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Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 253  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3163 journals]
  • Concentration dependent properties lead to plastic ratcheting in thin
           island electrodes on substrate under cyclic charging and discharging
    • Abstract: Publication date: Available online 19 October 2018Source: Acta MaterialiaAuthor(s): Kai Guo, Wei Zhang, Brian W. Sheldon, Huajian Gao It is known that the mechanical properties of electrodes in lithium-ion batteries, such as modulus, yield stress, and interfacial strength, can depend strongly on lithium concentration. Here we show that a thin film island electrode with properties dependent on lithium concentration naturally undergoes plastic ratcheting with accumulative deformation and failure during cyclic charging and discharging. Some key predictions from numerical simulations are validated by galvanostatic tests. Strategies to avoid ratcheting include limiting the electrode size and/or selecting a balanced combination of concentration dependent materials properties.Graphical abstractImage 1
       
  • The influence of Laves phases on the room temperature tensile properties
           of Inconel 718 fabricated by powder feeding laser additive manufacturing
    • Abstract: Publication date: Available online 19 October 2018Source: Acta MaterialiaAuthor(s): Shang Sui, Hua Tan, Jing Chen, Chongliang Zhong, Zuo Li, Wei Fan, Andres Gasser, Weidong Huang In this paper, a powder feeding laser additive manufacturing technology has been used for fabricating the Inconel 718 super-alloy. Laves phases of different sizes and morphologies have been obtained by using three types of heat treatments. The influence of Laves phases on the room temperature tensile properties of laser additive manufactured Inconel 718 has been investigated. The results show that small and granular Laves phase can be gained after heat treatment at 1050 °C for 15 min (S-15 sample). When the holding time extends to 45 min, the morphologies of Laves phases basically remain unchanged while its volume fraction further decreases (S-45 sample). Nevertheless, irregular and long-striped Laves phases still exist in the samples only after direct aging heat treatment (DA sample). The room temperature tensile results reveal that the S-15 samples have better tensile strength and ductility than that of S-45 samples. Besides, the DA samples with irregular and long-striped Laves phases show the lowest tensile strength and ductility. Hence, a certain amount of small and granular Laves phases are presumably beneficial for the room temperature tensile properties of Inconel 718. Moreover, a model has been established to describe the fracture of the Laves phase. On the basis of the fracture model, the critical stress needed for the fracture of long-striped Laves phases is lower than that needed for the fracture of granular Laves phases. Therefore, the former generally suffered internal fracture while the latter often fail by interfacial decohesion. Through influencing the volume fraction, the size and the distribution of γ" phase, the effect of the Laves phases on the room temperature tensile property is achieved. Furthermore, a yield strength model has been developed to reveal this influence in terms of numbers. The yield strength increments caused by grains, solution elements and γ' phase are almost the same for the three kinds of samples. The differences of the yield strength are mainly caused by γ" phase. In addition, in terms of ductility, granular Laves phases are more favorable than long-striped Laves phases.Graphical abstractImage 1
       
  • γ ' / γ ' ' +Coprecipitates+in+Ni-Base+Superalloys&rft.title=Acta+Materialia&rft.issn=1359-6454&rft.date=&rft.volume=">Growth Behavior of γ ' / γ ' ' Coprecipitates in Ni-Base Superalloys
    • Abstract: Publication date: Available online 19 October 2018Source: Acta MaterialiaAuthor(s): Rongpei Shi, Donald P. McAllister, Ning Zhou, Andrew J. Detor, Richard DiDomizio, Michael J. Mills, Yunzhi Wang Precipitation of the γ'' phase on {100} facets of preceding γ' precipitates is found to prevent the latter from overaging upon slow cooling from solution treatment in Ni-base superalloys based on the composition of alloy 718. By computer simulation using a multi-phase-field model, we find that the growth of a coprecipitate involves several concurrent and closely coupled processes, including thickening and lengthening of γ'' shells, growth of the γ' core along the 001γ', 011γ' and 111γ' directions, hard impingement between γ' and γ'' precipitates, and soft impingement among γ'' precipitates of different variants. These processes at different stages of growth are analyzed systematically as a function of coprecipitate size and configuration, and the results show that the growth kinetics of the γ' core in a coprecipitate is controlled by the interplay among: (1) partial removal of supersaturated γ matrix surrounding the γ' core by coprecipitation of γ'' shells, (2) cooperative growth of γ' and γ'' in the coprecipitates and (3) atomic mobility of γ'-formers in the γ'' phase. To maximize the effect of coprecipitation on preventing γ' from overaging upon slow cooling, the alloy composition and heat treatment schedule should be optimized to minimize the size of γ' cores at which coprecipitation of γ'' shells occurs and to reduce diffusion of γ'-formers through γ''.Graphical abstractImage 1
       
  • Phase field simulations of martensitic transformation in pre-strained
           nanocomposite shape memory alloys
    • Abstract: Publication date: Available online 17 October 2018Source: Acta MaterialiaAuthor(s): Dong Wang, Qianglong Liang, Shuangshuang Zhao, Pengyang Zhao, Tianlong Zhang, Lishan Cui, Yunzhi Wang We show in this paper how strain engineering alters the fundamental characteristic of a martensitic transformation (MT) and gives it a new set of properties including large quasi-linear elastic strain response with nearly vanishing hysteresis and low elastic modulus. The work is motivated and inspired by a recent experimental study on elastic and inelastic (transformation) strain matching in a pre-strained nano-composite with Nb nanowires embedded in a NiTi shape memory alloy matrix. In particular, we demonstrate by computer simulation that dislocations at Nb/NiTi interfaces produced by the pre-straining are responsible for the unprecedented properties. Microstructural evolution captured in the simulations reveals that local stress fields associated with the dislocations regulate the nucleation and growth of martensite, turning the otherwise sharp, strong first-order transition into a continuous, high-order like transition. The simulations predict that the stress-strain hysteresis and modulus of the composite decrease with increasing amount of pre-strain, which agrees well with the experimental measurement. This study suggests a design strategy by introducing non-uniform stress fields for enhanced properties of shape memory alloys.Graphical abstractImage 1
       
  • Grain size dependent physical properties in lead-free multifunctional
           piezoceramics: a case study of NBT-xST system
    • Abstract: Publication date: Available online 17 October 2018Source: Acta MaterialiaAuthor(s): Xing Liu, Saidong Xue, Feifei Wang, Jiwei Zhai, Bo Shen Grain size effect is one of the most important issues to develop next-generation functional devices. In this work, we firstly provide a systematic investigation on the grain size dependent physical properties based on a flexible (Na0.5Bi0.5)TiO3-xSrTiO3 (NBT-xST) system with multifunctionality. The NBT-20ST, -26ST and -35ST with multiple phase boundaries/structures were chosen as the studied compositions. The densified ceramics with a series of grain sizes were successfully fabricated by normal and two-step sintering method. For NBT-20ST and -26ST compositions, the coarse grain size is more favorable for improving the direct (small-signal d33) and converse (large-signal d33∗) piezoelectricity. The critical grain size of NBT-20ST and -26ST compositions for improving d33 and d33∗ is both around 1 μm. Rayleigh analysis and local PFM mapping indicate that the high d33 in coarse-grained NBT-20ST samples originates from the increased extrinsic contribution and easier domain wall motion, while the large d33∗ in coarse-grained NBT-26ST samples stems from the polarization enhancement through a linear electrostrictive effect. For NBT-35ST composition, an improved energy storage performance with high recoverable energy density over 1 J/cm3 was achieved in a 0.56 μm-sized sample owing to a large fraction of low-polarizability grain boundary layer. This study opens up a new way for designing novel lead-free multifunctional piezoceramics with superior electromechanical properties.Graphical abstractImage 1
       
  • Structure-Property Correlations in Thermally Processed Epitaxial LSMO
           Films
    • Abstract: Publication date: Available online 17 October 2018Source: Acta MaterialiaAuthor(s): Daniel Rasic, Ritesh Sachan, John Prater, Jagdish Narayan Mixed-valence perovskites have drawn significant research interest in the past due to their exotic properties. Lanthanum Strontium Manganese Oxide (LSMO) shows a ferromagnetic ordering that can be tuned with the control of defects and strain. Here, experiments were performed to decouple the effects of strain and oxygen content, which together control the magnetic properties of the LSMO (La0.7Sr0.3MnO3). In this work, thermal treatments show promise in effectively controlling the ferromagnetic response of LSMO films. A set of three samples were grown on the same substrate-buffer (Al2O3/MgO) platform with different oxygen partial pressures and annealed above their deposition temperature (∼900° C) in air. The physical and structural properties were measured and showed overall decrease in magnetization saturation as well as decrease in out-of-plane lattice spacing with decreasing oxygen partial pressure. A second anneal at lower (∼700° C) temperature with flow of pure oxygen was performed for six hours to allow for defect annihilation and grain growth. All three films remained epitaxial allowing for direct correlation of magnetic measurements with defect concentration. Partial recovery of the magnetic properties and a slight increase in interplanar spacing was observed. The inability of the films to fully recover their original magnetic properties suggests irreversible strain relaxation during the initial, high-temperature air anneal. This hypothesis was further supported by the in-situ XRD that showed a linear increase in the interplanar spacing with temperature until ∼520°C for LSMO and ∼690°C for MgO. With further increase in temperature, the films experienced both loss of oxygen and irreversible defect nucleation and recombination. High resolution high-angle annular dark field (HAADF) images showed uniform thickness and no interfacial mixing with subsequent annealing treatments while electron energy loss spectroscopy (EELS) showed a loss of characteristic pre-peak A in oxygen indicating formation of oxygen vacancies. Parallel annealing experiments in high vacuum instead of atmosphere were performed, which showed complete loss of crystal structure in the LSMO films due to significant loss of oxygen in the lattice that irreversibly collapsed the perovskite structure. Furthermore, a low-temperature (∼500° C) oxidation anneal was performed on a pristine sample with no change in the interplanar spacing observed indicating no change in the strain state of the film due to annealing below the deposition temperature. The reversibility of magnetic properties, which is observed as long as the crystal structure of the films is preserved, indicates the importance of bridging oxygen in controlling the magnetic behavior of mixed valence perovskites. Finally, it was determined that the highest magnetization saturation in the films is achieved with a high oxygen partial pressure during growth and subsequent thermal annealing below the deposition temperature.Graphical abstractImage 1
       
  • Phase-field simulation of solid state sintering
    • Abstract: Publication date: Available online 15 October 2018Source: Acta MaterialiaAuthor(s): Johannes Hötzer, Marco Seiz, Michael Kellner, Wolfgang Rheinheimer, Britta Nestler Manufacturing materials for high performance applications with tailored properties requires a deep knowledge about the sintering process and especially the underlying microstructure evolution. Due to the complex interplay of the material and process parameters as well as complex geometries it is challenging to predict the microstructure evolution during sintering with analytical models. A phase-field model based on the grand potential approach considering volume, surface and grain boundary diffusion is presented to describe the microstructural evolution during solid state sintering. To efficiently investigate realistic green bodies with multiple thousand particles in three dimensions, the model is implemented in a highly optimized manner in the massive parallel phase-field solver framework Pace3D. By comparing the neck growth rates and the particle approach in a two particle system for the different diffusion mechanisms a good agreement to analytic solutions is found. Based on a three dimensional green body of 24897 Al2O3-grains the densification is investigated with respect to the dominant diffusion mechanisms and compared with the analytic Coble model. Finally, the appearance of isolated pores in the microstructure is discussed.Graphical abstractImage 1
       
  • Indentation of a plastically deforming metal crystal with a self-affine
           rigid surface: a dislocation dynamics study
    • Abstract: Publication date: Available online 15 October 2018Source: Acta MaterialiaAuthor(s): S.P. Venugopalan, L. Nicola Although indentation of elastic bodies by self-affine rough indenters has been studied extensively, little attention has so far been devoted to plasticity. This is mostly because modeling plasticity as well as contact with a self-affine rough surface is computationally quite challenging. Here, we succeed in achieving this goal by using Green’s function dislocation dynamics, which allows to describe the self-affine rough surface using wavelengths spanning from 5 nm to 100μm. The aim of this work is to gain understanding in how plastic deformation affects the contact area, contact pressure and hardness, gap profile and subsurface stresses, while the roughness of the indenter is changed. Plastic deformation is found to be more pronounced for indenters with larger root-mean-square height and/or Hurst exponent, and to be size dependent. The latter means that it is not possible to scale observables, as typically done in elastic contact problems. Also, at a given indentation depth (interference) the contact area is smaller than for the corresponding elastic contact problem, but gap closure is more pronounced. Contact hardness is found to be much larger than what reported by classical plasticity studies. Primarily, this is caused by limited dislocation availability, for which the stiffness of the deforming crystal is in between that of a linear elastic and an elastic-perfectly plastic material. When calculating hardness and nominal contact pressure, including very small wavelength in the description of the surface is not necessary, because below a given wavelength the subsurface stresses become invariant to a further decrease in true contact area. This is true for both elastic and plastic materials. Considering small wavelengths is instead required to capture accurately roughening and contact stress distribution.Graphical abstractImage 1
       
  • Micromechanical modeling of non-linear stress-strain behavior of
           polycrystalline microcracked materials under tension
    • Abstract: Publication date: Available online 13 October 2018Source: Acta MaterialiaAuthor(s): Giovanni Bruno, Mark Kachanov, Igor Sevostianov, Amit Shyam The stress-strain behavior of microcracked polycrystalline materials (such as ceramics or rocks) under conditions of tensile, displacement-controlled, loading is discussed. Micromechanical explanation and modeling of the basic features, such as non-linearity and hysteresis in stress-strain curves, is developed, with stable microcrack propagation and “roughness” of intergranular cracks playing critical roles. Experiments involving complex loading histories were done on large- and medium grain size β-eucryptite ceramic. The model is shown to reproduce the basic features of the observed stress-strain curves.Graphical abstractThe stress-strain behavior of microcracked polycrystalline materials under conditions of tensile, displacement-controlled, loading is discussed. Micromechanical explanation and modeling of the basic features, such as non-linearity and hysteresis in stress-strain curves, is developed, with “roughness” of intergranular cracks playing critical roles: in forward loading, roughness profiles of crack faces get mismatched when nonlinearity starts due to crack propagation (point 2); at unloading (point 4) the faces get “stuck” (their displacement at peak load C is locked).Image 1
       
  • Point defect structure of La-doped SrTiO3 ceramics with
           colossal permittivity
    • Abstract: Publication date: Available online 13 October 2018Source: Acta MaterialiaAuthor(s): Mengjie Qin, Feng Gao, Jakub Cizek, Shengjie Yang, Xiaoli Fan, Lili Zhao, Jie Xu, Gaogao Dong, Mike Reece, Haixue Yan Sr1-xLaxTiO3 (SLTO) ceramics with colossal permittivity were fabricated by conventional solid-state reaction method. The point defects of pure STO and SLTO ceramics were analyzed by Positron Annihilation Lifetime Spectroscopy (PALS) and Coincidence Doppler Broadening (CDB). The charge compensation mechanisms and dielectric properties of ceramics were investigated. The results indicated that the intrinsic defects in pure STO ceramics were mainly VTi″″. The charge compensation mechanism of SLTO ceramics was predominantly formation of VSr″. With increasing La content, εr of SLTO ceramics increased up to ∼70000 at room temperature. The results of first-principle calculations indicated that the colossal permittivity came from a sharp polarization increase caused by dipole structure of defects. tanδ of SLTO ceramics showed obvious Debye relaxation at high temperatures and the relaxation showed a multiple relaxation times derived from different kinds of polarization mechanism. The main polarization mechanism of SLTO ceramics gradually changed from ion displacement polarization to defect dipole polarization influenced by the concentration of La dopants.Graphical abstractImage 1
       
  • Corrigendum to ‘Probing deformation mechanisms of a FeCoCrNi
           high-entropy alloy at 293 and 77 K using in situ neutron diffraction’
           [Acta Mater. 154C (2018) 79–89]
    • Abstract: Publication date: Available online 12 October 2018Source: Acta MaterialiaAuthor(s): Yiqiang Wang, Bin Liu, Kun Yan, Minshi Wang, Saurabh Kabra, Yu-Lung Chiu, David Dye, Peter D. Lee, Yong Liu, Biao Cai
       
  • Effects of the stacking fault energy fluctuations on the strengthening of
           alloys
    • Abstract: Publication date: Available online 12 October 2018Source: Acta MaterialiaAuthor(s): Yifei Zeng, Xiaorong Cai, Marisol Koslowski In alloys and high entropy alloys the stacking fault energy varies with the local composition. The effects of these energy fluctuations on the strengthening are studied using dislocation dynamics simulations that track the evolution of partial dislocations in FCC metals at zero temperature. Different values of the intrinsic stacking fault energy are assigned to regions with size in the range of 0.5 nm–12 nm in the slip plane, while the other mechanical properties are left constant. A theoretical model is derived and compared to the simulation results. In the model and the simulations the predicted value of the yield stress grows with larger fluctuations of the stacking fault energy. Furthermore, a strong size dependency is observed, with a maximum in the strength attained when the mean region size approaches the average equilibrium stacking fault width. In summary, the strength of high entropy alloys can be improved by introducing disorder in the chemical misfit with a characteristic length scale of the order of the average stacking fault width.Graphical abstractImage 1
       
  • Dislocation evolution at a crack-tip in a hexagonal close packed metal
           under plane-stress conditions
    • Abstract: Publication date: Available online 12 October 2018Source: Acta MaterialiaAuthor(s): Zhouyao Wang, Christopher Cochrane, Travis Skippon, Qingshan Dong, Mark R. Daymond Understanding the stress state and microstructural features at a growing crack-tip is critical to understanding the failure mechanisms of engineering structures. To investigate the strain and dislocation evolution at a crack-tip, electron backscatter diffraction and geometrically necessary dislocation analysis were performed on fully annealed zirconium foils at room temperature. Different levels of macroscopic plastic strain were applied: 0.0%, 0.22%, 0.84%, 1.2%. Based on their different Burgers vectors and line vectors, prismatic , basal , screw , screw and pyramidal geometrically necessary dislocation densities were estimated during crack blunting and subsequent propagation. Most of the plastic deformation was accommodated by screw and pyramidal dislocations. Screw dislocations were found to be dominant over the as might be expected. Instead of twinning, pyramidal slip accommodated the strain along the c-axis caused by contraction at the crack-tip. Dislocation densities at the crack-tip were plotted according to the angle relative to the applied tension direction and the distance from the tip, and were compared with plastic strains simulated from a 3D static finite element model. Crack-tip singularity was observed and total geometrically necessary dislocation densities were in qualitatively good agreement with the equivalent plastic strain distribution predicted by the finite element method (FEM).Graphical abstractImage 1
       
  • Vapor phase dealloying: a versatile approach for fabricating 3D porous
           materials
    • Abstract: Publication date: Available online 11 October 2018Source: Acta MaterialiaAuthor(s): Jiuhui Han, Cheng Li, Zhen Lu, Hao Wang, Zhili Wang, Kentaro Watanabe, Mingwei Chen Three-dimensional porous materials with bicontinuous open porosity represent a new class of functional materials for various applications. Top-down dealloying has been demonstrated to be one of the most effective ways to fabricate 3D porous materials. Vapor phase dealloying, which makes use of the saturated vapor pressure difference between the constituent components in an alloy for selectively removing a less stable element or phase, is a promising versatile method for fabricating porous materials from active metals to inorganic elements. Here, using nickel-zinc and germanium-zinc alloys as the prototypes of single-phase and two-phase precursors, respectively, we report the fabrication of 3D bicontinuous porous Ni and Ge by vapor phase dealloying on the basis of selective element or selective phase evaporations. We also show the incorporation of vapor phase dealloying with chemical vapor deposition for the one-pot growth of 3D nanoporous graphene and the functional applications of vapor phase dealloyed porous Ge as Li ion battery electrodes. This study shines lights on the versatility of vapor phase dealloying for the fabrication of bicontinuous porous materials for a wide range of functional applications.Graphical abstractImage 1
       
  • Application of Onsager's variational principle to the dynamics of a solid
           toroidal island on a substrate
    • Abstract: Publication date: Available online 11 October 2018Source: Acta MaterialiaAuthor(s): Wei Jiang, Quan Zhao, Tiezheng Qian, David J. Srolovitz, Weizhu Bao In this paper, we consider the capillarity-driven evolution of a solid toroidal island on a flat rigid substrate, where mass transport is controlled by surface diffusion. This problem is representative of the geometrical complexity associated with the solid-state dewetting of thin films on substrates. We apply Onsager's variational principle to develop a general approach for describing surface diffusion-controlled problems. Based on this approach, we derive a simple, reduced-order model and obtain an analytical expression for the rate of island shrinking and validate this prediction by numerical simulations based on a full, sharp-interface model. We find that the rate of island shrinking is proportional to the material constants B and the surface energy density γ0, and is inversely proportional to the island volume V0. This approach represents a general tool for modeling interface diffusion-controlled morphology evolution.Graphical abstractImage 1
       
  • Hierarchical microstructure design to tune the mechanical behavior of an
           interstitial TRIP-TWIP high-entropy alloy
    • Abstract: Publication date: Available online 11 October 2018Source: Acta MaterialiaAuthor(s): Jing Su, Dierk Raabe, Zhiming Li We demonstrate a novel approach of utilizing a hierarchical microstructure design to improve the mechanical properties of an interstitial carbon doped high-entropy alloy (HEA) by cold rolling and subsequent tempering and annealing. Bimodal microstructures were produced in the tempered specimens consisting of nano-grains (∼50 nm) in the vicinity of shear bands and recovered parent grains (10-35 μm) with pre-existing nano-twins. Upon annealing, partial recrystallization led to trimodal microstructures characterized by small recrystallized grains (
       
  • The influence of stacking fault energy on plasticity mechanisms in
           triode-plasma nitrided austenitic stainless steels: implications for the
           structure and stability of nitrogen-expanded austenite
    • Abstract: Publication date: Available online 11 October 2018Source: Acta MaterialiaAuthor(s): Xiao Tao, Xingguang Liu, Allan Matthews, Adrian Leyland Austenitic stainless steels (ASSs), especially AISI type 304 and 316 ASSs, have been extensively studied after thermochemical diffusion treatments (e.g. nitriding, carburising) to resolve the anomalous lattice expansion after supersaturation of interstitial elements under paraequilibium conditions. The known issues are i) plastic deformation of surfaces under nitrogen-introduced strain at low treatment temperatures and ii) degradation in surface corrosion performance in association with chromium nitride formation at elevated treatment temperatures (and/or longer treatment times). In this study, a nitrogen-containing high-manganese ASS and a high-nickel ASS (i.e. Fe-17Cr-20Mn-0.5N and Fe-19Cr-35Ni, in wt.%) were triode-plasma nitrided under a high nitrogen gas volume fraction and low (and close to monoenergetic) ion energy of ∼200 eV at 400°C, 425°C and 450°C for 4hrs and 20hrs, respectively. Auxiliary radiant heating was used to facilitate different treatment temperatures at a deliberately controlled and constant substrate current density of ∼0.13 mA/cm2, under which material surface crystallographic structure was mainly influenced by the different treatment temperatures and times applied during nitriding. With respect to stacking fault energy (SFE), we illustrate and discuss i) the analogy of composition-induced plastic deformation phenomena to mechanical deformation processes, ii) two possible types of dislocation-mediated plasticity mechanism in γN, iii) two possible types of diffusional decomposition mechanism for γN, and iv) the lattice structures formed at low to moderate nitriding temperatures.Graphical abstractImage 1
       
  • Characterization of (Ti,Mo,Cr)C nanoprecipitates in an austenitic
           stainless steel on the atomic scale
    • Abstract: Publication date: Available online 11 October 2018Source: Acta MaterialiaAuthor(s): N. Cautaerts, R. Delville, E. Stergar, D. Schryvers, M. Verwerft Nanometer sized (Ti,Mo,Cr)C (MX-type) precipitates that grew in a 24% cold worked Ti-stabilized austenitic stainless steel (grade DIN 1.4970, member of the 15-15Ti alloys) after heat treatment were fully characterized with transmission electron microscopy (TEM), probe corrected high angle annular dark field scanning transmission electron microscopy (HR-HAADF STEM), and atom probe tomography (APT). The precipitates shared the cube-on-cube orientation with the matrix and were facetted on {111} planes, yielding octahedral and elongated octahedral shapes. The misfit dislocations were believed to have burgers vectors a/6 which was verified by geometrical phase analysis (GPA) strain mapping of a matrix-precipitate interface. The dislocations were spaced five to seven atomic planes apart, on average slightly wider than expected for the lattice parameters of steel and TiC. Quantitative atom probe tomography analysis of the precipitates showed that precipitates were significantly enriched in Mo, Cr and V, and that they were hypostoichiometric with respect to C. These findings were consistent with a reduced lattice parameter. The precipitates were found primarily on Shockley partial dislocations originating from the original perfect dislocation network. These novel findings could contribute to the understanding of how TiC nanoprecipitates interact with point defects and matrix dislocations. This is essential for the application of these Ti-stabilized steels in high temperature environments or nuclear fast reactors.Graphical abstractImage 1
       
  • Investigation of Interactions between Defect Clusters in Stainless Steels
           by In Situ Irradiation at Elevated Temperatures
    • Abstract: Publication date: Available online 10 October 2018Source: Acta MaterialiaAuthor(s): Dongyue Chen, Kenta Murakami, Hiroaki Abe, Zhengcao Li, Naoto Sekimura In recent years, modeling studies on the interactions between defect clusters have been extensively conducted to predict the behavior of stainless steels in reactors. However, comparable experimental results are desired to validate existing results and provide guidelines for future modeling. The size of defect clusters is expected to be a key factor influencing cluster interactions. Thus, in this work, the effects of growing clusters on their surrounding microstructure were quantitatively examined by in situ transmission electron microscopy during ion irradiation. To this end, a high-purity 316L stainless steel model alloy was irradiated by 2 MeV Fe2+ to 0.2 dpa at 400°C and 300°C separately. At 300°C, the number density of defect clusters monotonically increased with increasing fluence, whereas at 400°C, the number density decreased almost immediately after the rapid nucleation regime. This decrease could be explained by the distinct growth of some clusters, which suppressed the nucleation of clusters around them as well as the lifetime of neighboring tiny clusters. Large interstitial-type clusters approximately 6–11 nm in size could be absorbed by neighboring interstitial-type clusters of similar sizes via interstitial crowdion diffusion. The absorption would not occur until both clusters grew large enough to permit a diffusion path between them.Graphical abstractImage 1
       
  • Morphological instability of iron-rich precipitates in Cu-Fe-Co alloys
    • Abstract: Publication date: Available online 10 October 2018Source: Acta MaterialiaAuthor(s): K.X. Chen, P.A. Korzhavyi, G. Demange, H. Zapolsky, R. Patte, J. Boisse, Z.D. Wang The mechanical properties of metallic materials are determined by their microstructure, and in particular, the different morphologies of precipitates lead to distinct strengthening effects. Usually, the shape of precipitates changes during growth and coarsening regimes, leading to modification of the macroscopic properties of the materials. Thus, understanding of this phenomenon is key to tailoring the precipitate strengthening of industrial alloys. In this article, a general approach to explain the shape instability of iron-rich nanoparticles in Cu-Fe-Co alloys during casting and ageing processes is proposed. The evolution of particle shape from sphere to cuboid to petal and finally splitting into eight sub-nanoparticles is observed using transmission electron microscopy. Phase-field modelling and thermodynamic calculations are combined into a general model that describes and elucidates the morphological evolution of precipitates in alloys in terms of particle size, interfacial and elastic strain energy, and chemical driving force. These findings have the potential to promote new microstructural design approaches for a wide range of materials.Graphical abstractImage 1
       
  • Differentiation of γ′- and γ″- precipitates in Inconel 718 by a
           complementary study with small-angle neutron scattering and analytical
           microscopy
    • Abstract: Publication date: Available online 10 October 2018Source: Acta MaterialiaAuthor(s): R. Lawitzki, S. Hassan, L. Karge, J. Wagner, D. Wang, J. von Kobylinski, C. Krempaszky, M. Hofmann, R. Gilles, G. Schmitz We present an experimental method to distinguish and quantify the two strengthening phases γ′ and γ″ in the nickel-based superalloy Inconel 718. So far, this was only achieved by techniques that evaluated sample volumes in the nano-to micrometer range. In this study, reliable volume fractions of the precipitates were obtained and calculated from ex-situ small-angle neutron scattering (SANS) on differently heat treated specimens. For interpretation of the SANS curves, a structural model was set up, using complementary information from measurements by transmission electron microscopy (TEM) and atom probe tomography (APT). Phase identification, as well as size distributions, morphologies and crystallographic information of the precipitates were obtained by TEM. APT provided compositional information, which is necessary to calculate the scattering contrast of each phase. As a benefit of using bulk neutron diffraction for quantification, volumes of a few tens of cubic millimeters are analyzed and thus, significantly better statistics are obtained. The measured γ″ volume fractions are remarkably lower than stated in previous works, but now well fulfill the chemical mass balance.Graphical abstractImage 1
       
  • Strengthening mechanisms acting in extruded Mg-based long-period stacking
           ordered (LPSO)-phase alloys
    • Abstract: Publication date: Available online 10 October 2018Source: Acta MaterialiaAuthor(s): Koji Hagihara, Zixuan Li, Michiaki Yamasaki, Yoshihito Kawamura, Takayoshi Nakano The unusual increase in the strength by extrusion is a unique feature of recently developed Mg alloys containing the LPSO phase. In this study, we first elucidated the detailed mechanisms that induce this drastic strengthening. The dependencies of the deformation behavior of a Mg88Zn4Y7 extruded alloy, which contains ∼86-vol% LPSO phase, on the temperature, loading orientation, and extrusion ratio were examined. It was found that the yield stress of the alloy is drastically increased by extrusion, but the magnitude of the increase in the yield stress is significantly different depending on the loading orientation. That is, the strengthening of the LPSO phase by extrusion shows a strong anisotropy. By the detailed analyses, this was clarified to be derived from the variation in the deformation mechanisms depending on the loading orientation and extrusion ratio. Basal slip was found to govern the deformation behavior when the loading axis was inclined at a 45° to the extrusion direction, while the predominant deformation mechanism varies from basal slip to the formation of deformation kink bands as the extrusion ratio increased when the loading axis was parallel to the extrusion direction. Moreover, it was clarified in this study that the introduction of a deformation-kink-band boundary during extrusion effectively acts as a strong obstacle to basal slip. That is, "the kink band strengthening" was first quantitatively elucidated, which contributes to the drastic increase in the yield stress of the extruded LPSO-phase alloys in the wide temperature range below 400 ºC.Graphical abstractImage 1
       
  • Determination of the structure and properties of an edge dislocation in
           rutile TiO2
    • Abstract: Publication date: Available online 9 October 2018Source: Acta MaterialiaAuthor(s): Emile Maras, Mitsuhiro Saito, Kazutoshi Inoue, Hannes Jónsson, Yuichi Ikuhara, Keith P. McKenna A global optimization procedure is used to predict the structure and electronic properties of the b=c[001] edge dislocation in rutile TiO2. Over 1,000 different atomic configurations have been generated using both semi-empirical and density functional theory estimates of the energy of the system to identify the most stable structure. Both stoichiometric and oxygen deficient dislocation core structures are predicted to be stable depending on the oxygen chemical potential. The latter is associated with Ti3+ species in the dislocation core. The dislocation is predicted to act as a trap for electrons but not for holes suggesting they are not strong recombination centers. The predicted structures and properties are found to be consistent with experimental results obtained using scanning transmission electron microscopy and electron energy loss spectroscopy on samples produced using the bicrystal approach.Graphical abstractImage 1
       
  • Quantification of precipitate hardening of twin nucleation and growth in
           Mg and Mg-5Zn using micro-pillar compression
    • Abstract: Publication date: Available online 9 October 2018Source: Acta MaterialiaAuthor(s): Jiangting Wang, Mahendra Ramajayam, Eric Charrault, Nicole Stanford In polycrystalline materials, the stress corresponding to twin nucleation is difficult to separate from twin growth because these events occur concurrently during deformation. In this work, we separate the nucleation stress and growth stress of {101¯2} twinning by compression of micro-pillars containing pre-existing twins through their centre. Micro-pillar compression results showed a strong size effect on both twin nucleation and twin growth stresses for pure Mg and Mg-5Zn alloys. Taking this into account, the critical stress for growth of twins in pure magnesium is found to be ∼7 MPa which is consistent with previously published measurements on macroscopic single crystals. These experiments have been used to deduce the precipitate hardening of twin growth, and for the present precipitate dispersion this has been measured to be ∼30 MPa. Back-stress calculations based on elastic bending of the precipitates showed close agreement to the measured precipitate hardening, and this model therefore accounts well for the observed strengthening. Site-specific atom probe tomography of the twin boundaries showed that room temperature ageing is sufficient to produce segregation of zinc to the twin boundary. This was found to immobilize the twin, and is believed to be the first report of solute locking of twins from room temperature exposure.Graphical abstractImage 1
       
  • Semi-solid deformation of Al-Cu alloys: a quantitative comparison between
           real-time imaging and coupled LBM-DEM simulations
    • Abstract: Publication date: Available online 9 October 2018Source: Acta MaterialiaAuthor(s): T.C. Su, C. O’Sullivan, T. Nagira, H. Yasuda, C.M. Gourlay Semi-solid alloys are deformed in a wide range of casting processes; an improved understanding and modelling capability is required to minimise defect formation and optimise productivity. Here we combine thin-sample in-situ X-ray radiography of semisolid Al-Cu alloy deformation at 40 – 70% solid with 2D coupled lattice Boltzmann method - discrete element method (LBM-DEM) simulations. The simulations quantitatively capture the key features of the in-situ experiments, including (i) the local contraction and dilation of the grain assembly during shear deformation; (ii) the heterogeneous strain fields and localisation features; (iii) increases in local liquid pressure in regions where liquid was expelled from the free surface in the experiment; and (iv) decreases in liquid pressure in regions where surface menisci are sucked-in in experiments. The verified DEM simulations provide new insights into the role of initial solid fraction on the stress-deformation response and support the hypothesis that the behaviour of semi-solid alloys can be described using critical state soil mechanics.Graphical abstractImage 1
       
  • Morphological Control and Kinetics in Three Dimensions for Hierarchical
           Nanostructures Growth by Screw Dislocations
    • Abstract: Publication date: Available online 8 October 2018Source: Acta MaterialiaAuthor(s): Yanhui Chu, Siyi Jing, Da Liu, Jinchao Liu, Yunlong Zhao The precise control and in-depth understanding of the anisotropic crystal screw dislocation growth could yield further optimization of nanomaterial design and broader applications, yet the studies of rational control and kinetics for more complex nanostructures are still insufficient. In this work, by programming synthesis conditions, we achieve a controllable three-dimensional (3D) screw dislocation growth of hierarchical nanostructures, including nanowires, nanoplates, and previously unreported hierarchical hollow nanobelts, via a facile chemical vapor deposition approach. Notably, the screw dislocation growth in nanobelts is confirmed by the clear observations of the stepwise spiral terraces with initial hexagonal to octagonal shapes and the hollow cores in the growth spiral centers, as well as the fundamental Burton-Cabrera-Frank crystal growth theoretical calculations. The formation of the nanowires and nanoplates can be well interpreted by the previously reported screw dislocation growth model, while a new 3D screw dislocation growth model with considering of transition in growth velocities and directions is proposed to interpret the formation of the nanobelts and other potential complex nanostructures. This study not only enriches our understanding of the screw dislocation growth kinetics but also guides us to achieve the precise morphological design and control in nanosynthesis.Graphical abstractBy programming synthesis conditions, we achieve a controllable 3D screw dislocation growth of previously unreported hierarchical hollow nanobelts via a facile CVD approach. More importantly, we propose a new 3D screw dislocation growth model with considering of transition in growth velocities and directions to interpret the formation of the nanobelts.Image 1
       
  • Influence of metal/semiconductor interface on attainable piezoelectric and
           energy harvesting properties of ZnO
    • Abstract: Publication date: Available online 6 October 2018Source: Acta MaterialiaAuthor(s): Nikola Novak, Peter Keil, Till Frömling, Florian H. Schader, Alexander Martin, Kyle G. Webber, Jürgen Rödel The piezoelectric coefficient is a measure to quantify the potential use of a material in energy harvesting and sensor applications. High concentration of free charge carriers in piezoelectric materials can significantly impede the use of generated piezoelectric charge. However, in piezoelectric semiconductors such as ZnO, high conductivity drastically reduces the attainable piezoelectric coefficient and consequently the harvesting performance. Typically, acceptor doping and a decrease in operation temperature are employed to reduce the conductivity and retain the piezoelectric properties of the material. In piezotronics, however, the creation of a resistive space charge layer at the metal-semiconductor interface (Schottky contact) retains piezoelectric properties in spite of a highly conductive bulk material. Nevertheless, the piezoelectric coefficient has never been determined using a Schottky contact. Thus, the energy harvesting properties of a single contact have so far not been quantified. To this end In this study, undoped semiconducting ZnO single crystals with both Ohmic and Schottky contacts were prepared to quantify the effective piezoelectric response at temperatures from 20020 °C to -140 °C and frequencies of mechanical loading from 0.5 Hz to 160 Hz. It was demonstrated that the formation of an electrostatic potential barrier at the metal-semiconductor interface increases the overall resistance, which provides access to unbiased piezoelectric coefficients of ZnO single crystals even at room temperature. These findings were verified using semiconducting ZnO for energy harvesting at room temperature and relatively low loading frequency.Graphical abstractImage 1
       
  • On the heterogeneous nature of deformation in a strain-transformable beta
           metastable Ti-V-Cr-Al alloy
    • Abstract: Publication date: Available online 6 October 2018Source: Acta MaterialiaAuthor(s): L. Lilensten, Y. Danard, C. Brozek, S. Mantri, P. Castany, T. Gloriant, P. Vermaut, F. Sun, R. Banerjee, F. Prima Ti-10V-4Cr-1Al wt% (TVCA) is a new grade of titanium alloy, developed to combine twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) effects. The TVCA alloy exhibits a very high strain-hardening rate and an excellent balance between strength and ductility for great potential in aerospace applications. Deformation mechanisms are investigated using in-situ techniques as synchrotron X-ray diffraction (SXRD) and in-situ electron backscatter diffraction (EBSD) analysis during tensile strain, as well as transmission electron microscopy (TEM). The results reveal that permanent {332} mechanical twinning and an unstable orthorhombic α” martensite are the major deformation products. This study aims at unveiling the interaction and co-deformation of the various deformation features, that lead to the outstanding mechanical properties of the alloy. The very high strain hardening rate could be explained by the simultaneous activation of two different deformation modes, the primary TRIP mode on one side, and the hybrid TWIP and secondary TRIP mode on the other one, in different grains, resulting in in-grain dynamic hardening (Hall-Petch)/softening (α” martensite) effects and meso-scale dynamic mechanical contrast. Selection of the deformation mechanism – TRIP or TWIP –, which seems to be inhomogeneous, at both the inter- and the intra-granular level, is investigated.Graphical abstractImage 1
       
  • The formation mechanism of a novel interfacial phase with high thermal
           stability in a Mg-Gd-Y-Ag-Zr alloy
    • Abstract: Publication date: Available online 6 October 2018Source: Acta MaterialiaAuthor(s): L.R. Xiao, Y. Cao, S. Li, H. Zhou, X.L. Ma, L. Mao, X.C. Sha, Q.D. Wang, Y.T. Zhu, X.D. Han Due to their unique precipitation behavior, magnesium-rare earth (Mg-RE) alloys exhibit excellent mechanical properties and decent thermal stability. In this work, a Mg-Gd-Y-Ag-Zr alloy was employed to investigate the segregation and interfacial phase formation at grain boundaries after plastic deformation and heat treatment. The interfacial phase was unequivocally investigated by aberration-corrected high-angle annular dark-filed scanning transmission electron microscopy (HAADF-STEM) from three different crystal directions and modeling, which reveals a hitherto unknown crystal structure (monoclinic: β = 139.1°, a = 1.20 nm, b = 1.04 nm and c = 1.59 nm). Its orientation relationship with the Mg matrix is: [101]//[110]α, [302]//[100]α and (010)//(0001)α. Different from the precipitates in matrix, the size of the interfacial phase was not sensitive to annealing temperature between 250 °C and 400 °C. Transformation of twin boundaries to coaxial grain boundaries via multiple twinning led to the generation of many high strain sites along the boundaries, which promoted the formation of the interfacial phase. The interfacial phase was stable up to 400 °C, which was about 100 °C higher than the dissolution temperature of β′ and γ" precipitates.Graphical abstractImage 1
       
  • Low-Temperature-Solderable Intermetallic Nanoparticles for 3D Printable
           Flexible Electronics
    • Abstract: Publication date: Available online 4 October 2018Source: Acta MaterialiaAuthor(s): Ying Zhong, Rong An, Huiwen Ma, Chunqing Wang Functional materials for flexible and wearable smart devices have attracted much attention in recent years. This paper describes structure and properties of uniquely prepared, functional interconnectable nanoparticles (NPs) of Cu6Sn5 intermetallic compound that can allow 3D flexible packaging and nano-circuits. In situ TEM analysis confirms that size-controllable Cu6Sn5 NPs as small as ∼6.40 nm can be made sinterable at the start temperature as low as ∼130 °C, which is much lower than its bulk melting point (MP) of 415 oC. After sintering, its high MP provides mechanical and thermal stability. Based on the in situ TEM observation and calculation, particle size and distribution affects the sintering process. More interestingly, the relative orientations of adjacent particles also play an important role. A new orientation related sintering mechanism noted as orientation unification (OU) is revealed as two adjacent particles exhibit orientation change to slowly match their orientation with each other during the heating process. The interesting interaction between nano-Cu6Sn5 and micro-Cu substrate during in situ TEM heating gives first hand atomic level proof of the formation of Cu3Sn. The nano-Cu6Sn5 joints possess high enough bonding strength and great high temperature working capability. This intermetallic nano-soldering approach can pioneer a novel strategy of circuit connection, by providing high working temperature interconnection materials for 3D flexible packaging and ultra-high-density micro/nano interconnections.Graphical abstractImage 1
       
  • An Investigation of the Microstructure and Ductility of Annealed
           Cold-Rolled Tungsten
    • Abstract: Publication date: Available online 4 October 2018Source: Acta MaterialiaAuthor(s): Chai Ren, Z. Zak Fang, Lei Xu, Jonathan P. Ligda, James D. Paramore, Brady G. Butler Tungsten is notoriously brittle metal at room temperature. Furthermore, contrary to most metals, plastic deformation increases ductility and recrystallization decreases ductility of tungsten. The fundamentals that govern this behavior have challenged academia and industry for decades. This paper focuses on understanding the controlling factors of ductility through a systematic investigation of the changes in microstructure and mechanical properties of cold-rolled tungsten that occur during annealing. Cold-rolled tungsten samples were annealed at temperatures up to 1400 °C, and mechanical testing and microstructural analysis was performed before and after annealing. Furthermore, a dislocation mobility model based on the Orowan equation was applied. The mechanisms of deformation are discussed within the context of deformed and annealed microstructures. The high fraction of low angle grain boundaries and high density of edge dislocations were found to be the most important factors for ductility. Although there were gradual changes in microstructure and mechanical properties, the ductility of cold-rolled tungsten was maintained up to 1300 °C. The material recrystallized when annealed above this temperature, had no ductility, and suffered brittle fracture. Microstructural characterizations of the as-rolled material revealed a typical BCC texture, with grains elongated in rolling direction and a large amount of edge dislocations and low angle grain boundaries. The level of texturing and the fraction of low angle grain boundaries diminished after recrystallization. It was found that, compared to the recrystallized material, as-rolled tungsten can accommodate over 7 orders of magnitude higher deformation velocity due to the high density of edge dislocations.Graphical abstractImage 1
       
  • Elucidation of Cold-Spray Deposition Mechanism by Auger Electron
           Spectroscopic Evaluation of Bonding Interface Oxide Film
    • Abstract: Publication date: Available online 4 October 2018Source: Acta MaterialiaAuthor(s): Yuji Ichikawa, Ryotaro Tokoro, Masatoshi Tanno, Kazuhiro Ogawa The relationship between the cold spray deposition mechanism, microstructure, and strength of the resulting film must be understood for this innovative process to be practical. Previous studies have suggested that the coating mechanism is reliant on breaking the natural oxide film such that metallic bonding occurs through direct contact between the metal surfaces. In this study, the proposed model was experimentally verified by a small tensile adhesion test and auger electron spectroscopy analysis of the bonding interface. Since shear deformation does not occur at the tip (south pole) of the incoming particle, the oxide film is not broken, such that the bonding strength is weak. In contrast, at the outer edge of the particle, metallic bonding occurs, attaining a level of strength that exceeds that of the base material due to the huge plastic deformation. This phenomenon is known as the “south-pole problem,” and can lead to a decrease in the overall adhesion strength despite the local adhesion being strong. However, detailed observations revealed, in parts of the deposits, particles that had adhered across their entire surface. This suggests that, provided the collision state can be controlled, it is possible to overcome the south-pole problem and improve the adhesion strength.Graphical abstractImage 1
       
  • Architected Cellular Piezoelectric Metamaterials:
           Thermo-Electro-Mechanical Properties
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): J. Shi, A.H. Akbarzadeh Advances in additive manufacturing have recently made possible the manufacturing of smart materials with arbitrary microarchitectures, which leads to developing lightweight smart metamaterials with unprecedented multifunctional properties. In this paper, asymptotic homogenization (AH) method is developed for predicting the effective thermo-electro-mechanical properties of architected cellular piezoelectric metamaterials. The effect of pore microarchitecture (relative density and cell topology) and polarization direction on elastic, dielectric, piezoelectric, pyroelectric and thermal properties of periodic cellular piezoelectric metamaterials is explored. The pore topology is determined by Fourier series expansion. Alternative pore microarchitectures are considered by tailoring shape parameters, scaling factor, and rotation angle of the constitutive pore. Smart cellular metamaterials made of both single-phase (BaTiO3) and bi-phase (BaTiO3-expoy) piezoelectric materials are considered. Apart from effective thermo-electro-mechanical properties, a series of figures of merit for the cellular piezoelectric metamaterials, i.e. piezoelectric coupling constant, acoustic impedance, piezoelectric charge coefficient, hydrostatic figure of merit, current responsivity, voltage responsivity and pyroelectric harvesting figures of merit, are presented and the reason for difference between the figures of merit of different types of piezoelectric metamaterials is discussed. The figures of merit shed lights on the effect of microarchitecture on optimizing the multifunctional performance of smart cellular metamaterials for applications as structurally efficient multifunctional energy harvesters. It is found that the piezoelectric and pyroelectric figures of merit of cellular piezoelectric metamaterials can be significantly improved compared to the commonly used honeycomb cellular materials and composite materials with solid circular inclusion if an appropriate microarchitecture is selected for the pore. For example, piezoelectric charge coefficient (dh) for a transversely polarized single-phase cellular piezoelectric metamaterial with a solid volume fraction of 0.4 can be 350% higher than the corresponding figures of merit of honeycomb piezoelectric material; voltage responsivity of transversely polarized bi-phase cellular piezoelectric metamaterials with an inclusion volume fraction of 0.3 can be also 249% higher than the corresponding value of composite materials with a solid circular inclusion.Graphical abstractImage 1
       
  • hcp → ω phase transition mechanisms in shocked zirconium: A machine
           learning based atomic simulation study
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): Hongxiang Zong, Yufei Luo, Xiangdong Ding, Turab Lookman, Graeme J. Ackland There has been much controversy over the behavior of zirconium under shock strong enough to cause the pressure-induced hcp → ω phase transformation. Due to the short time- and length scales involved, direct measurements of the microstructure are extremely challenging. We have performed molecular dynamics simulations to investigate this issue, with Zr described by a machine-learned interatomic potential. Two different orientation relationships (ORs) between the hcp and ω phases are observed under shock driven conditions. Unlike the case with Ti that is in the same group, the ORs between the hcp and ω phases show less anisotropic phase transition sensitivity and in most cases follow the Silcock relationship with(0001)α (12¯10)ω. Furthermore, we find that the α→ ω transformation in shocked Zr occurs via an intermediate metastable bcc structure during the loading process, whereas no such intermediate is found during the reverse ω→α transition when the shock releases.Graphical abstractImage 1
       
  • Plastic Flow Resistance of NiTiCu Shape Memory Alloy-Theory and
           Experiments
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): S. Alkan, H. Sehitoglu The NiTiCu alloys belong to a class of materials with excellent shape memory properties. The limitations in shape memory properties arise due to onset of slip at interfaces and also in austenite domains. As slip mediated plasticity is a source of irreversibility, it is imperative to understand the glide resistance of austenite which can be rather complex. In this paper, we develop a model to precisely derive the CRSS for slip substantiated with the uniaxial loading experiments on single crystals in a wide range of orientations employing Digital Image Correlation. We illustrate the core spreading of {011} dislocations and evaluate the role of non-Schmid stress components which introduces profound anisotropy in CRSS levels. The model matches the experimental findings in single crystals with excellent agreement. The theory and experiments show significant crystal orientation dependence of plasticity which must be taken into account when designing with these alloys.Graphical abstractImage 1
       
  • Graphene-Size-Tuned Mechanical Serration Behaviors in Nanocarbons
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): Bo Li, Yanli Nan, Xiang Zhao, Peng Zhang, Xiaolong Song Two vastly different types of load-displacement responses observed in graphitic nanostructures under nano-compression are compared in terms of serration behaviors. Different from commonly encountered linear/nonlinear elastic deformation, a periodic serration behavior related to plastic flow is observed in amorphous carbon nanospheres. The true stress-strain relation exhibits a sole feature of type C serration, and comprehensive statistical, dynamical and fractal analyses further demonstrate a chaotic characteristic of dynamics for those serration events. When entering a quasi-steady flow stage, the elastic stress in each serration event could maintain a relatively stable level near ∼135 MPa, very close to the interlayer shear stress (ISS) of single crystalline graphite (∼140 MPa). This finding indicates the dependence of shear deformation on weak van der Waals interaction (elastic constant C44), instead of other structural factors associated with high elastic constants of graphite cells. Based on the experimental results, a microscale ISS-driven shearing mechanism is proposed. The local flexibility induced by small graphene lamellas may facilitate interfacial slip between neighboring domains with commensurate contact. Such slip mode may be responsible for the mechanical serration phenomenon in graphitic materials.Graphical abstractImage 1
       
  • Atomistic insight into the dislocation nucleation at
           crystalline/crystalline and crystalline/amorphous interfaces without full
           symmetry
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): Y.Y. Xiao, X.F. Kong, B.N. Yao, D. Legut, T.C. Germann, R.F. Zhang Misfit dislocations at bimetal interfaces play a decisive role in determining various deformation behaviors by carrying the shear sliding, serving as a barrier for dislocation transmission and a source of dislocation nucleation. However, when the interface does not possess the distinct feature of misfit dislocations, the nucleation mechanism of lattice dislocations at the interfaces cannot be simply quantified by previously developed atomistic mechanisms based on characteristic misfit dislocations. Using crystalline/crystalline interfaces with a large lattice mismatch and crystalline/amorphous interfaces without local symmetry as prototypes, we show for the first time that the dislocation nucleation at such interfaces is attributable to the localized strain heterogeneities by modifying the volumetric and shear strain components at the atomic level to mechanically respond to different loadings. Using atomic strain tensor analysis, we found that in-plane localized shearing plays a critical role in the emission of lattice dislocations from interfaces, while the corresponding normal components of the volumetric strain tensor will dominate the character of the nucleated lattice dislocation by modifying the atomic excess volume at the interface to overcome the barrier to dislocation nucleation. Further exploration of various crystalline/amorphous interfaces by varying the chemical composition of the amorphous side indicates that chemical heterogeneity may substantially change the strain heterogeneity by forming a different clustered structure at the interface, resulting in the preferred choice of nucleation sites at the boundary regions that can be defined as nano shear traces (NSTs). These results provide a foundation to investigate the effects of strain and chemical heterogeneities in order to provide a realistic explanation of interface mediated deformation mechanisms and an efficient solution to tune interface dominated plasticity.Graphical abstractImage 1
       
  • Helium induced microstructure damage, nano-scale grain formation and
           helium retention behaviour of ZrC
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): Shradha Agarwal, Arunodaya Bhattacharya, Patrick Trocellier, Steven J. Zinkle Transition-metal ultra-high temperature ceramics are promising materials for nuclear structural applications. However, an understanding of their response to high-temperature irradiation and helium is vastly limited. This paper presents a study of helium effects in zirconium carbide (ZrC) by performing room temperature 3 MeV 3He+ ion irradiations up to 5x1020 ions.m-2 and high-temperature annealing (1273–1873 K), coupled with state-of-art characterization using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and nuclear reaction analysis (NRA). We reveal that ZrC is susceptible to irradiation damage in terms of helium bubble formation. After annealing at 1373 K, tiny bubbles (1-2 nm) formed aligned clusters which were highly over-pressurized, producing strain contrast in TEM. At 1773 K, significant bubble growth occurred. Additionally, at 1773 K, a combined TEM/SEM analysis revealed dramatic matrix damage due to helium-induced surface blistering. Underneath blister caps, the microstructure evolved into ultra-fine nano-scale grains, similar to high burn-up structures observed in nuclear fuels, but consisting of numerous nano-cracks. We hypothesize that such structures are formed due to high gas pressure build-up and its subsequent release. This phenomenon initiated at the grain boundaries. Blister top surface consisted of inter-granular and trans-granular cracks. NRA depth profiling revealed that helium was present as double peaks with major portion lying at the end-of-the-range (EOR) and the rest as TEM invisible clusters in a shoulder extending to the surface. ZrC started releasing helium after 1373 K. Helium release increased significantly at higher temperatures, with majority helium loss occurring from EOR, rather than from near-surface regions.Graphical abstractImage 1
       
  • Globularization using Heat Treatment in Additively Manufactured Ti-6Al-4V
           for High Strength and Toughness
    • Abstract: Publication date: Available online 3 October 2018Source: Acta MaterialiaAuthor(s): Rushikesh Sabban, Sumit Bahl, Kaushik Chatterjee, Satyam Suwas A bimodal globularized microstructure in contrast to martensitic laths is known to impart high strength and toughness in Ti-6Al-4V. Heat treatment for the phase transformation of the laths to the globularized microstructure must be preceded by plastic deformation. This work reports an innovative strategy to obtain the bimodal microstructure consisting of globular α in additively manufactured Ti-6Al-4V alloy by heat treatment alone. The heat treatment schedule involves repeated thermal cycling close to but below the β transus temperature to form globular α eliminating the need for plastic deformation prior to heat treatment. A new mechanism of globularization other than known in literature is proposed to explain the formation of globular α. The inherent dislocation sub-structure of the martensitic laths initiates globularization by thermal grooving and boundary splitting but is unable to completely globularize the microstructure. Mechanisms such as cylinderization and edge spheroidization also do not lead to globularization. The purposefully designed thermal cycling causes oscillations in the volume fractions of α and β phases that in synergism with the slow cooling segments of the cycle globularize the α phase by epitaxial growth. The bimodal microstructure thus produced led to a significant improvement in the ductility by 80% and the toughness by 66 %, which are desirable for structural applications. Furthermore, beneficial compressive stresses were generated in the alloy because of cyclic heat treatment. It is envisaged that the exceptional combination of mechanical properties observed here will lead to the fabrication of SLM printed Ti-6Al-4V parts that could leverage the advantages of additive manufacturing with material properties that are comparable to those obtained by conventional fabrication routes.Graphical abstractImage 1
       
  • Deformation of lamellar γ-TiAl below the general yield stress
    • Abstract: Publication date: Available online 2 October 2018Source: Acta MaterialiaAuthor(s): Thomas Edward James Edwards, Fabio Di Gioacchino, Amy Jane Goodfellow, Gaurav Mohanty, Juri Wehrs, Johann Michler, William John Clegg The occurrence of plasticity below the macroscopic yield stress during tensile monotonic loading of nearly lamellar Ti-45Al-2Nb-2Mn(at%)-0.8vol% TiB2 at both 25 °C and 700 °C, and in two conditions of lamellar thickness, was measured by digital image correlation strain mapping of a remodelled Au surface speckle pattern. Such initial plasticity, not necessarily related to the presence of common stress concentrators such as hard particles or cracks, could occur at applied stresses as low as 64 % of the general yield stress. For a same applied strain it was more prominent at room temperature, and located as slip and twinning parallel to, and near to or at (respect.) lamellar interfaces of all types in soft mode-oriented colonies. These stretched the full colony width and the shear strain was most intense in the centre of the colonies. Further, the most highly operative microbands of plasticity at specimen fracture were not those most active prior to yielding. The strain mapping results from polycrystalline tensile loading were further compared to those from microcompression testing of soft-mode stacks of lamellae milled from single colonies performed at the same temperatures. Combined with post-mortem transmission electron microscopy of the pillars, the initial plasticity by longitudinal dislocation glide was found to locate within 30 – 50 nm of the lamellar interfaces, and not at the interfaces themselves. The highly localised plasticity that precedes high cycle fatigue failure is therefore inherently related to the lamellar structure, which predetermines the locations of plastic strain accumulation, even in a single loading cycle.Graphical abstractImage 1
       
  • An atomistic investigation of the interaction of dislocations with
           Guinier-Preston zones in Al-Cu alloys
    • Abstract: Publication date: Available online 1 October 2018Source: Acta MaterialiaAuthor(s): G. Esteban-Manzanares, E. Martínez, J. Segurado, L. Capolungo, J. LLorca The interaction between edge dislocations and Guinier-Preston zones in an Al-Cu alloy was analyzed by means of atomistic simulations. The different thermodynamic functions that determine the features of these obstacles for the dislocation glide were computed using molecular statics, molecular dynamics and the nudged elastic band method. It was found that Guinier-Preston zones are sheared by dislocations and the rate at which dislocations overcome the precipitate is controlled by the activation energy, ΔU, in agreement with the postulates of the harmonic transition state theory. Moreover, the entropic contribution to the Helmholtz activation free energy was in the range 1.3–1.8 kb, which can be associated with the typical vibrational entropy of solids. Finally, an estimation of the initial shear flow stress as a function of temperature was carried out from the thermodynamic data provided by the atomistic simulations. Comparison with experimental results showed that the effect of the random precipitate distribution and of the dislocation character and dislocation/precipitation orientation has to be taken into account in the simulations to better reproduce experiments.Graphical abstractImage 1
       
  • Dynamic precipitation, segregation and strengthening of an Al-Zn-Mg-Cu
           alloy (AA7075) processed by high-pressure torsion
    • Abstract: Publication date: Available online 28 September 2018Source: Acta MaterialiaAuthor(s): Yidong Zhang, Shenbao Jin, Patrick W. Trimby, Xiaozhou Liao, Maxim Y. Murashkin, Ruslan Z. Valiev, Jizi Liu, Julie M. Cairney, Simon P. Ringer, Gang Sha Combining transmission Kikuchi diffraction, high resolution transmission electron microscopy and atom probe tomography, we investigated an Al-Zn-Mg-Cu alloy (AA7075) processed by high-pressure torsion (HPT) at room temperature and 200 °C, with an objective to reveal the deformation-induced precipitation and segregation of elements at grain boundaries, and to study their appearance at different processing regimes. Although HPT processing at the two temperatures both induced the formation of ŋ phase, ŋ precipitates formed at the two temperatures have different chemical compositions. The increase of the HPT processing temperature increased significantly segregation of Mg and Cu at grain boundaries. The HPT–induced segregation and decomposition of the alloy have a significant effect on its mechanical strength. Our results open a way for achieving advanced mechanical properties in nanostructured metals and alloys by designing their precipitation and segregation through the control of SPD processing regimes.Graphical abstractImage 1
       
  • Two Way Shape Memory Effect in NiTiHf High Temperature Shape Memory Alloy
           Tubes
    • Abstract: Publication date: Available online 27 September 2018Source: Acta MaterialiaAuthor(s): C. Hayrettin, O. Karakoc, I. Karaman, J.H. Mabe, R. Santamarta, J. Pons Two-way shape memory effect (TWSME) in nano-precipitation hardened, Ni50.3Ti29.7Hf20 high temperature shape memory alloy (HTSMA) thin walled tubes and its thermal stability were investigated. Torsional TWSME was induced in the thin wall tubes by repeated thermal cycling across their martensitic transformation under applied shear stress. The effects of training parameters and geometric factors, such as the number of training cycles, shear stress levels, and thickness of the tube walls, on the resulting TWSME were evaluated. Thermal stability of TWSME was characterized as a function of annealing treatments at elevated temperatures. It was found that under 200MPa, 600 thermal cycles were sufficient to reach a two-way shape memory strain (TWSMS) as high as 2.95%, which was shown to be stable upon annealing up to 400°C for 30 minutes. This TWSMS was 85% of the maximum measured actuation strain under 200MPa. The microstructure after thermo-mechanical training was investigated using transmission electron microscopy (TEM), which did not indicate a significant change in precipitate structure and size after the training. However, small amount of remnant austenite was revealed at 100°C below the martensite finish temperature, with notable amount of dislocations. Overall, it was found that nano-precipitation hardened Ni50.3Ti29.7Hf20 shows relatively high TWSMS and stable actuation response after much less number of training cycles as compared to binary NiTi and nickel lean NiTiHf compositions. Tube wall thickness and training stress levels have been found to have negligible effect on shape memory strains and number of cycles to reach the desired training level, for the ranges studied.Graphical abstractImage 1
       
  • The Formation of Highly Ordered Graphitic Interphase Around Embedded CNTs
           Controls the Mechanics of Ultra-Strong Carbonized Nanofibers
    • Abstract: Publication date: Available online 27 September 2018Source: Acta MaterialiaAuthor(s): Jizhe Cai, Mohammad Naraghi Templating graphitization process, i.e., the transformation of certain polymers to highly-ordered graphitic (HOG) domains upon pyrolysis in the vicinity of graphitic nanomaterials, such as carbon nanotubes (CNTs), is known to be an effective approach to modify the microstructure of carbon nanofibers (CNFs). In this work, the microstructure of CNFs subjected to the templating effect of functionalized single-walled CNTs (f-SWNTs) and the effect of templating on mechanical properties of CNF/f-SWNTs hybrid nanofiber are studied. The CNF/f-SWNTs were fabricated via pyrolysis of electrospun polyacrylonitrile precursors with CNT inclusions. Prior to pyrolysis, the precursors were subjected to thermomechanical treatments, known as hot-drawing, to enhance chain and CNT alignment and packing. The study of the microstructure of the precursor and CNFs indicates the crucial role of precursor hot-drawing in enhancing the microstructure of the precursor and CNFs, leading to drastically enhanced templating effect, as evidenced from the thickness of the HOG that forms around CNTs. Mechanical tests on single nanofibers using custom-designed microdevices led to the realization that the templating effect of CNTs on CNFs, when properly implemented via precursor hot-drawing, can considerably increase the strength of CNFs. The average tensile strength and modulus of CNF/f-SWNTs in which HOG domains had clearly formed were measured to be 7.6 and 268 GPa, respectively, which are the highest value reported to date among similar types of materials. The existence and evolution of the HOG around CNTs inside CNFs and mechanical reinforcing of HOG were thoroughly discussed in conjunction with finite element models of building blocks of CNFs, alluding to the stress fields around HOG and CNTs in the CNF. The high-performance 1-D hybrid graphitic nanostructure developed here, CNF/f-SWNTs, can serve as an outstanding reinforcement material for weight sensitive applications.Graphical abstractImage 1
       
  • First-order reversal curve analysis of a Nd-Fe-B sintered magnet with soft
           X-ray magnetic circular dichroism microscopy
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): Kazunori Miyazawa, Satoshi Okamoto, Takahiro Yomogita, Nobuaki Kikuchi, Osamu Kitakami, Kentaro Toyoki, David Billington, Yoshinori Kotani, Tetsuya Nakamura, Taisuke Sasaki, Tadakatsu Ohkubo, Kazuhiro Hono First-order reversal curve (FORC) diagram, which visualizes the variation of magnetic susceptibility on a field plane, has been applied to a Nd-Fe-B sintered magnet. The FORC diagram exhibits the characteristic behavior of two remarkable spots in low-field and high-field regions. The high-field FORC spot corresponds to the irreversible magnetization reversal at a coercive field, whereas the low-field FORC spot indicates the appearance of a large magnetic susceptibility state during the demagnetization process. Moreover, this low-field FORC spot becomes dominant at high temperature, accompanied by a significant reduction in coercivity. These results suggest that the low-field FORC spot has a strong correlation with the degradation of magnetic properties of a Nd-Fe-B sintered magnet. To clarify the actual magnetization reversal processes corresponding to these two FORC spots, soft X-ray magnetic circular dichroism (XMCD) microscopy observation was employed with similar field sequences of the FORC measurements. Consequently, the low-field FORC spot is mainly attributed to the domain wall motion in multi-domain grains, whereas the high-field FORC spot corresponds to the magnetization reversal of single-domain grains. These indicate that a FORC diagram is a powerful evaluation method for the magnetization reversal processes of permanent magnets.Graphical abstractImage 1
       
  • Structural evolution of directionally freeze-cast iron foams during
           oxidation/reduction cycles immediately prior to returning your
           corrections. -->
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): Stephen K. Wilke, David C. Dunand Cyclical oxidation/reduction behavior of iron-based powders and porous pellets is of great interest for iron-air batteries, steam-iron, and chemical looping processes, but extended cycling is limited by degradation via sintering or pulverization. To address these problems, we use directional freeze casting to fabricate porous iron foams, consisting of colonies of parallel iron lamellae and open channels of sufficient width (20–40 and 20–50 μm, respectively) to accommodate iron/iron oxide volume changes during redox cycling. Iron foams of three different initial channel porosities (48, 61 and 65 vol.%) are fabricated via water-based freeze casting of Fe2O3 powders followed by reduction with H2 and sintering. The evolution of these iron foams is examined after 5 and 10 redox cycles between Fe3O4 and Fe at 800 °C (via steam and H2) using optical microscopy, scanning electron microscopy, and synchrotron X-ray tomography. Redox cycling causes a macroscopic foam shrinkage as the iron lamellae grow closer together, decreasing (and even sometimes eliminating) the channel width between lamellae. Smaller micropores within individual iron lamellae are partially preserved, consistent with new porosity formation via vacancy diffusion and clustering in the oxide phase. Additionally, a dense Fe shell forms on the exterior surface of most samples, caused by lamellae contacting and sintering during oxidation, followed by formation of an impermeable Fe layer during reduction. Strategies are proposed to reduce both channel constriction and shell formation, which are undesirable as they restrict gas phase transport.Graphical abstractImage 1
       
  • Fracture Toughness of NiTi–Towards Establishing Standard Test Methods
           for Phase Transforming Materials
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): Behrouz Haghgouyan, Ceylan Hayrettin, Theocharis Baxevanis, Ibrahim Karaman, Dimitris C. Lagoudas A new test methodology for measuring the fracture toughness of shape memory alloys using the critical value of J-integral as the fracture criterion is proposed. The method relies on the ASTM standard method for measuring the fracture toughness of conventional ductile materials extended to account for the martensitic transformation/martensite reorientation-induced changes in the apparent elastic properties. A comprehensive set of nominally-isothermal fracture experiments is carried out on near-equiatomic NiTi compact tension specimens at three distinct temperatures: (i) below the martensite-finish temperature, Mf; (ii) between the martensite-start temperature, Ms, and the martensite desist temperature, Md, above which the stress-induced martensitic transformation is suppressed; and (iii) above Md. At these temperatures, the material either remains in the martensite state throughout the loading (martensitic material, case (i)) or transforms from austenite to martensite close to the crack tip (transforming material, case (ii)) or remains always in the austenite state (austenitic material, case (iii)), respectively. The critical J-values for crack growth, i.e., the fracture toughness, reported in all three cases, result in extrapolated stress intensity factors that are much higher than the corresponding values reported in literature on the basis of linear elastic fracture mechanics. Moreover, contrary to literature, the fracture toughness of martensitic and transforming materials is found to be approximately the same while the fracture toughness of stable austenite is considerably higher. This mechanics-aided test method can be potentially utilized for measuring the fracture toughness of martensitically transforming materials beyond shape memory alloys.Graphical abstractImage 1
       
  • Site occupancy of alloying elements in the L12 structure determined by
           channeling enhanced microanalysis in γ/γ’ Co-9Al-9W-2X alloys
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): Li Wang, Michael Oehring, Yong Liu, Uwe Lorenz, Florian Pyczak Knowledge about the sublattice site preference of alloying elements in the L12-γ’ phase of novel Co-base superalloys is a necessary pre-requisite to understand their influence on the properties of the alloys in general and the γ’ phase in particular. In the present study, the atomic site occupancy of the alloying elements in the L12-γ’ structure in Co-9Al-9W-2X quaternary alloys after long-term annealing at 900 °C for 5000 hours was determined using the atom location by channeling enhanced microanalysis (ALCHEMI) technique in combination with energy-dispersive X-ray spectroscopy (EDX) composition analysis in a transmission electron microscope (TEM). The experimental ALCHEMI data were evaluated by comparing them with those calculated by the program ‘Inelastic Cross Section Calculator’ (ICSC). The results show that Co mainly occupies one sublattice site and Al/W are located at the other sublattice site in the L12 unit cell in the ternary alloy. The additional elements Ti, V, Mo and Ta which partition strongly to the γ’ phase tend to occupy the Al/W sublattice site, and Cr which partitions more to the γ phase also favors the Al/W sublattice site, while Ni weakly partitions into the γ’ phase and favors the Co sublattice site. The results of this study can provide evidence to the predictions on the site preference in literature based on the phase composition or on theoretical studies.Graphical abstractImage 1
       
  • Interplay between chemical strain, defects and ordering in Sr1-xLaxFeO3
           materials
    • Abstract: Publication date: Available online 26 September 2018Source: Acta MaterialiaAuthor(s): V.V. Sereda, D.S. Tsvetkov, I.L. Ivanov, A.Yu. Zuev Different point defect interactions were found to govern the defect chemistry of SrFeO3-δ and La0.6Sr0.4FeO3-δ. Conventional defect structure model based on two reactions – oxygen release by oxide lattice and the charge disproportionation in Fe-sublattice – can be applied successfully to La0.6Sr0.4FeO3-δ. Successful verification of this model using the available data on the oxygen nonstoichiometry gives virtually zero standard entropy and comparatively high standard enthalpy of iron disproportionation (116.54±1.14 kJ/mol). In turn, the chemical strain of La0.6Sr0.4FeO3-δ was predicted successfully using the simple dimensional model, based on the ionic radii formalism and verified defect structure model. For SrFeO3-δ, a reference set of nonstoichiometry data was chosen from among the multitudinous literature data by comparing the calculated and calorimetrically determined oxide’s reduction enthalpies. Some aspects of perovskite – brownmillerite phase transition in SrFeO3-δ were discussed and the defect structure model for this oxide was proposed and then verified using the chosen data set. Introduction of the vacancy cluster formation in the defect structure model was shown to be necessary since SrFeO3-δ is highly nonstoichiometric with respect to oxygen and tends to form various ordered structures. As a consequence, chemical expansivity of SrFeO3-δ with respect to the oxygen nonstoichiometry was found to be much more complex than that of La0.6Sr0.4FeO3-δ. According to our findings, unusually high values and the anomalous character of chemical strain of SrFeO3-δ are likely to be attributed to the vacancy cluster formation (i.e. short-range ordering) and some degree of long-range vacancy ordering, respectively.Graphical abstractImage 1
       
  • Evolution of structure and residual stress of a fcc/hex-AlCrN
           multi-layered system upon thermal loading revealed by cross-sectional
           X-ray nano-diffraction
    • Abstract: Publication date: Available online 25 September 2018Source: Acta MaterialiaAuthor(s): N. Jäger, S. Klima, H. Hruby, J. Julin, M. Burghammer, J.F. Keckes, C. Mitterer, R. Daniel Understanding the influence of process conditions and coating architecture on the microstructure and residual stress state of multi-layered coatings is essential for the development of novel thermally and mechanically stable coatings and requires advanced depth resolving characterization techniques. In this work, an arc-evaporated multi-layered coating, consisting of alternating Al70Cr30N and Al90Cr10N sublayers with an individual layer thickness between 120nm and 380nm, was investigated. The as-deposited state of the multi-layered coating and the state after vacuum annealing at 1000°C for 30min was studied along its cross-section by synchrotron X-ray nano-diffraction using a beam with a diameter of 50nm. The results revealed sublayers with alternating cubic and hexagonal phase, causing repeated interruption of the grain growth at the interfaces. The in-plane residual stress depth distribution across the coating thickness could be tuned in a wide range between pronounced compressive and slight tensile stress by combining the effects of the coating architecture and the modulated incident particle energy controlled by the substrate bias voltage ranging from -30V to -250V. This resulted in an oscillatory stress profile fluctuating between -2 GPa and -4.5 GPa or pronounced stress gradients with values between -4 GPa and 0.5 GPa. Finally, the decomposition routes of the metastable cubic Al70Cr30N phase could be controlled by the Al90Cr10N sublayers which acted as nucleation sites and governed the texture of the decomposition products as Cr2N. The results demonstrate that the cross sectional combinatorial approach, relying on a sophisticated multi-layer architecture combining various materials synthesized under tailored conditions, allowed for resolving structural variations and stress profiles in the individual layers within the complex architecture and pioneers the path for knowledge-based development of multi-layered coatings with predefined microstructure and a dedicated stress design.Graphical abstractImage 1
       
  • Micromechanical behavior and thermal stability of a dual-phase α+α’
           titanium alloy produced by additive manufacturing
    • Abstract: Publication date: Available online 25 September 2018Source: Acta MaterialiaAuthor(s): Charlotte de Formanoir, Guilhem Martin, Frédéric Prima, Sébastien Allain, Thibaut Dessolier, Fan Sun, Solange Vivès, Benjamin Hary, Yves Bréchet, Stéphane GodetABSTRACTIn order to improve the tensile properties of additively manufactured Ti-6Al-4V parts, specific heat treatments have been developed. Previous work demonstrated that a sub-transus thermal treatment at 920 °C followed by water quenching generates a dual-phase α+α' microstructure with a high work-hardening capacity inducing a desirable increase in both strength and ductility. The present study investigates the micromechanical behavior of this α+α' material as well as the thermal stability of the metastable α’ martensite. To that end, annealing of the α+α' microstructure is performed and the resulting microstructural evolution is analyzed, along with its impact on the tensile properties. A deeper understanding of the micromechanics of the multiphase microstructure both before and after annealing is achieved by performing in-situ tensile testing within a SEM, together with digital image correlation for full-field local strain measurements. This approach allows the strain partitioning to be quantified at a microscale and highlights a significant mechanical contrast between the two phases. In the α+α' microstructure, the α' phase is softer than the α phase, which is confirmed by nanoindentation measurements. Partial decomposition of the martensite during annealing induces a substantial hardening of the α' phase, which is attributed to fine-scale precipitation and solution strengthening. A scale transition model based on the iso-work assumption and describing the macroscopic tensile behavior of the material depending on the individual mechanical behavior of each phase is also proposed. This model enables to provide insights into the underlying deformation and work-hardening mechanisms.Graphical abstractImage 1
       
  • Elemental site occupancy in the L12 A3B ordered intermetallic phase in
           Co-based superalloys and its influence on the microstructure
    • Abstract: Publication date: Available online 25 September 2018Source: Acta MaterialiaAuthor(s): P. Pandey, S.K. Makineni, A. Samanta, A. Sharma, S.M. Das, B. Nithin, C. Srivastava, A.K. Singh, D. Raabe, B. Gault, K. Chattopadhyay We explore the effects of the elemental site occupancy in γ'-A3B (L12) intermetallic phases and their partitioning across the γ/γ' interface in a class of multicomponent W-free Co-based superalloys. Atom probe tomography and first principles density functional theory calculations (DFT) were used to evaluate the Cr site occupancy behavior in the γ' phase and its effect on the γ/γ' partitioning behavior of other solutes in a series of Co-30Ni-10Al-5Mo-2Ta-2Ti-xCr alloys, where x is 0, 2, 5, and 8 at.% Cr, respectively. The increase in Cr content from 0 to 2 to 5 at.% leads to an inversion of the partitioning behavior of the solute Mo from the γ' phase (KMo>1) into the γ matrix (KMo
       
  • Multi-scale modeling of the complex microstructural evolution in
           structural phase transformations
    • Abstract: Publication date: Available online 25 September 2018Source: Acta MaterialiaAuthor(s): Kang Wang, Lin Zhang, Feng Lius Modeling the microstructural evolution in structural phase transformations remains challenging, mostly due to the competitions among the potential product phases and the multi-scale nature. To develop a practical tool for such a scientifically and technologically important issue, a multi-scale framework is proposed, where a coarse graining scheme based on the probability density distribution of the representative volume elements (RVEs) of product phases is coupled with the maximal entropy production principle (MEPP) to model the competitions among the multiple product phases as the selection of dissipative paths, and a Fokker-Planck type equation is obtained for the evolution of multiple microstructural parameters (MPs) for the product phases. Applied to precipitation in Al-Cu alloys, the present model, free of adjustable parameters, predicts a correct sequence of precipitation, i.e. GP zone → θꞌꞌ → θꞌ, and yields the accurate precipitation kinetics for θꞌꞌ and θꞌ as compared with the previous experimental data, thus demonstrating the inherent correlation between the MPs and thermodynamics and kinetics of the transformation. For the complex transformations in engineering alloys, the current framework, starting from the general statistical principles and the MEPP, can incorporate the specific MPs for a given transformation following the same scheme.Graphical abstractImage 1
       
  • Strain relaxation in low-mismatched
           GaAs/GaAs1-xSbx/GaAs heterostructures
    • Abstract: Publication date: Available online 22 September 2018Source: Acta MaterialiaAuthor(s): Abhinandan Gangopadhyay, Aymeric Maros, Nikolai Faleev, David J. Smith The creation of structural defects in low-mismatched GaAs/GaAs0.92Sb0.08/GaAs(001) heterostructures and their evolution during strain relaxation have been studied using transmission electron microscopy as well as high-resolution x-ray diffraction and atomic force microscopy. These GaAsSb films had thicknesses in the range of 50 to 4000 nm with 50-nm-thick capping layers and were grown using molecular beam epitaxy. The strain relaxation had three distinct phases as the film thickness was increased, whereas the thin GaAs capping layers exhibited only the initial sluggish stage of relaxation in heterostructures with thick GaAsSb films. The character of the misfit dislocations at the two interfaces was determined using g.b analysis, and atomic-scale structural information was obtained using aberration-corrected electron microscopy. Stage-I relaxation took place primarily by glide of dissociated 60° dislocations. Although the films were mostly free of threading dislocations, many curved dislocations extended into the substrate side for heterostructures that had undergone Stage-II and Stage-III relaxation. Investigation of dislocation density evolution at the cap/film interface and morphological evolution of the growth surface revealed a strong correlation. The smoother growth surface in the heterostructure with 4000-nm-thick film resulted in a reduced areal density of surface troughs that acted as nucleation sites for dislocations, which explained the decreased dislocation density at the cap/film interface. Overall, these results prove that heterogeneously nucleated surface half-loops are the primary source of threading dislocations in low-mismatched heterostructures.Graphical abstractImage 1
       
  • Microstructural effects on strain rate and dwell sensitivity in dual-phase
           titanium alloys
    • Abstract: Publication date: Available online 20 September 2018Source: Acta MaterialiaAuthor(s): Sana Waheed, Zebang Zheng, Daniel S. Balint, Fionn P.E. Dunne In this study, stress relaxation tests are performed to determine and compare the strain rate sensitivity of different α−β titanium alloy microstructures using discrete dislocation plasticity (DDP) and crystal plasticity finite element (CPFE) simulations. The anisotropic α and β phase properties of alloy Ti-6242 are explicitly included in both the thermally-activated DDP and CPFE models together with direct dislocation penetration across material-interfaces in the DDP model. Equiaxed pure α, colony, Widmanstatten and basketweave microstructures are simulated together with an analysis of the effect of α grain size and dislocation penetration on rate sensitivity. It is demonstrated that alloy morphology and texture significantly influence microstructural material rate sensitivity in agreement with experimental evidence in the literature, whereas dislocation penetration is found not to be as significant as previously considered for small deformations. The mechanistic cause of these effects is argued to be changes in dislocation mean free-path and the total propensity for plastic slip in the specimen. Comparing DDP results with corresponding CPFE simulations, it is shown that discrete aspects of slip and hardening mechanisms have to be accounted for to capture experimentally observed rate sensitivity. Finally, the dwell sensitivity in a polycrystalline dual-phase titanium alloy specimen is shown to be strongly dependent on its microstructure.Graphical abstractImage 1
       
  • Grain Boundary Segregation and Intermetallic Precipitation in Coarsening
           Resistant Nanocrystalline Aluminum Alloys
    • Abstract: Publication date: Available online 20 September 2018Source: Acta MaterialiaAuthor(s): A. Devaraj, W. Wang, R. Vemuri, L. Kovarik, X. Jiang, M. Bowden, J.R. Trelewicz, S. Mathaudhu, A. Rohatgi In-spite of all of the unique properties of nanocrystalline materials, they are notorious when it comes to their susceptibility to thermally induced grain coarsening, thus imposing an upper limit to their application temperature. In this study, we demonstrate a coupled Monte Carlo-molecular dynamics simulation-guided experimental approach of improving the resistance to thermally induced grain coarsening in light-weight nanocrystalline Al-Mg alloys. The structure, grain boundary segregation of Mg, and extent of grain coarsening of the Al-Mg alloys were characterized using plan view and cross-sectional transmission electron microscopy and atom probe tomography. Coarsening resistance is attributed to a combination of thermodynamic stabilization of grain boundaries by controlled Mg segregation, and kinetic stabilization through pinning of the boundaries with nanoscale intermetallic precipitates. Thus, we highlight the opportunities in extending the upper limit of application temperature for nanocrystalline alloys by using a complementary thermodynamic and kinetic stabilization approach.Graphical abstractImage 1
       
  • In situ STEM/SEM Study of the Coarsening of Nanoporous Gold
    • Abstract: Publication date: Available online 4 September 2018Source: Acta MaterialiaAuthor(s): A.A. El-Zoka, J.Y. Howe, R.C. Newman, D.D. Perovic Nanoporous gold (NPG), formed by the chemical or electrochemical dealloying of binary or ternary solid solutions, has strong prospects as a catalyst, sensor substrate, membrane, actuator, and other applications. In view of the wide range of thermochemical conditions expected, understanding the evolution of NPG during thermal coarsening is necessary to assess and improve the prospects for its functionality. The addition of Pt to NPG (NPG-Pt) was shown in recent years to have an effect of inhibtion of coarsening, both during dealloying and during oxidizing post-treatment. In this study, the direct observation of the coarsening of nanoligaments by simultaneous transmitted electron (TE) and secondary electron (SE) imaging reveals the complex nature of the coarsening process. As a result of adding Pt, alterations in the volume fraction of ligaments are shown to have a strong impact on the operating coarsening mechanism and kinetics in a low-pressure hydrogen environment. Pt was also shown to increase the temperature for the onset of thermal coarsening. The important roles of surface diffusion and the possibility of coalescence-controlled coarsening kinetics are discussed in detail. Reproducible observations of ligament collapse and void annihilation serve to expand the current understanding of thermal coarsening of NPG.Graphical abstractImage 1
       
  • Hashin-Shtrikman bounds with eigenfields in terms of texture coefficients
           for polycrystalline materials
    • Abstract: Publication date: Available online 3 September 2018Source: Acta MaterialiaAuthor(s): Mauricio Lobos Fernández, Thomas Böhlke The Hashin-Shtrikman bounds accounting for eigenfields are represented in terms of tensorial texture coefficients for arbitrarily anisotropic materials and arbitrarily textured polycrystals. This requires a short review of the Hashin-Shtrikman bounds with eigenfields, an investigation of the polarization field determined by the stationarity condition and, finally, the analysis of the resulting expressions of the Hashin-Shtrikman bound of the effective potential. The resulting expressions are given naturally in terms of symmetric second-order tensors and minor and major symmetric fourth-order tensors. These properties induce, based on the tensorial Fourier expansion of the crystallite orientation distribution function, a dependency of all Hashin-Shtrikman properties in terms of solely the second- and the fourth-order texture coefficients. This is a new result, which is not self-evident, since an alternative formulation of the polarization field would alter the implied algebraic properties of the Hashin-Shtrikman functional. The results obtained by the polarization field, determined through the stationarity condition of the Hashin-Shtrikman functional, are discussed and demonstrated with an example for linear thermoelasticity in which bounds for elastic and thermoelastic properties are illustrated.Graphical abstract(Left) Set of texture coefficients of second- and fourth-order for polycrystals of hexagonal materials with macroscopic hexagonal symmetry. The blue region depictes the region described by the Frobenius norm of the textures coefficients bounded by unity. The orange region describes the region of all possible texture coefficients of second- and fourth-order. The texture coefficients for the single crystal are marked by the blue point, while the red point marks the texture coefficients for a uniform distribution.(Right) Properties-closure for a linear thermoelastic polycrystal of a hexagonal material with macroscopic hexagonal symmetry. The bounds of Voigt, Reuss and Hashin-Shtrikman have been evaluated with the representations presented in this work using the set of all possible texture coefficients depicted in orange in the left graphic. The bounds for the effective stiffness component C_1111 and for the effective thermal expansion coefficient beta_11 have been investigated. The properties-closure for these properties based on the Voigt and Reuss region (first-order bounds) delivers the green region. The properties-closure based on the Hashin-Shtrikman bounds (second-order bounds) delivers the red region. A propertiesprofile with accepted tolerances is depicted by the black point. The Hashin-Shtrikman bounds help material scientists in order to check if aimed properties-profile are reachable or not by polycrystals of considered materials.Image 1
       
 
 
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