K can account for the surprisingly low value of the flow stress extrapolated to 0K (much lower than the theoretical Peierls stress).Kinetics of dislocations in pure Fe. Part II. In situ straining experiments at low temperature - Uncorrected ProofD. Caillard10.1016/j.actamat.2010.02.024Acta Materialia (2010)2010-03-02Acta Materialia2010-03-02http://www.actamat.org/article/PIIS1359645410001114/abstract?rss=yesAbstract: Single crystals of γ-TiAl cannot be grown in the near-stoichiometric compositions that are present inside two-phase -microstructures with attractive mechanical properties. Therefore, the single-crystal constitutive behavior of γ-TiAl was studied by nanoindentation experiments in single-phase regions of these -microstructures. The experiments were characterized by orientation microscopy and atomic force microscopy to quantify the orientation-dependent mechanical response during nanoindentation. Further, they were analyzed by a three-dimensional crystal plasticity finite element model that incorporated the deformation behavior of γ-TiAl. The spatially resolved activation of competing deformation mechanisms during indentation was used to assess their relative strengths. A convention was defined to unambiguously relate any indentation axis to a crystallographic orientation. Experiments and simulations were combined to study the orientation-dependent surface pile-up. The characteristic pile-up topographies were simulated throughout the unit triangle of γ-TiAl and represented graphically in the newly introduced inverse pole figure of pile-up patterns. Through this approach, easy activation of ordinary dislocation glide in stoichiometric γ-TiAl was confirmed independently from dislocation observation by transmission electron microscopy.Plastic anisotropy of γ-TiAl revealed by axisymmetric indentation - Uncorrected ProofC. Zambaldi, D. Raabe10.1016/j.actamat.2010.02.025Acta Materialia (2010)2010-03-02Acta Materialia2010-03-02http://www.actamat.org/article/PIIS1359645410001126/abstract?rss=yesAbstract: Substructure development in an austenitic Ni–30%Fe model alloy was investigated within a dynamic recrystallization (DRX) regime. The substructure characteristics of the deformed matrix and DRX grains were markedly different regardless of the grain size and orientation. The former largely displayed ‘organized’, banded subgrain arrangements with alternating misorientations, resulting from a limited number of active slip systems. In contrast, the substructure of DRX grains was generally more ‘random’ and exhibited complex subgrain/cell arrangements characterized by local accumulation of misorientations, suggesting multiple slip. The proposed mechanism of the unique substructure development within DRX grains suggests that the DRX nuclei, forming along pre-existing grain boundaries and triple points, essentially represent grain boundary regions, which experience multiple slip to preserve the compatibility with neighbouring deformed grains. This results in the formation of a complex cell/subgrain structure, which progressively extends as the grain boundary regions expands outwards during DRX growth.On the characteristics of substructure development through dynamic recrystallization - Uncorrected ProofHossein Beladi, Pavel Cizek, Peter D. Hodgson10.1016/j.actamat.2010.02.026Acta Materialia (2010)2010-03-02Acta Materialia2010-03-02http://www.actamat.org/article/PIIS1359645410001138/abstract?rss=yesAbstract: The effect of a magnetic field on the development of texture and microstructure in cold-rolled (80%) commercially pure zirconium (Zr701) was investigated. The specifically oriented sheet specimens were annealed at 550°C for 15, 30 and 45min and at 700°C for 60, 90 and 180min in a magnetic field of 19T and 17T, respectively. X-ray diffraction and electron backscatter diffraction measurements were used to characterize the crystallographic texture and the grain microstructure. The results revealed that the magnetic annealing promotes grain growth in the investigated material. This becomes apparent from the faster development of specific “grain growth” texture components and the bigger mean grain size after magnetic annealing. Magnetic annealing at 700°C resulted in asymmetry of the two major texture components that constantly increased with annealing time. This effect is attributed to a magnetic driving force for grain growth arising from the anisotropic magnetic susceptibility of zirconium.Observations on the effect of a magnetic field on the annealing texture and microstructure evolution in zirconium - Uncorrected ProofD.A. Molodov, N. Bozzolo10.1016/j.actamat.2010.02.027Acta Materialia (2010)2010-03-02Acta Materialia2010-03-02http://www.actamat.org/article/PIIS135964541000114X/abstract?rss=yesAbstract: A general computational alloy design approach based on thermodynamic and physical metallurgical principles, and coupled with a genetic optimization scheme, is presented. The method is applied to the design of new ultra-high-strength maraging stainless steels strengthened by Ni3Ti intermetallics. In the first design round, the alloy composition is optimized on the basis of precipitate formation at a fixed ageing temperature without considering other steps in the heat treatment. In the second round, the alloy is redesigned, applying an integrated model which allows for the simultaneous optimization of alloy composition and the ageing temperature as well as the prior austenitization temperature. The experimental characterizations of prototype alloys of both the first and second rounds clearly demonstrate that alloys designed by the proposed approach achieve the desired microstructures.Genetic design and characterization of novel ultra-high-strength stainless steels strengthened by Ni3Ti intermetallic nanoprecipitates - Uncorrected ProofW. Xu, P.E.J. Rivera-Díaz-del-Castillo, W. Wang, K. Yang, V. Bliznuk, L.A.I. Kestens, S. van der Zwaag10.1016/j.actamat.2010.02.028Acta Materialia (2010)2010-03-02Acta Materialia2010-03-02http://www.actamat.org/article/PIIS1359645410001151/abstract?rss=yesAbstract: The role of interface morphology in the magnetic behavior of FePt/Fe exchange-coupled systems has been considered. Hard/soft bilayers with different interface morphologies were obtained by depositing Fe thin layers on FePt epitaxial layers with different morphologies, but all with a high degree of L10 ordering and high coercivity. It was found that the hard/soft coupling can be tuned by changing the FePt morphology, allowing tailoring of the hysteresis loops and modification of the magnetic regime at a fixed Fe thickness (from rigid magnet to exchange-spring magnet). Remarkably, all bilayer series showed a drastic reduction in coercivity with increasing Fe thickness to 3.5nm (μ0HC⩽1.1T).Role of interface morphology in the exchange-spring behavior of FePt/Fe perpendicular bilayers - Uncorrected ProofF. Casoli, F. Albertini, L. Nasi, S. Fabbrici, R. Cabassi, F. Bolzoni, C. Bocchi, P. Luches10.1016/j.actamat.2010.02.029Acta Materialia (2010)2010-03-02Acta Materialia2010-03-02http://www.actamat.org/article/PIIS1359645410000996/abstract?rss=yesAbstract: A commercially available granulated TZ3Y powder has been sintered by hot-pressing (HP). The “grain size/relative density” relationship, referred to here as the “sintering path”, has been established for a constant value of the heating rate (25°Cmin−1) and a constant value of the macroscopic applied pressure (100MPa). It has then been compared that obtained previously on the same powder but sintered by spark plasma sintering (SPS, heating rate 50°Cmin−1, same applied macroscopic pressure). By coupling the analysis of a sintering law (derived from creep rate equations) and comparative observations of sintered samples using transmission electron microscopy, a hypothesis about the densification mechanism(s) involved in SPS and HP has been proposed. Slight differences in the densification mechanisms lead to scars in the microstructure that explain the higher total ionic conductivity measured, in the temperature range 300–550°C, when SPS is used for sintering.Spark plasma sintering of a commercially available granulated zirconia powder: Comparison with hot-pressing - Uncorrected ProofGuillaume Bernard-Granger, Ahmed Addad, Gilbert Fantozzi, Guillaume Bonnefont, Christian Guizard, Dorothée Vernat10.1016/j.actamat.2010.02.013Acta Materialia (2010)2010-02-26Acta Materialia2010-02-26http://www.actamat.org/article/PIIS1359645410001059/abstract?rss=yesAbstract: High-precision data on phase transformation temperatures in NiTi, including numerical expressions for the effect of Ni on MS, MF, AS, AF and T0, are obtained, and the reasons for the large experimental scatter observed in previous studies are discussed. Clear experimental evidence is provided confirming the predictions of Tang et al. 1999 regarding deviations from a linear relation between the thermodynamic equilibrium temperature and Ni concentration. In addition to affecting the phase transition temperatures, increasing Ni contents are found to decrease the width of thermal hysteresis and the heat of transformation. These findings are rationalized on the basis of the crystallographic data of Prokoshkin et al. 2004 and the theory of Ball and James . The results show that it is important to document carefully the details of the arc-melting procedure used to make shape memory alloys and that, if the effects of processing are properly accounted for, precise values for the Ni concentration of the NiTi matrix can be obtained.Influence of Ni on martensitic phase transformations in NiTi shape memory alloys - Uncorrected ProofJ. Frenzel, E.P. George, A. Dlouhy, Ch. Somsen, M.F.-X. Wagner, G. Eggeler10.1016/j.actamat.2010.02.019Acta Materialia (2010)2010-02-26Acta Materialia2010-02-26http://www.actamat.org/article/PIIS1359645410001072/abstract?rss=yesAbstract: The objective of this work is to study the influence of the maximum Cu content during the deposition of Cu(In,Ga)Se2 (CIGSe) by multi-stage co-evaporation on the phases present in the final film, the film structure and the electrical properties of resulting solar cell devices. The variation of the composition is controlled by the Cu content in stage 2 of the deposition process. The different phases are identified by Raman spectroscopy. The in-depth Ga gradient distribution is investigated by in-depth resolved Raman scattering and secondary neutral mass spectroscopy. The morphology of the devices is studied by scanning electron microscopy. Efficiencies of 9.2% are obtained for ordered-vacancy-compound-based cells with a Cu/(In+Ga) ratio=0.35, showing the system’s flexibility. This work supports the current growth model: a small amount of Cu excess during the absorber process is required to obtain a quality microstructure and high performance devices.Cu deficiency in multi-stage co-evaporated Cu(In,Ga)Se2 for solar cells applications: Microstructure and Ga in-depth alloying - Uncorrected ProofR. Caballero, V. Izquierdo-Roca, X. Fontané, C.A. Kaufmann, J. Álvarez-García, A. Eicke, L. Calvo-Barrio, A. Pérez-Rodríguez, H.W. Schock, J.R. Morante10.1016/j.actamat.2010.02.021Acta Materialia (2010)2010-02-26Acta Materialia2010-02-26http://www.actamat.org/article/PIIS1359645410000820/abstract?rss=yesAbstract: Being able to predict the grain formation process and attendant grain size has been a central topic in solidification. Such an analytical model is presented for constitutional supercooling (CS)-driven grain formation with several simplifications. The model links the nucleation of new grains to the growth of a larger neighbouring grain. The average grain size () is thus determined by two components: the minimum growth (rcs) necessary to establish sufficient CS (ΔTn) for nucleating new grains, and the spatial mean distance () to the most potent available nucleants. Both spherical and planar growth fronts are considered, covering growth curvatures from small to infinite. Two distinct fundamental approaches are used, which result in identical descriptions of , where (D is the diffusion coefficient, v is the growth velocity, Q is the growth restriction factor). The model is compared with literature data produced under various conditions and demonstrated on aluminium alloys as an example.An analytical model for constitutional supercooling-driven grain formation and grain size prediction - Uncorrected ProofM. Qian, P. Cao, M.A. Easton, S.D. McDonald, D.H. StJohn10.1016/j.actamat.2010.01.052Acta Materialia (2010)2010-02-25Acta Materialia2010-02-25http://www.actamat.org/article/PIIS135964541000087X/abstract?rss=yesAbstract: Tensile tests on single crystals of Gum Metal (Ti–36Nb–2Ta–3Zr–0.3O (wt.%)) showed, anomalously, that while a stress-induced β(bcc)→α″(orthorhombic) transformation occurred in a crystal pulled in the 〈110〉 direction, (1) no transformation was observed in crystals pulled in the 〈100〉 or 〈111〉 directions and (2) little or no transformation soccurred in severely worked rods, which are polycrystals with very strong 〈110〉 texture. Analysis of the energetics of the β→α″ transformation offers straightforward explanations: (1) an α″ precipitate has zero elastic energy if it forms as a thin plate with the habit {111.5}; a 〈110〉 tensile load significantly decreases the energy of this plate; loading along 〈100〉 or 〈111〉 is less effective; (2) while worked rods have a strong 〈110〉 axial texture, the perpendicular planes are severely distorted, increasing the elastic energy of α″ and inhibiting the transformation.Anomalous transformation-induced deformation in 〈110〉 textured Gum Metal - Uncorrected ProofJ.W. Morris, Y. Hanlumyuang, M. Sherburne, E. Withey, D.C. Chrzan, S. Kuramoto, Y. Hatashi, M. Hara10.1016/j.actamat.2010.02.001Acta Materialia (2010)2010-02-25Acta Materialia2010-02-25http://www.actamat.org/article/PIIS1359645410000911/abstract?rss=yesAbstract: In the present paper, an elasto-viscoplastic model is proposed to describe the behavior of hexagonal close-packed materials where multiple deformation modes, including plastic slip and twinning, coexist. The model assists in interpreting the experimental lattice strains whose evolution is often complex because of the many deformation modes involved. The proposed rate-dependent model is compared with a previously developed rate-independent model by studying the development of internal strains in a moderately textured Zircaloy-2 slab. The comparison shows that most of the inflexions that are experimentally observed are well reproduced by the proposed model and the qualitative and quantitative agreement is generally better with the new rate-dependent formulation.Study of internal strain evolution in Zircaloy-2 using polycrystalline models: Comparison between a rate-dependent and a rate-independent formulation - Uncorrected ProofC. Mareau, M.R. Daymond10.1016/j.actamat.2010.02.005Acta Materialia (2010)2010-02-24Acta Materialia2010-02-24http://www.actamat.org/article/PIIS1359645410000935/abstract?rss=yesAbstract: This study presents a simplified micromechanical model to predict electromechanical behaviors of piezocomposites, having ferroelectric fibers and polymer matrix. A nonlinear electromechanical constitutive model is formulated for the ferroelectric fibers, i.e. PZT fibers. The nonlinearity is due to polarization switching in the PZT materials under high electric field and compression stress. Phenomenological models are used to represent stress–strain and polarization–electric field hysteresis responses during polarization switching. The nonlinear electromechanical constitutive relation is verified using hysteresis polarization and strain responses of PZT-51. Effective responses of piezocomposites at various fiber volume contents, generated using the simplified micromechanical model, are also compared with available experimental data. The effects of viscoelastic polymer matrix on the overall electromechanical hysteresis and creep behaviors of piezocomposites are also examined.A micromechanical formulation for piezoelectric fiber composites with nonlinear and viscoelastic constituents - Uncorrected ProofAnastasia H. Muliana10.1016/j.actamat.2010.02.007Acta Materialia (2010)2010-02-24Acta Materialia2010-02-24http://www.actamat.org/article/PIIS1359645410000959/abstract?rss=yesAbstract: In situ electron backscattering diffraction (EBSD) investigations were conducted on polycrystalline NiTi tube specimens during tensile and compressive deformation. The long-range cooperative and catalytic martensitic transformation under tension induces the transformation to proceed in the form of helical Lüders band. Propagation of the band is closely related to the spatial distribution of the orientations of individual grains. In uniaxial compression, the larger variation in Schmid factors, and consequently the larger variation in the critical transformation stresses among grains, leads to a homogeneous martensitic transformation, and therefore the absence of the Lüders band. To interpret the observed tension–compression asymmetry, a crystallographic model of the critical transformation stress and transformation strain for polycrystalline NiTi under tension and compression is proposed. The model defines three crystallographic regions: tension-favorable, compression-favorable and neutral zones. The orientation population in which tensile strains are larger than compressive strains is much higher than that of orientations with higher compressive strains. For resolved shear stress, orientation populations favoring tension and compression do not show any great difference.EBSD studies of the stress-induced B2–B19′ martensitic transformation in NiTi tubes under uniaxial tension and compression - Uncorrected ProofS.C. Mao, J.F. Luo, Z. Zhang, M.H. Wu, Y. Liu, X.D. Han10.1016/j.actamat.2010.02.009Acta Materialia (2010)2010-02-24Acta Materialia2010-02-24http://www.actamat.org/article/PIIS135964541000100X/abstract?rss=yesAbstract: A cluster dynamics model has been parametrized to quantitatively reproduce results obtained by atomistic kinetic Monte Carlo (AKMC) modeling on the precipitation of Cu in α-Fe under thermal aging. The cluster mobility, highlighted by AKMC, is shown to have a significant effect on the precipitation kinetics and can reconcile the experimentally observed fast kinetics with the relatively low diffusivity of Cu monomers.Influence of cluster mobility on Cu precipitation in α-Fe: A cluster dynamics modeling - Corrected ProofT. Jourdan, F. Soisson, E. Clouet, A. Barbu10.1016/j.actamat.2010.02.014Acta Materialia (2010)2010-02-24Acta Materialia2010-02-24http://www.actamat.org/article/PIIS1359645410001023/abstract?rss=yesAbstract: In order to use the magnetic shape memory alloy Ni–Mn–Ga with its high achievable strain of up to 10% for microactuators, freestanding epitaxial films are required. Here we show that these conditions can be fulfilled when using chromium as a sacrificial layer. The low misfit towards Ni–Mn–Ga enables epitaxial growth. Furthermore, Cr can afterwards be removed selectively by wet-chemical etching and during deposition no significant interdiffusion is observed. The structure, microstructure and magnetic properties of micrometer thick films are not affected by the etching process. Films are ferromagnetic at room temperature and we observe the coexistence of non-modulated (NM) and seven layered modulated (14M) martensite. Tensile stress–strain measurement of a freestanding bridge reveals a broad strain plateau of 12% at a twinning stress of 25MPa, indicating reorientation of NM variants.Comparing properties of substrate-constrained and freestanding epitaxial Ni–Mn–Ga films - Uncorrected ProofAnja Backen, SrinivasaReddy Yeduru, Manfred Kohl, Stefan Baunack, Anett Diestel, Bernhard Holzapfel, Ludwig Schultz, Sebastian Fähler10.1016/j.actamat.2010.02.016Acta Materialia (2010)2010-02-24Acta Materialia2010-02-24http://www.actamat.org/article/PIIS1359645410001047/abstract?rss=yesAbstract: The kinetics of the interfacial reaction of a thin layer of Sn sandwiched between two pieces of Ag foil has been investigated at temperatures of 260°C, 300°C and 340°C. A time dependence of the form t1/n with n=3 was obtained for the kinetics of both the consumption of the Sn remaining and the thickening growth of the Ag3Sn scallops formed between Sn and Ag. Such a result can be explained well using the model of grain boundary/molten channel-controlled growth of intermetallic compounds. In this case, the diffusion of Ag atoms through the molten channels existing between the previously formed Ag3Sn scallops is the controlling mechanism for the kinetics. We also report here the derived kinetic constants including reaction constants and the associated activation energy for guiding the practical transient liquid phase soldering of the Ag–Sn–Ag system.Kinetics of Ag3Sn growth in Ag–Sn–Ag system during transient liquid phase soldering process - Corrected ProofJ.F. Li, P.A. Agyakwa, C.M. Johnson10.1016/j.actamat.2010.02.018Acta Materialia (2010)2010-02-24Acta Materialia2010-02-24http://www.actamat.org/article/PIIS1359645410000972/abstract?rss=yesAbstract: Nickel sulphide inclusions are known to be responsible for delayed fracture in tempered glasses due to phase transformation within the inclusion. Microstructural identification of the phase transformation mechanisms in the Ni–S system close to the NiS composition were carried out on a series of partially transformed states. observations allow to investigate The morphological evolution during transformation, the phase orientation relationships and the first stages of the transformation were investigated by optical microscopy, electron backscatter diffraction, and scanning and transmission electron microscopy. The transformation mechanisms change significantly with the change in sulphur content of the α-NiS phase. Massive transformation is observed for near-stoichiometric composition. For overstoichiometric composition, the transformation is controlled by a long-range diffusion mechanism. The influence of stoichiometry and impurities (Fe) on the microstructural evolution and transformation mechanisms has also been studied.Phase transformations in nickel sulphide: Microstructures and mechanisms - Uncorrected ProofOussama Yousfi, Yves Brechet, Florence Robaut, Frederic Charlot, Andreas Kasper, Francis Serruys, Patricia Donnadieu10.1016/j.actamat.2010.02.011Acta Materialia (2010)2010-02-23Acta Materialia2010-02-23http://www.actamat.org/article/PIIS1359645410000984/abstract?rss=yesAbstract: The influence of additives on the reaction kinetics and microstructure refinement in LiBH4–MgH2 composites is investigated in detail. Indications of the rate-limiting processes during the reactions are obtained by comparison of the measured reaction kinetics with simulations with one specific rate-limiting process. The kinetics of the sorption reactions are derived from volumetric measurements as well as from in situ X-ray diffraction measurements. During desorption, the hydrogen is released at a constant rate, which is possibly correlated with the one-dimensional growth of MgB2 platelets. In contrast, the kinetic curves of the absorption reactions exhibit the typical shape of contracting-volume controlled kinetics. The microscopical interpretation of kinetic measurements are supported by transmission electron microscopy images confirming the formation of additive-nanostructures in the grain boundaries upon cycling. The present investigations underline the importance of the additives as nucleation substrates and the influence of microstructure on the reaction kinetics.Role of additives in LiBH4–MgH2 reactive hydride composites for sorption kinetics - Uncorrected ProofU. Bösenberg, J.W. Kim, D. Gosslar, N. Eigen, T.R. Jensen, J.M. Bellosta von Colbe, Y. Zhou, M. Dahms, D.H. Kim, R. Günther, Y.W. Cho, K.H. Oh, T. Klassen, R. Bormann, M. Dornheim10.1016/j.actamat.2010.02.012Acta Materialia (2010)2010-02-23Acta Materialia2010-02-23http://www.actamat.org/article/PIIS1359645410001011/abstract?rss=yesAbstract: Surface and bulk liquid phase transitions are measured by a unique method currently used to determine surface and interfacial tension of liquid alloys. Focusing on the Al–In system, the location of the liquid miscibility gap was determined from the critical to the monotectic temperatures. The surface tensions of nine liquid alloys, the interfacial tension between coexisting liquids and their densities were measured as a function of temperature. Implementing the bulk data extracted from the asymmetric miscibility gap into a sub-regular model reproduced the experimental surface and interfacial tensions. The wetting temperature was estimated to lie well below the monotectic temperature. The micrometer thickness of the In-rich films which wet the surface of the Al-rich liquid phase after solidification is suggested to be due to the growth of the equilibrium wetting film by diffusion from the Al-rich phase during cooling.Surfaces, interfaces and phase transitions in Al–In monotectic alloys - Uncorrected ProofI. Kaban, S. Curiotto, D. Chatain, W. Hoyer10.1016/j.actamat.2010.02.015Acta Materialia (2010)2010-02-23Acta Materialia2010-02-23http://www.actamat.org/article/PIIS1359645410000893/abstract?rss=yesAbstract: A precipitation model for an event-based kinetic Monte Carlo (EKMC) method is presented. It is based on atomic-scale computations of the emission and absorption rates of monomers by clusters. Clusters are considered as single objects that emit monomers close to them, at higher rates than predicted by the mean field cluster dynamics method. We show that a law based on continuous diffusion equation can be used to account for absorption, provided the reaction distances between clusters are accurately computed. The model is shown to reproduce quantitatively results obtained by atomistic kinetic Monte Carlo methods when only monomers are mobile. The kinetics obtained by EKMC is faster than the one given by cluster dynamics, which highlights the limits of such a mean-field method, especially at high solute concentrations. When applied to the precipitation of Cu in Fe, which involves the mobility of clusters, the EKMC model shows good agreement with experimental results.Modeling homogeneous precipitation with an event-based Monte Carlo method: Application to the case of Fe–Cu - Uncorrected ProofT. Jourdan, J.-L. Bocquet, F. Soisson10.1016/j.actamat.2010.02.003Acta Materialia (2010)2010-02-22Acta Materialia2010-02-22http://www.actamat.org/article/PIIS135964541000090X/abstract?rss=yesAbstract: Selective laser melting (SLM) is an additive manufacturing technique in which functional, complex parts can be created directly by selectively melting layers of powder. This process is characterized by highly localized high heat inputs during very short interaction times and will therefore significantly affect the microstructure. In this research, the development of the microstructure of the Ti–6Al–4V alloy processed by SLM and the influence of the scanning parameters and scanning strategy on this microstructure are studied by light optical microscopy. The martensitic phase is present, and due to the occurrence of epitaxial growth, elongated grains emerge. The direction of these grains is directly related to the process parameters. At high heat inputs it was also found that the intermetallic phase Ti3Al is precipitated during the process.A study of the microstructural evolution during selective laser melting of Ti–6Al–4V - Uncorrected ProofLore Thijs, Frederik Verhaeghe, Tom Craeghs, Jan Van Humbeeck, Jean-Pierre Kruth10.1016/j.actamat.2010.02.004Acta Materialia (2010)2010-02-22Acta Materialia2010-02-22http://www.actamat.org/article/PIIS1359645410000923/abstract?rss=yesAbstract: The ability of second phase particles to migrate along with grain boundaries is shown to be determined not only by the particle mobility but also by the migration rate of the grain boundary where they locate. This leads to a duality in the mobile particle behaviour: they behave as either movable or immovable depending on the boundary migration rate. In the first case, they reduce the boundary mobility; in the second one they decrease the driving force for boundary migration. It is demonstrated by numerical modeling that mobile particles with low mobility can suppress grain growth even in nanocrystalline material, the limiting grains size being several times smaller than in the case of randomly distributed immobile particles. It is also shown that the Zener solution to the problem of the grain growth retardation by disperse particles is a specific case of the proposed approach.On grain growth in the presence of mobile particles - Uncorrected ProofV.Yu. Novikov10.1016/j.actamat.2010.02.006Acta Materialia (2010)2010-02-22Acta Materialia2010-02-22http://www.actamat.org/article/PIIS1359645410001035/abstract?rss=yesAbstract: This paper presents the thermodynamic evaluation of A390 hypereutectic Al–Si alloy (Al–17%Si–4.5%Cu–0.5%Mg) and alloys up to 10% Mg, using the Factsage® software. Two critical compositions were detected at 4.2% and 7.2% Mg where the temperatures of the liquidus, the start of the binary and of the ternary eutectic reaction are changed. These critical compositions show differences in the formation of Mg2Si intermetallic particles during the solidification interval. For compositions up to 4.2% Mg, the Mg2Si intermetallic phase first appears in the ternary eutectic zone. With Mg contents between 4.2% and 7.2%, Mg2Si particle appears in both the binary and ternary eutectic reactions. Above 7.2% Mg, it solidifies as a primary phase and also during the binary and ternary reactions. The calculated liquid fraction vs. temperature curves also showed a decrease of the eutectic formation temperature (knee point temperature) with the addition of Mg content up to 4.2% Mg. This temperature becomes almost constant up to 10% Mg. The calculation of eutectic formation temperature shows a good agreement with differential scanning calorimetry (DSC) tests.Thermodynamic evaluation of hypereutectic Al–Si (A390) alloy with addition of Mg - Uncorrected ProofAlireza Hekmat-Ardakan, Frank Ajersch10.1016/j.actamat.2010.02.017Acta Materialia (2010)2010-02-22Acta Materialia2010-02-22http://www.actamat.org/article/PIIS1359645410000881/abstract?rss=yesAbstract: This work reveals the formation mechanisms of saturation dislocation patterns in three typical multiple-slip oriented [001], [011] and copper single crystals. Compared with the single-slip oriented copper single crystals, the three multiple-slip oriented ones show very different dislocation patterns. It was found that the dislocation patterns in cyclically saturated copper single crystals are the Labyrinth structure for [001], wall structure for [011] and cell structure for , respectively. Based on a two-phase structure consisting of persistent slip bands and veins for single-slip orientation, the formation mechanisms of the dislocation patterns in multiple-slip oriented crystals are proposed as follows: the formation of the complex dislocation patterns depends on the activating slip system. The easy operation of the critical secondary slip system will contribute to the formation of the Labyrinth structure. The activation of the coplanar secondary slip system will be beneficial to formation of the cell structure. If no secondary slip system is activated, the wall structure is more prone to appear. Finally, the intrinsic relationship between various dislocation patterns and face centered cubic crystal structure was established.Formation mechanisms of cyclic saturation dislocation patterns in [001], [011] and copper single crystals - Corrected ProofP. Li, S.X. Li, Z.G. Wang, Z.F. Zhang10.1016/j.actamat.2010.02.002Acta Materialia (2010)2010-02-18Acta Materialia2010-02-18http://www.actamat.org/article/PIIS1359645410000844/abstract?rss=yesAbstract: We use molecular statics simulations with the tight-binding potential to analyze stress evolution in nanosize square prismatic gold specimens of different aspect ratios (length/width) deformed in either simple tension/compression or tension/compression. In the former case atoms on end faces are displaced axially but are free to move laterally, and in the latter case atoms on end faces are restrained from moving laterally during their axial displacement. It is found that the stress distribution in the unloaded reference configuration is non-uniform, and it satisfies the local and the global equilibrium equations. Large values of the von Mises stress and the maximum shear stress occur on atoms located at the third layer beneath the traction free surfaces forming different patterns for specimens loaded in tension and compression. The specimen is assumed to yield when its total strain energy drops noticeably. Maximum values of the von Mises stress and the maximum shear stress at yielding are essentially independent of specimen’s length for specimens deformed in tension. For specimens deformed in compression, wave-like patterns of stresses along the axial centroidal axis are observed when the specimen yields.Changes in internal stress distributions during yielding of square prismatic gold nano-specimens - Corrected ProofR.C. Batra, A.A. Pacheco10.1016/j.actamat.2010.01.054Acta Materialia (2010)2010-02-17Acta Materialia2010-02-17http://www.actamat.org/article/PIIS1359645410000832/abstract?rss=yesAbstract: Co–30wt.% Cr alloy was prepared by electro-deoxidation in molten calcium chloride at 1123K. A preliminary study was conducted into the preparation of the mixture of the Co3O4 and Cr2O3 and the formation of the non-stoichiometric, spinel structured, mixed oxide nominally labeled CoxCryO4. Constant voltage chronoamperometry was used both to prepare the alloy and to investigate its mechanism of formation. Electro-deoxidation proceeds by the simultaneous rapid reduction of CoO to Co and the slower reduction/substitution of CoxCryO4 to CaCr2O4 and Co metal. The final step of the electro-deoxidation is the reduction of CaCr2O4 to Cr metal, which alloys with the Co metal, and release of Ca2+ back into the electrolyte.Electrochemical synthesis of a biomedically important Co–Cr alloy - Corrected ProofD.J.S. Hyslop, A.M. Abdelkader, A. Cox, D.J. Fray10.1016/j.actamat.2010.01.053Acta Materialia (2010)2010-02-16Acta Materialia2010-02-16http://www.actamat.org/article/PIIS1359645410000868/abstract?rss=yesAbstract: The correlation between stacking fault energy (SFE) and deformation microstructure of high-interstitial-alloyed austenitic Fe–18Cr–10Mn–(N or N+C) alloys was investigated. As the content of the interstitial elements increased, the deformation microstructure changed in a sequence strain-induced martensitic transformation, mixture of martensite and twin, and finally deformation twin. The SFE, playing an important role in the transition of deformation microstructure, was evaluated by the Rietveld whole-profile fitting combined with the double-Voigt size–strain analysis for neutron diffraction profiles of tensile-strained bulk samples. At fixed N+C content, the ratio of mean-squared strain to stacking fault probability remained constant regardless of the accumulated strain, whereas the ratio gradually increased with increasing N+C content. Almost linear dependence of measured SFE on N+C content could be established. According to the SFE, deformation bands exhibited distinct substructures, and their particular intersecting behavior resulted in the formation of different types of products (secondary ε martensite, α′ martensite and secondary twin) at the intersecting regions.Correlation between stacking fault energy and deformation microstructure in high-interstitial-alloyed austenitic steels - Corrected ProofTae-Ho Lee, Eunjoo Shin, Chang-Seok Oh, Heon-Young Ha, Sung-Joon Kim10.1016/j.actamat.2010.01.056Acta Materialia (2010)2010-02-16Acta Materialia2010-02-16http://www.actamat.org/article/PIIS1359645410000522/abstract?rss=yesAbstract: The microstructure evolution of aluminium alloy 6016 processed by equal channel angular pressing (ECAP) was investigated for different processing parameters. A number of heat treatments, including F, W, O, T4, T6 and T7, were evaluated for workability required to withstand the severe plastic deformation. It was found that this aluminium alloy had limited workability at room temperature in the F, W and T4 tempers, especially at low levels of applied back pressure, while both the O and T7 tempers can be pressed to a very high strain (∼1800%) without failure. Considering that the O temper has better utility for industry because of decreased preparation time, a complete study of microstructure after ECAP processing with and without back pressure was carried out for the O temper. The thermal stability of microstructure after 16 ECAP passes with 200MPa of back pressure was also studied.ECAP with back pressure for optimum strength and ductility in aluminium alloy 6016. Part 1: Microstructure - Corrected ProofP.W.J. Mckenzie, R. Lapovok10.1016/j.actamat.2010.01.038Acta Materialia (2010)2010-02-15Acta Materialia2010-02-15http://www.actamat.org/article/PIIS1359645410000765/abstract?rss=yesAbstract: It is often necessary to identify close-packed or nearly close-packed planes in a crystal. This can be done by inspection in crystal structures where one atom occupies each lattice point – i.e. face-centred cubic and body-centred cubic crystals. However, in more complex crystal structures intuitive inspection becomes rather difficult. The present paper describes a simple method for identifying close-packed or nearly close-packed planes in crystals containing more than one atom per lattice point. The method also distinguishes between “flat” planes, where all the atom centres lie in the plane, and “rumpled” planes, where the atom centres do not lie in the plane. The extent of the departure from true “flatness” can also be estimated.Identifying close-packed planes in complex crystal structures - Corrected ProofP.M. Kelly, H.-P. Ren, D. Qiu, M.-X. Zhang10.1016/j.actamat.2010.01.046Acta Materialia (2010)2010-02-15Acta Materialia2010-02-15http://www.actamat.org/article/PIIS1359645410000789/abstract?rss=yesAbstract: Nanoscale incipient plastic deformation in crystalline metals occurs as the result of the collective motion of dislocations. It is known as “nanoplasticity” and is recognized as the elementary process of the macroscopic deformation. Abrupt increases in indent displacements called displacement bursts were observed in recent nanoindentation experiments; that is, the specific behavior for nanoplasticity. In the present study, experimental tests are first conducted to educe the unique nature of the nanoscale deformation. Subsequently, large-scale atomistic simulations are performed to predict the incipient plastic deformation and a new discrete dislocation model combined with the boundary element analysis is constructed to capture the collective motion of the dislocations. Our results suggest that the incipient plastic deformation requires much higher critical shear stress than the theoretical shear strength due to high compressive stress distribution beneath the indenter, and that the displacement burst is induced by surface rearrangement corresponding to hundreds of dislocation dipoles.Nanoscale contact plasticity of crystalline metal: Experiment and analytical investigation via atomistic and discrete dislocation models - Corrected ProofT. Tsuru, Y. Shibutani, Y. Kaji10.1016/j.actamat.2010.01.048Acta Materialia (2010)2010-02-15Acta Materialia2010-02-15http://www.actamat.org/article/PIIS1359645410000790/abstract?rss=yesAbstract: Microstructural evolution associated with the shear banding in nano-scale twin/matrix (T/M) lamellae of a Cu–Al alloy processed by means of dynamic plastic deformation was investigated using transmission electron microscopy (TEM) and high-resolution TEM. The development of a shear band was found to be a two-stage process, namely a nucleation stage resulting in a narrow band composed of nano-sized (sub)grains intersecting the T/M lamellae, followed by a thickening stage of the narrow band into adjacent T/M lamellae regions. The nucleation stage occurred within a narrow region of an almost constant thickness (100–200nm thick, referred to as “core” region) and consisted of three steps: (1) initiation of localized deformation (bending, necking, and detwinning) against the T/M lamellae, (2) evolution of a dislocation structure within the detwinned band, and (3) transformation of the detwinned dislocation structure (DDS) into a nano-sized (sub)grain structure (NGS). On the two sides of a core region, two transition layers (TRLs) exist where the T/M lamellae experienced much less shear strain. The interface boundaries separating the core region and the TRLs are characterized by very large shear strain gradients accommodated by high density of dislocations. Increasing shear strains leads to thickening of shear bands at the expense of the adjoining T/M lamellae, which is composed of thickening of the core region by transforming the TRLs into the core region with DDS and NGS, analogous to steps (2) and (3) of the nucleation process, and outward movement of the TRLs by deforming the adjoining T/M lamellae. Grain sizes in the well-developed shear bands are obviously larger than the lamellar thickness of original T/M lamellae.Nucleation and thickening of shear bands in nano-scale twin/matrix lamellae of a Cu–Al alloy processed by dynamic plastic deformation - Corrected ProofC.S. Hong, N.R. Tao, X. Huang, K. Lu10.1016/j.actamat.2010.01.049Acta Materialia (2010)2010-02-15Acta Materialia2010-02-15http://www.actamat.org/article/PIIS1359645410000807/abstract?rss=yesAbstract: The influence of a triaxial stress applied normally to shear planes and shear direction during affine shear deformation of face-centered cubic crystals on the theoretical shear strength is studied for the shear system using first-principles methods. The applied relaxation procedure guarantees that the modeled system is subjected to a superposition of shear, normal and in-plane stresses with individually adjustable in-plane and normal stress values. The theoretical shear strengths of individual elements prove to be qualitatively different functions of the superimposed stresses. In the special case of hydrostatic loading, however, these functions are qualitatively uniform. This behavior is discussed in terms of the electronic structure.The theoretical shear strength of fcc crystals under superimposed triaxial stress - Corrected ProofM. Černý, J. Pokluda10.1016/j.actamat.2010.01.050Acta Materialia (2010)2010-02-15Acta Materialia2010-02-15http://www.actamat.org/article/PIIS1359645410000819/abstract?rss=yesAbstract: In this paper, we demonstrate a way to impart severe plastic deformation to magnesium at room temperature to produce ultrafine grain size of ∼250nm through equal channel angular extrusion (ECAE). The strategy to deform magnesium at lower temperature or to achieve such grain sizes has been proposed as: (i) to obtain a suitable initial orientation with high Schmid factor for basal slip and low Schmid factor for pyramidal/prismatic slip; (ii) to take advantage of low stacking fault energy of basal and high stacking fault energies of prismatic/pyramidal planes in order to relatively work-harden the basal plane with respect to the pyramidal/prismatic plane; and (iii) to lower the temperature of deformation in steps, leading to continual refinement of grains, resulting in finer grain size. The experimental as well as simulated texture of ECAE-processed samples indicate that the deformation mechanism leading to ultrafine grain size is slip-dominated. The recrystallization mechanism during ECAE has been found to be orientation-dependent.Room-temperature equal channel angular extrusion of pure magnesium - Corrected ProofSomjeet Biswas, Satyaveer Singh Dhinwal, Satyam Suwas10.1016/j.actamat.2010.01.051Acta Materialia (2010)2010-02-15Acta Materialia2010-02-15http://www.actamat.org/article/PIIS1359645410000856/abstract?rss=yesAbstract: Cryo-stage transmission electron microscopy (TEM), supported by Density Functional Theory (DFT), is employed to explore the microstructure of magnesium hydride (MgH2) powders. Mechanical milling results in deformation twinning of the hydride. The crystallography of the twins is established. DFT analysis shows that the twin unit cell is just as thermodynamically stable as the undeformed α-MgH2 matrix. It is hypothesized that the twins contribute significantly to the observed milling-induced kinetic enhancement by acting as high diffusivity paths for hydrogen. Energy-filtered TEM analysis on partially desorbed MgH2 demonstrates that nucleation and growth of metallic magnesium occurs non-uniformly. Larger powder particles are a composite of isolated magnesium grains heterogeneously nucleated on the remaining hydride. Smaller particles are either fully transformed to magnesium or remain entirely a hydride. There is little evidence for any “core–shell” structure. It is also shown that in situ hydrogen desorption in the TEM is not representative of the elevated-temperature ex situ sequence.Analysis of deformation twins and the partially dehydrogenated microstructure in nanocrystalline magnesium hydride (MgH2) powder - Corrected ProofM. Danaie, S.X. Tao, P. Kalisvaart, D. Mitlin10.1016/j.actamat.2010.01.055Acta Materialia (2010)2010-02-15Acta Materialia2010-02-15http://www.actamat.org/article/PIIS1359645410000418/abstract?rss=yesAbstract: Soldering between solid Ni and liquid Sn is studied, including late stages of reflow. During the very early stages of the process, intermetallic grains grow with a 1/3 power dependence on time. Later, kinetics clearly changes to parabolic growth, but remarkably in two subsequent regimes distinguished by different rate constants. The observed kinetics is discussed with respect to recent flux-driven ripening theory. This theory is only valid, if at all, for short reflows up to about 4min. Transmission electron microscopy reveals the predicted scallop-like microstructure only at the very beginning. A sponge-like structure of equiaxed grains then develops, triggered by permanent nucleation of new grains at the Ni/Ni3Sn4 interface. Wetting of grain boundaries appears only up to a certain depth in the reaction zone. This remarkable behavior is explained by thermodynamic arguments.Reaction kinetics of Ni/Sn soldering reaction - Uncorrected ProofJens Görlich, Dietmar Baither, Guido Schmitz10.1016/j.actamat.2010.01.027Acta Materialia (2010)2010-02-12Acta Materialia2010-02-12http://www.actamat.org/article/PIIS1359645410000741/abstract?rss=yesAbstract: The martensite start temperature and driving force were calculated for the austenite to martensite transformation of a dual-phase bainitic ferrite–austenite steel at room temperature. The mechanical energy was estimated during the first and the second pass of equal channel angular pressing (ECAP). The applied mechanical energy during the first pass is not enough to induce the austenite to martensite transformation. However, the mechanical energy during the second pass can provide the required energy for the martensite formation. Microstructural observations by transmission electron microscopy and scanning electron microscopy of as-received and ECAPed samples confirm the formation of martensite during the second pass.Control of austenite to martensite transformation through equal channel angular pressing aided by thermodynamic calculations - Corrected ProofF. Hajiakbari, M. Nili-Ahmadabadi, B. Poorganji, T. Furuhara10.1016/j.actamat.2010.01.044Acta Materialia (2010)2010-02-12Acta Materialia2010-02-12http://www.actamat.org/article/PIIS1359645410000753/abstract?rss=yesAbstract: Microstructure and texture evolution during cold rolling and subsequent annealing were studied in an Fe–22 wt.% Mn–0.376 wt.% C alloy. During rolling the deformation mechanisms were found to be dislocation slip, mechanical twinning, deformation-induced ε-martensite transformation and shear banding. At higher strains, the brass-type texture with a spread towards the Goss-type texture dominated. A decrease in the Cu- and S- components was attributed to the preferential transformation to ε-martensite in Cu- and S-oriented grains. The texture of ε-martensite was sharp and could be described as {1129}〈3362〉. The orientation relationship {111}γ//{0001}ε and 〈110〉γ//〈11–20〉ε between ε-martensite and austenite was observed but only certain variants were selected. On subsequent annealing, the ε-martensite transformed reversely to austenite by a diffusionless mechanism. Changes in length along rolling, normal and transverse directions on heating were anisotropic due to a combination of volume expansion and shape memory effects. The S-texture component increased significantly due to transformation from the ε-martensite.Effect of deformation and annealing on the formation and reversion of ε-martensite in an Fe–Mn–C alloy - Corrected ProofYaping Lü, Bevis Hutchinson, Dmitri A. Molodov, Günter Gottstein10.1016/j.actamat.2010.01.045Acta Materialia (2010)2010-02-12Acta Materialia2010-02-12http://www.actamat.org/article/PIIS1359645410000777/abstract?rss=yesAbstract: Elastic strain has been investigated by transmission electron microscopy in nanometric InAs layers grown on Ga0.47In0.53As/InP(001) by molecular beam epitaxy using a residual Sb flux. Deposits of 10 and 15 monolayers of InAs (3 and 4.5nm) remain elastically stressed with a two-dimensional growth mode. The out-of-plane strain in the layers is analyzed by cross-sectional high-resolution electron microscopy. A distortion of the substrate below and on top of the InAs layers is detected and is attributed to a significant surface relaxation effect due to thinning. Surface relaxation is modeled by three-dimensional finite element modeling. An additional relaxation effect is obtained when the sample is not infinite along the direction perpendicular to the thinning. This effect enhances the buffer distortion of the buffers below and on top of the strained layers. Taking into account thin foil effects, the experimental out-of-plane strain is in excellent agreement with the theoretical value calculated for a pure InAs layer (i.e. 0.035), demonstrating the high level of strain and stress in the layers.Analysis by high-resolution electron microscopy of elastic strain in thick InAs layers embedded in Ga0.47In0.53As buffers on InP(001) substrate - Corrected ProofC. Gatel, H. Tang, C. Crestou, A. Ponchet, N. Bertru, F. Doré, H. Folliot10.1016/j.actamat.2010.01.047Acta Materialia (2010)2010-02-12Acta Materialia2010-02-12http://www.actamat.org/article/PIIS1359645410000571/abstract?rss=yesAbstract: A phase-field model simulating vacancy diffusion in a solid with a strong vacancy mobility inhomogeneity is presented. The model is used to study void migration via bulk and surface diffusion in a temperature gradient. The simulations demonstrate that voids migrate up the temperature gradient, and the migration velocity varies inversely with the void size, in agreement with theory. It is also shown that the current model has the capability to investigate the effects of surface diffusion, temperature gradient and vacancy concentration on the void migration velocity. An interesting potential application of the model is to study the kinetics of void migration and the formation of a central hole in nuclear fuels.Phase-field simulation of void migration in a temperature gradient - Corrected ProofS.Y. Hu, C.H. Henager10.1016/j.actamat.2010.01.043Acta Materialia (2010)2010-02-11Acta Materialia2010-02-11http://www.actamat.org/article/PIIS1359645410000510/abstract?rss=yesAbstract: The simultaneous increase in strength and ductility of aluminium alloy 6016 processed by equal channel angular pressing (ECAP) was investigated. A complete study of microstructure, texture and mechanical properties after ECAP processing with and without back pressure was carried out for the O temper. The simultaneous increase in strength and ductility of AA6016-O with number of ECAP passes was explained by the use of back pressure during ECAP. A maximum ductility of ∼100% was obtained at the temperature of 200°C and strain rate of 10−4s−1, which is a significant improvement on the ductility exhibited by AA6016 (∼89%) after a conventional thermomechanical treatment at a much higher temperature of 500°C. The mechanical behaviour was interpreted in the context of the textures developed in the material. A significant amount of texture rotation due to applied back pressure was found.ECAP with back pressure for optimum strength and ductility in aluminium alloy 6016. Part 2: Mechanical properties and texture - Corrected ProofP.W.J. Mckenzie, R. Lapovok10.1016/j.actamat.2010.01.037Acta Materialia (2010)2010-02-10Acta Materialia2010-02-10http://www.actamat.org/article/PIIS1359645410000534/abstract?rss=yesAbstract: The size, surface and interface effects on the magnitude and stability of spontaneous polarization in a symmetric nanoscale ferroelectric capacitor were studied by analyzing its evolutionary trajectory based on a thermodynamic model. Analytic expressions of the Curie temperature, spontaneous polarization, critical thickness and the Curie–Weiss relation were derived, taking into account the effects of the depolarization field, built-in electric field, interfaces and surfaces. Our results show that the critical properties are not only functions of the ambient temperature, misfit strain and electromechanical boundary conditions, but also depend on the characteristics of electrodes, surfaces and interfaces, through the incomplete charge compensation, near-surface variation of polarization and work function steps of ferroelectric–electrode interfaces, which are adjustable.Critical properties of symmetric nanoscale metal–ferroelectric–metal capacitors - Corrected ProofYue Zheng, M.Q. Cai, C.H. Woo10.1016/j.actamat.2010.01.039Acta Materialia (2010)2010-02-10Acta Materialia2010-02-10http://www.actamat.org/article/PIIS1359645410000558/abstract?rss=yesAbstract: Small-sized indium tin oxide (ITO) nanowires were fabricated using the electron beam evaporation (EBE) technique at low temperature (∼150°C) without adding any catalyst. The ITO nanowires have a typical diameter of around 10nm and a length of more than 100nm, with body-centered cubic crystal structures that grow along the 〈100〉 directions, as revealed by transmission electron microscopy. The growth mechanism of the branched ITO nanowires was found to be a vapor–solid process. The nanowire films show a broadband anti-reflection property due to the graded refraction index from the film surface to the substrate. Enhanced field emission properties with a low turn-on electric field and a high field enhancement factor were also observed in the ITO nanowires.Broadband anti-reflection and enhanced field emission from catalyst-free grown small-sized ITO nanowires at a low temperature - Corrected ProofNeng Wan, Jun Xu, Guran Chen, Xinhui Gan, Sihua Guo, Ling Xu, Kunji Chen10.1016/j.actamat.2010.01.041Acta Materialia (2010)2010-02-10Acta Materialia2010-02-10http://www.actamat.org/article/PIIS135964541000056X/abstract?rss=yesAbstract: Dislocation density and crystal orientation were investigated for a set of multicrystalline silicon ingots grown in a pilot scale furnace. Both low and high dislocation density ingots were observed. The low dislocation density ingots showed a dominating orientation close to (211) in contrast to the high dislocation density ingots. The orientations are consistent with growth on dendritic crystals formed along the crucible bottom and dendritic crystals with an angle towards the melt, respectively. During crystal growth, the power that was dissipated to the crystallization furnace showed a marked drop shortly after the onset of crystallization for low dislocation density ingots, an indication of fast release of crystallization heat from dendritic growth. Ingots that were not dominated by a high dislocation density instead had a high occurrence of twinned areas. Favourable orientation of the [10] vector in the growth plane is suggested to be the cause of growth dominated by multiple twin faceting. This favourable orientation existed for crystals grown from dendrites grown along the crucible bottom, and this is suggested as an explanation for why these crystals are dominated by multiple twins rather than dislocations.Relationship between dislocation density and nucleation of multicrystalline silicon - Corrected ProofG. Stokkan10.1016/j.actamat.2010.01.042Acta Materialia (2010)2010-02-10Acta Materialia2010-02-10