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Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 266  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3161 journals]
  • High permeability and low loss bioinspired soft magnetic composites with
           nacre-like structure for high frequency applications
    • Abstract: Publication date: 1 April 2019Source: Acta Materialia, Volume 167Author(s): Wangchang Li, Haowen Cai, Yue Kang, Yao Ying, Jing Yu, Jingwu Zheng, Liang Qiao, Ye Jiang, Shenglei Che Bioinspired Soft Magnetic Composites (SMCs) with nacre-like structure were fabricated using highly planar arranged flaky-Sendust. These SMCs show particularly high permeability and low loss, thus exhibiting great potential for high frequency magnetic components. Due to the high saturation magnetization and permeability of metal soft magnetic materials, it is possible to miniaturize magnetic devices by reducing the total loss, especially the eddy current loss at high frequency. The new SMCs with nacre-like structure exhibited a high permeability of up to 600 at 1 MHz, which was ten times that of common Sendust composites. The total loss decreased to 470.82 and 1162.60 kW/m3 at 100 and 200 kHz, respectively (stimulated at 100 mT), and the maximum magnetic induction increased to 714 mT at 8000 A/m, which was superior to that of ferrites. This outstanding comprehensive property is a result of the anisotropic demagnetizing factor of the flaky particles in a planar arrangement, which can be derived from the Aharoni's formula. The minimum total loss is a result of the balance of hysteresis and eddy current loss for the small thickness, which is below skin depth. In addition, the three-dimensional loss separation characteristic was analyzed using the nonlinear regression method to theoretically evaluate the influencing factors related to the morphology of the materials. This approach provides small, high frequency, and high saturation magnetic devices for switching regulators, photovoltaic inverter boost inductors, and noise filters.Graphical abstractImage 1
       
  • Comparison between Stress-Strain Plots obtained from Indentation
           
    • Abstract: Publication date: Available online 14 February 2019Source: Acta MaterialiaAuthor(s): J.E. Campbell, R.P. Thompson, J. Dean, T.W. Clyne This paper is focused on comparisons between stress-strain plots from conventional uniaxial (tensile or compressive) testing and those obtained from indentation experiments, via iterative FEM modeling of the process in which the plasticity is represented using a constitutive law. Both Ludwik-Hollomon and Voce equations are used in the current work. Advantages of a spherical indenter shape, and of using the residual indent profile as the main experimental outcome, are highlighted. It is shown via detailed study of two different materials, with low and high work hardening rates, that the methodology (here termed indentation plastometry) can be used to obtain (nominal) tensile stress-strain curves, which incorporate the onset of necking and the ultimate tensile strength. High levels of fidelity are observed between these and corresponding plots obtained by conventional tensile testing. It is noted that, while there is also excellent consistency with the outcomes of uniaxial compression tests, the latter inevitably involve some experimental complications that are best avoided. It is concluded that indentation plastometry has the potential to become a mainstream testing methodology in the near future.Graphical abstractImage 1
       
  • Structure-Property Relationships in the Lead-free Piezoceramic System
           K0.5Bi0.5TiO3 - BiMg0.5Ti0.5O3
    • Abstract: Publication date: Available online 14 February 2019Source: Acta MaterialiaAuthor(s): Aurang Zeb, David A. Hall, Zabeada Aslam, Jennifer Forrester, Jing-Feng Li, Yizhe L. Li, Chiu C. Tang, Ge Wang, Fangyuan Zhu, Steven J. Milne Distinctive structure-property relationships are revealed in the relaxor ferroelectric ceramic solid solution, (1-x)K0.5Bi0.5TiO3 - xBiMg0.5Ti0.5O3: 0.02 < x < 0.08. The constructed phase diagram and results of in-situ synchrotron X-ray diffraction provide explanations for temperature and electric field dependent anomalies in dielectric, ferroelectric and electromechanical properties. At room temperature a mixed phase tetragonal and pseudocubic phase field occurs for compositions 0> x ≤ 0.07. As temperature rises to ≥ 150 °C, the ferroelectric tetragonal relaxor phase changes to a pseudocubic ergodic relaxor phase; this change in length scale of polar order is responsible for an inflection in relative permittivity - temperature plots. The transition is reversed by a sufficient electric field, thereby explaining the constricted form of polarisation-electric field loops measured at>150°C. It is also responsible for a change in slope of the strain-electric field (S-E) plots which are relatively linear in the ferroelectric regime i.e. at temperatures up to 150 °C, giving unipolar strains of 0.11 % at 20 °C and 0.14% at 150 °C (50 kV cm-1 field). The additional contribution from the effect of the field-induced pseudocubic to tetragonal transition, generates strains of ∼ 0.2 % at 185 °C. Unusual for a piezoelectric solid solution, the maximum strains and charge coefficients (d33 =150 pC N-1, 20 °C) do not coincide with a morphotropic or polymorphic phase boundary.Graphical abstractImage 1
       
  • Analysis of the Partial Molar Excess Entropy of Dilute Hydrogen in Liquid
           Metals and Its Change at the Solid-Liquid Transition
    • Abstract: Publication date: Available online 14 February 2019Source: Acta MaterialiaAuthor(s): Andrew H. Caldwell, Antoine Allanore A systematic change in the partial molar enthalpy of mixing (Δh¯Hmix) and partial molar excess entropy (Δs¯Hex) for dilute hydrogen-metal systems at the solid-liquid transition is reported. Expressions for Δh¯Hmixand Δs¯Hexare derived from the Fowler model of hydrogen solubility, and the change in Δs¯Hexat melting is bounded. The theoretical bound is in agreement with measured data. A connection is made between the change in Δs¯Hexand short range order in the metal-hydrogen system.Graphical abstractImage 1
       
  • Interplay between the Effects of Deformation Mechanisms and Dynamic
           Recrystallization on the Failure of Mg-3Al-1Zn
    • Abstract: Publication date: Available online 13 February 2019Source: Acta MaterialiaAuthor(s): M.W. Vaughan, W. Nasim, E. Dogan, J.S. Herrington, G. Proust, A.A. Benzerga, I. Karaman This work focuses on the relationship between deformation mechanisms, dynamic recrystallization (DRX), and failure mechanisms in Mg-3Al-1Zn under tension at relatively low temperatures (25-200°C). The loading orientation was selected to favor either prismatic slip, basal slip, or extension twinning as the primary active deformation mechanism upon yielding. The tensile response was accurately simulated at various temperatures using visco-plastic self-consistent crystal plasticity modeling coupled with optimization under constraints. The relative deformation mechanism activities were predicted to gain insight into the role of microstructure evolution and DRX in failure. Extensive microstructural investigation was performed to identify characteristics of failure and categorize them according to slip, twin, and DRX activities, in an attempt to ultimately provide insights for achieving better formability at low temperatures. Failure processes were categorized according to four primary microstructural features observed during deformation prior to failure: (1) extension twinning without DRX, (2) extension twinning with DRX, (3) contraction twinning followed by double twinning, and (4) abundant DRX and high microcrack density along DRX regions. Prismatic slip was found to promote homogeneous DRX via the continuous DRX mechanism, resulting in ductile fracture. Similarly, basal slip allows for high elongation to failure but low yield strengths while promoting discontinuous DRX, whereas extension twinning contributed to quasi-brittle failure at low temperatures and minimal DRX activity. As the yield strength becomes orientation-independent at 150°C, with increased elongation, transition from twinning-to slip-dominant microstructures, and higher DRX activity, the findings indicate that the flow anisotropy in Mg-3Al-1Zn can be mitigated beginning at 150°C.Graphical abstractImage 1
       
  • Quantitative phase field modeling of solute trapping and continuous growth
           kinetics in quasi-rapid solidification
    • Abstract: Publication date: Available online 12 February 2019Source: Acta MaterialiaAuthor(s): Tatu Pinomaa, Nikolas Provatas Solute trapping is an important phenomenon in rapid solidification of alloys, for which the continuous growth model (CGM) of Aziz et al. [1] is a popular sharp interface theory. By modulating the so-called anti-trapping current and using asymptotic analysis, we show how to quantitatively map the thin interface behavior of an ideal dilute binary alloy phase field model onto the CGM kinetics. We present the parametrizations that allow our phase field model to map onto the sharp interface kinetics of the CGM, both in terms of partition coefficient k(V) and kinetic undercooling. We also show that the mapping is convergent for different interface widths, both in transient and steady state simulations. Finally we present the effect that solute trapping can have on cellular growth in directional solidification. The presented treatment for solute trapping can be easily implemented in different phase field models, and is expected to be an important feature in future studies of quantitative phase field modeling in quasi-rapid solidification regimes, such as those relevant to metal additive manufacturing.Graphical abstractImage 1
       
  • Grain Growth and Solid-State Dewetting of Bi-Crystal Ni-Fe Thin Films on
           Sapphire
    • Abstract: Publication date: Available online 12 February 2019Source: Acta MaterialiaAuthor(s): Amit Sharma, Aakash Kumar, Nimrod Gazit, David J. Srolovitz, Eugen Rabkin We studied the solid-state dewetting behavior of thin Ni80Fe20 films deposited on basal plane oriented sapphire substrate and annealed in the range of temperatures of 1023-1323 K. All studied films exhibited strong texture and maze bicrystal microstructure, with only two grains misoriented by 60° around the common axis present in the film. The morphology of partially dewetted films changed from the one typical for polycrystalline thin films to the one typical for single crystalline heteroepitaxial films with increasing temperatures and annealing times. This change of dewetting behavior was associated with the fast grain growth in the films. The films of pure Ni of identical thickness, annealed under identical conditions exhibited significantly slower grain growth and lower thermal stability. Both the high-resolution X-ray diffraction and the cross-sectional high-resolution transmission electron microscopy observations revealed the phase separation of the Ni80Fe20 films into two parallel layers of the face-centered cubic (adjacent to the substrate) and hexagonal close-packed (on the top of the film) phases of similar compositions. Our density functional theory (DFT) calculations indicated that this phase separation is driven by the decrease of the film surface and interface energy, leading to the thermodynamically equilibrium thickness of the metastable hexagonal close-packed phase. This phase exhibits higher surface anisotropy than its stable face-centered cubic counterpart and is instrumental in accelerating the grain growth in the film via suppression of grain boundary grooving.Graphical abstractImage 1
       
  • Materials Informatics: From the Atomic-Level to the Continuum
    • Abstract: Publication date: Available online 30 January 2019Source: Acta MaterialiaAuthor(s): J.M. Rickman, T. Lookman, S.V. Kalinin In recent years materials informatics, which is the application of data science to problems in materials science and engineering, has emerged as a powerful tool for materials discovery and design. This relatively new field is already having a significant impact on the interpretation of data for a variety of materials systems, including those used in thermoelectrics, ferroelectrics, battery anodes and cathodes, hydrogen storage materials, polymer dielectrics, etc. Its practitioners employ the methods of multivariate statistics and machine learning in conjunction with standard computational tools (e.g., density-functional theory) to, for example, visualize and dimensionally reduce large data sets, identify patterns in hyperspectral data, parse microstructural images of polycrystals, characterize vortex structures in ferroelectrics, design batteries and, in general, establish correlations to extract important physics and infer structure-property-processing relationships. In this Overview, we critically examine the role of informatics in several important materials subfields, highlighting significant contributions to date and identifying known shortcomings. We specifically focus attention on the difference between the correlative approach of classical data science and the causative approach of physical sciences. From this perspective, we also outline some potential opportunities and challenges for informatics in the materials realm in this era of big data.Graphical abstractImage 1
       
  • Origin of an unusual systematic variation in the heteroepitaxy of Ag on Ni
           – the roles of twinning and step alignment
    • Abstract: Publication date: Available online 30 January 2019Source: Acta MaterialiaAuthor(s): P. Wynblatt, D. Chatain, A.D. Rollett, U. DahmenABSTRACTA systematic variation in the orientation relationship (OR) of Ag films grown on Ni substrates previously discovered by a combinatorial approach is analyzed using concepts of grain boundaries, surface science and phase transformations. On roughly half of all Ni substrate orientations, Ag adopts a “special” OR that varies systematically from a twin OR, which develops on substrates lying along the (111)-(210) line of the standard stereographic triangle (SST), to the so-called oct-cube OR, which arises exclusively on (100) substrates. On the other half of Ni substrate orientations, Ag adopts the standard cube-on-cube OR. The special ORs are modeled by a) linear interpolation, b) 1D edge-to-edge matching and c) 2D transformation strains, using the twin relationship as a reference. The 1D model shows that the systematic variation in the special ORs can be understood in terms of a growth mechanism by attachment at steps and a lattice rotation due to the difference between the step heights in the substrate and the film. The 2D model explains why this mechanism does not apply to substrates on which Ag displays the cube-on-cube OR.Graphical abstractImage 1
       
  • Dislocation Dynamics in a Nickel-Based Superalloy via In-Situ Transmission
           Scanning Electron Microscopy
    • Abstract: Publication date: Available online 29 January 2019Source: Acta MaterialiaAuthor(s): J.C. Stinville, Eric R. Yao, Patrick G. Callahan, Jungho Shin, Fulin Wang, McLean P. Echlin, Tresa M. Pollock, Daniel S. Gianola Micro-tensile specimens of nickel based-superalloy oligocrystals were tested in-situ in an SEM in transmission mode (TSEM) enabling observation of dislocations. The dynamics of dislocation motion during tensile loading were captured and correlated with the measured intermittencies during plastic flow recorded by high load- and temporal-resolution sensors. This investigation in particular focused on the dislocation behavior near twin boundaries with different slip configurations. A multiplicity of deformation mechanisms at the dislocation scale were observed within individual slip bands, including precipitate shearing, dislocation decorrelation and APB-coupled shearing. These processes affect strain localization near twin boundaries and provide new defect-level insights on strain localization and fatigue crack initiation in these alloys.Graphical abstractImage 1
       
  • New tessellation-based procedure to design perfectly hyperuniform
           disordered dispersions for materials discovery
    • Abstract: Publication date: Available online 29 January 2019Source: Acta MaterialiaAuthor(s): J. Kim, S. Torquato Disordered hyperuniform dispersions are exotic amorphous two-phase materials characterized by an anomalous suppression of long-wavelength volume-fraction fluctuations, endowing them with novel physical properties. While such unusual materials have received considerable attention, a stumbling block has been an inability to create large samples that are truly hyperuniform due to current computational and experimental limitations. To overcome such barriers, we introduce a new, simple construction procedure that guarantees perfect hyperuniformity for very large sample sizes. It involves tessellating space into cells and then inserting a particle into each cell such that the local-cell particle packing fractions are identical to the global packing fraction. We analytically prove that such dispersions are perfectly hyperuniform in the infinite-sample-size limit. Our methodology enables a remarkable mapping that converts a very large nonhyperuniform disordered dispersion into a perfectly hyperuniform one, which we numerically demonstrate in two and three dimensions. A similar analysis also establishes the hyperuniformity of the famous Hashin-Shtrikman multiscale dispersions, which possess optimal transport and elastic properties. Our hyperuniform designs can be readily fabricated using modern photolithographic and 3D printing technologies. The exploration of the enormous class of hyperuniform dispersions that can be designed and tuned by our tessellation-based methodology paves the way for accelerating the discovery of novel hyperuniform materials.Graphical abstractImage 1
       
  • Morphological Evolution of Transformation Products and Eutectoid
           Transformation(s) in a Hyper-Eutectoid Ti-12 at% Cu Alloy
    • Abstract: Publication date: Available online 29 January 2019Source: Acta MaterialiaAuthor(s): Harish Donthula, B. Vishwanadh, T. Alam, T. Borkar, R.J. Contieri, R. Banerjee, R. Tewari, G.K. Dey, S. Banerjee Evidences of both sluggish eutectoid and active eutectoid (not suppressible under rapid cooling) transformations have been found for the first time in a single hyper-eutectoid Ti-Cu alloy. Both of these types of eutectoid reactions have been investigated in detail, at different length scales, by coupling scanning electron microscopy (SEM),transmission electron microscopy (TEM) and atom probe tomography (APT). The unique three phase crystallographic relationship between the parent β (bcc) and the two product phases, α (hcp) and Ti2Cu, has been established. The extent of partitioning of the solute (Cu) between the two product phases has been determined by APT and is rationalised in terms of thermodynamic considerations. Based on the observed lattice site correspondence and the extent of solute partitioning, a possible mechanism of active eutectoid transformation is proposed.Graphical abstractImage 1
       
  • Boundary micro-cracking in metastable Fe45Mn35Co10Cr10 high-entropy alloys
    • Abstract: Publication date: Available online 25 January 2019Source: Acta MaterialiaAuthor(s): Shaolou Wei, Jinwoo Kim, Cemal Cem Tasan Mechanically-induced martensitic transformation can be a double-edged sword: depending on composition and processing history it can either lead to various beneficial mechanical effects (e.g. transformation-induced plasticity, transformation-toughening), or induce local brittleness and damage nucleation. While several corresponding guidelines are presented in steels research, controlling microstructure metastability has not drawn sufficient attention in the quick-emerging field of high-entropy alloys. In the present work, we investigated the damage mechanisms of a mechanically metastable Fe45Mn35Co10Cr10 high-entropy alloy under uniaxial tensile loading. Our integrated in-situ scanning electron microscopy/electron backscatter diffraction experiments revealed a two-fold effect of the highly localized strain, induced by asynchronously transformed martensite, leading to boundary damage nucleation and dissimilarly oriented martensitic variant formation. The latter suppresses slip transfer between adjacent grains, further expediting the growth of the nucleated damage incidents. Based on these experimental observations and corresponding theoretical calculations, we discuss the underlying mechanisms and propose a sequence of micro-events that create the observed phenomena.Graphical abstractImage 1
       
  • Sintering processes in direct ink write additive manufacturing: A
           mesoscopic modeling approach
    • Abstract: Publication date: Available online 10 January 2019Source: Acta MaterialiaAuthor(s): Fadi Abdeljawad, Dan S. Bolintineanu, Adam Cook, Harlan Brown-Shaklee, Christopher DiAntonio, Daniel Kammler, Allen Roach Direct ink write (DIW) is an emerging additive manufacturing technique that allows for the fabrication of arbitrary complex geometries required in many technologies. DIW of metallic or ceramic materials involves a sintering step, which greatly influences many of the microstructural features of the printed object. Herein, we explore solid-state sintering in DIW through a mesoscopic modeling framework that is capable of capturing bulk and interface thermodynamics and accounting for various mass transport mechanisms. Simulation results of idealized geometries identify regimes in materials parameter space, where densification rates are enhanced. With the aid of several statistical and topological descriptors, the role of particle size distribution (PSD) on the microstructural evolution is explored and quantified. More specifically, it is found that a bi-dispersed PSD enhances pore shrinkage kinetics. However, bi-dispersity yields microstructures with pores that are highly eccentric, an effect that could be detrimental to the mechanical properties of the printed material. On the whole, our modeling approach provides a capability to explore the phase space of DIW process parameters and determine ones that lead to optimal microstructures.Graphical abstractImage 1
       
  • Ultrahigh-strength AISI-316 austenitic stainless steel foils through
           concentrated interstitial carbon
    • Abstract: Publication date: 1 April 2019Source: Acta Materialia, Volume 167Author(s): Z. Ren, A.H. Heuer, F. Ernst This research explores intrinsic properties of the carbon-rich subsurface zone (“case”) that low-temperature carburization generates in an austenitic stainless steel (AISI-316). Foils of this steel were carburized to obtain concentrated interstitially dissolved carbon distributed uniformly throughout their thickness. Compared to the as-received foils, such “through” carburization increases the ultimate tensile strength to 3 times, the yield strength to 4 times, and Young's modulus to 1.5 times, respectively. On the other hand, the strain to failure decreases to (9 ± 1) × 10−3. For comparison, foils with larger thickness were carburized as well. Those foils retained a core of low carbon level. Decreasing the ratio of mean carbon depth to foil thickness was found to decrease the ultimate tensile strength, the yield strength, and Young's modulus, while increasing the strain to failure. Tensile testing partially carburized foils to failure confirmed ductility in the core, but revealed reduced ductility in the carbon-rich zone near the surface. On the surface, intergranular cracks were observed to propagate perpendicular to the tensile direction, indicating brittle fracture initiated by crack nucleation at grain boundaries. The isolated concentrated solid solution of interstitial carbon in austenite can be regarded as a new material with exceptional properties, particularly ultrahigh tensile strength, high yield strength, and high Young's modulus.Graphical abstractImage 1
       
  • Revealing the factors influencing grain boundary segregation of P, As in
           Si: Insights from first-principles
    • Abstract: Publication date: Available online 12 February 2019Source: Acta MaterialiaAuthor(s): Dongdong Zhao, Yanjun Li Phosphorous (P) and arsenic (As) segregation at grain boundaries (GBs) usually deteriorate the electrical performance of n-type doped Si-wafers. In order to probe the factors influencing P and As segregation behaviors along Si GBs, a systematic investigation upon the interactions between P, As atoms and a series of coincidence site lattice Si GBs, including Σ3 {111}, Σ9 {221}, Σ27 {552}, Σ3 {112}, was carried out via first-principles calculations. It is revealed that the segregation behaviors of P, As along different GBs are different, which is dependent on both GB characteristics, i.e. the intrinsic lattice distortion, number of dangling/extra bonds, deep levels in the bandgap of density of states, and also the impurity properties. It reveals that smaller P atoms tend to segregate to the compressed atomic sites in GBs, by which the intrinsic GB lattice distortion is reduced. However, GB lattice distortion hardly induce As segregation owing to the similar atomic size between As and Si atoms. Calculations also indicate that under-coordinated core sites with dangling bond along GBs are remarkably attractive for P and As segregation, as a result of the nature of group-V elements to be three-coordinated rather than four-coordinated. Furthermore, GBs having deep levels in the bandgap of density of states, produced either by dangling or extra bonds, show a strikingly high attractive strength for both P and As segregation. The present work is supposed to provide important insights on substitutional impurity segregation along GBs in multi-crystalline Si in the atomic and electronic level.Graphical abstractImage 1
       
  • Formation Mechanism of Abnormally Large Grains in a Polycrystalline
           Nickel-based Superalloy during Heat Treatment Processing
    • Abstract: Publication date: Available online 11 February 2019Source: Acta MaterialiaAuthor(s): Xin Wang, Zaiwang Huang, Biao Cai, Ning Zhou, Oxana Magdysyuk, Yanfei Gao, Shesh Srivatsa, Liming Tan, Liang Jiang Controlling the final grain size in a uniform and controlled manner in powder metallurgy nickel-based superalloys is important since many mechanical properties are closely related to it. However, it has been widely documented that powder metallurgy superalloys are prone to suffer from growth of abnormally large grains (ALGs) during supersolvus heat treatment, which is harmful to in-service mechanical performance. The underlying mechanisms behind the formation of ALGs are not yet fully understood. In this research, ALGs were intentionally created using spherical indentation applied to a polycrystalline nickel-based superalloy at room temperature, establishing a deformation gradient underneath the indentation impression, which was quantitatively determined using finite element modelling, electron backscatter diffraction (EBSD) and synchrotron diffraction. Subsequent supersolvus heat treatment leads to the formation of ALGs in a narrow strain range, which also coincides with the contour of residual plastic strain in a range of about 2% to 10%. The formation mechanisms can be attributed to: (1) nucleation sites available for recrystallization are limited, (2) gradient distribution of stored energy across grain boundary. The proposed mechanisms were validated by the phase-field simulation. This research provides a deeper insight in understanding the formation of ALGs in polycrystalline nickel-based superalloy components during heat treatment, when subsurface plastic deformation caused by (mis)handling before super-solvus heat treatment occurs.Graphical abstractImage 1
       
  • Crystallographic orientation dependent maximum layer thickness of cubic
           AlN in CrN/AlN multilayers
    • Abstract: Publication date: Available online 10 February 2019Source: Acta MaterialiaAuthor(s): Zhuo Chen, David Holec, Matthias Bartosik, Paul H. Mayrhofer, Zaoli Zhang Metastable rock-salt (face centered cubic, c-) AlN can be grown in CrN/AlN multilayers when the AlN layer is thin enough. Exceeding a certain critical thickness, the thermodynamically stable wurtzite (w) structure grows. In this work, a bilayer-period-gradient (21 repeated blocks, each consisting of 10 bilayers with AlN layer-thicknesses ranging from 1.0 nm to 10.0 nm), ∼2.0-μm-thick, reactively magnetron sputtered multilayer was characterized in detail with a spherical aberration-corrected transmission electron microscope (TEM). The studies are complemented by DFT (density functional theory) calculations.The high resolution TEM (HRTEM) studies reveal that the growth-orientation is not as effective as the and growth-orientations in stabilizing the metastable c-AlN. The critical thickness for the c-AlN layers (before the thermodynamically stable w-AlN forms) is around ∼2.0 nm for the growth-orientation but reaches as high as 4.1 nm for both and growth-orientations. Contrary to the orientation, in both and orientations several unusually highly mismatched c-CrN/w-AlN interface structures form as soon as w-AlN is present. DFT studies suggest that the larger critical thickness of the AlN layers in and orientation is allowed by the lower surface energy and higher cubic/wurtzite interfacial energy. The combination of HRTEM and DFT studies allows answering open questions on the impact of crystallographic orientations and interface structures, and also provides a better understanding on the growth mechanisms of c-AlN, necessary for the outstanding mechanical properties of AlN-containing multilayers.Graphical abstractImage 1
       
  • Tilt strain glass in Sr and Nb co-doped LaAlO3 ceramics
    • Abstract: Publication date: Available online 10 February 2019Source: Acta MaterialiaAuthor(s): Yuanchao Ji, Pei Zhang, Liqiang He, Dong Wang, Hanyu Luo, Kazuhiro Otsuka, Yunzhi Wang, Xiaobing Ren Strain glass, a glassy state of lattice strain, has been found in Ti50-xNi50+x alloys and later in many metallic ferroelastic/martensitic systems, where shear or shuffle serves as a primary order parameter (POP). Another class of non-metallic ferroelastic ceramics are also known to widely exist and commonly possess a polyhedral tilt as the POP. So far, it is unclear whether a “tilt” strain glass exists. Here, we report a finding of a tilt strain glass in La1-xSrxAl0.95Nb0.05O3-δ ceramics. With increasing Sr2+ dopants, the ferroelastic transition from cubic to rhombohedral phases is gradually suppressed. At a critical concentration (xc ∼10%), a strain glass transition emerges, characterized by five sets of evidence: (I) an invariance of average structure; (II) frequency dependence of elastic moduli at a strain glass transition temperature Tg; (III) non-ergodicity; (IV) formation of rhombohedral nanodomains; (V) a gradual increase of tilt angle upon cooling. Surprisingly, the established phase diagram shows an increase of Tg with increasing dopants (a positive correlation), which is different from previous strain glass phase diagrams. The positive and negative correlations can be explained as a balance between two factors of strain glass transition: a global transition factor producing a negative contribution competes with a local field one producing a positive contribution. Our discovery of strain glass in ceramics may also bring novel properties as in metals.Graphical abstractImage 1
       
  • Simultaneous enhancement of stress- and strain-controlled fatigue
           properties in 316L stainless steel with gradient nanostructure
    • Abstract: Publication date: Available online 10 February 2019Source: Acta MaterialiaAuthor(s): Y.B. Lei, Z.B. Wang, J.L. Xu, K. Lu A gradient nanostructured (GNS) surface layer with full austenitic phase was synthesized on AISI 316L stainless steel by surface mechanical rolling treatment at ∼280 °C. The mean grain size is ∼45 nm at the top surface and increases gradually with depth. Deformation-induced martensite (DIM) transformation was suppressed and the microstructural refinement was dominated by dislocation activities and twinning during the formation of the GNS layer. Axial tension-compression fatigue tests showed that fatigue strength and life are simultaneously enhanced in the GNS samples relative to the corresponding coarse-grained counterparts in both stress- and strain-controlled tests. This is very different from fatigue behavior of conventional nanostructured materials, i.e. an enhanced stress-controlled fatigue strength with a decreased strain-controlled fatigue life. Besides contributions from the enhanced mechanical properties and the suppressed surface defects formation, analyses of fatigue mechanisms demonstrated that the promoted formation of DIM during cyclic strain plays a crucial role in enhancing fatigue properties of the GNS samples in strain-controlled tests.Graphical abstractImage 1
       
  • Thermodynamics of grain boundary segregation, interfacial spinodal and
           their relevance for nucleation during solid-solid phase transitions
    • Abstract: Publication date: Available online 8 February 2019Source: Acta MaterialiaAuthor(s): A. Kwiatkowski da Silva, R. Darvishi Kamachali, D. Ponge, B. Gault, J. Neugebauer, D. Raabe Grain boundary segregation, embrittlement and phase nucleation are interconnected phenomena that are often treated separately, which is partly due to limitations of the current models to predict grain boundary segregation in non-ideal solid solutions. Here, a simple model is introduced to predict grain boundary segregation in solid solutions by coupling available bulk thermodynamic data with a mean-field description of the grain boundary character. The model is confronted with experimental results obtained in Fe-Mn alloys analysed by atom probe tomography. This model successfully predicts a first order transition or a discontinuous jump in the composition of the grain boundary which kinetically implies the formation of spinodal Mn fluctuations that tend to grow further with time within the segregated region. The increase in solute concentration at the grain boundary leads to an increase of the enthalpy of the boundary and to its embrittlement at lower temperatures. Once austenite is formed, the amount of segregated solute Mn on the grain boundaries is drastically reduced and the toughness of the grain boundary is increased.Graphical abstractImage 1
       
  • Linking microstructure and local mechanical properties in SiC-SiC fiber
           composite using micromechanical testing
    • Abstract: Publication date: Available online 6 February 2019Source: Acta MaterialiaAuthor(s): Y. Zayachuk, P. Karamched, C. Deck, P. Hosemann, D.E.J. Armstrong Local mechanical properties of SiC-SiC fiber-reinforced composite – matrix, fiber and interphases – were evaluated using nanoindentation and microcantilever fracture testing. The fracture toughness was found to be ∼4.25 MPa*m1/2 in the matrix, ∼2 MPa*m1/2 in the fibers and ∼0.8 MPa*m1/2 at the interphases. Nanoindentation hardness was found to vary from ∼17 GPa in the center of the fibers to ∼40 GPa in the matrix. Values obtained with micromechanical testing were found to be in good agreement with the available data on bulk mechanical properties. The mechanical property variations in the different components of the composite can be explained by the variations in the microstructure. The matrix has complex hierarchical microstructure with elongated grains, often featuring twinning, growing radially from the fibers in predominantly direction and forming sets of concentric rings around them. The fibers contain equiaxed grains with carbon precipitates at the grain boundaries. It was found that in the matrix fracture is transgranular, while in the fibers it can be both trans- and intergranular; at the interphases the fracture occurs at the carbon-fiber boundary. The differences in mechanical properties between the matrix and the fibers are attributed to the presence of carbon inclusions in the fibers, which reduce both hardness and fracture toughness.Graphical abstractImage 1
       
  • Tuning the microstructure and metastability of β-Ti for simultaneous
           enhancement of strength and ductility of Ti-based bulk metallic glass
           composites
    • Abstract: Publication date: Available online 5 February 2019Source: Acta MaterialiaAuthor(s): L. Zhang, R.L. Narayan, H.M. Fu, U. Ramamurty, W.R. Li, Y. Li, H.F. Zhang A parametric experimental study on the role played by the size, metastability, and volume fraction of the dendritic β-Ti phase on the tensile properties of amorphous matrix composites is conducted. Towards this end, several bulk metallic glass composites (BMGCs) with varying compositions were synthesized, processed under different cooling rates and tensile tested. Results show that the stress induced martensitic transformation, from β to α″, of the dendritic Ti phase enhances the resistance to shear band propagation and, in turn, imparts significant strain hardening capability to the composite. This transformation was found to be favored in BMGCs in which the size of the dendrites is relatively coarse and Co content is ∼1 at.%. Furthermore, a volume fraction of the dendritic phase between 34% and 45% was found to result in optimum combination of strength and ductility. The utility of these microstructural design principles learned from this study was demonstrated by design, synthesis, and testing of a BMGC containing transformable β-Ti with a volume fraction of ∼38% that simultaneously exhibits high strength and ductility.Graphical abstractImage 1
       
  • Design of D022 superlattice with superior strengthening effect
           in high entropy alloys
    • Abstract: Publication date: Available online 4 February 2019Source: Acta MaterialiaAuthor(s): Feng He, Da Chen, Bin Han, Qingfeng Wu, Zhijun Wang, Shaolou Wei, Daixiu Wei, Jincheng Wang, C.T. Liu, Ji-jung Kai Precipitation strengthening is one of the most promising mechanisms to develop high-performance high entropy alloys (HEAs). However, the design of a reinforcing phase with an excellent strengthening effect is still one of the most pivotal challenges. In the present study, a design strategy based on overall valence electron concentration (OVEC) is developed, and a coherent D022 superlattice (noted as γ″ phase) with superior strengthening effect is designed. The newly developed γ″ phase is systematically characterized using transmission electron microscope and atom probe tomography. Differentiating from the traditional Ni3Nb γ″ phase, the present high-entropy γ″ phase contains ∼7.7% Co and follows the (Ni,Co,Cr,Fe)3(Nb,Fe) stoichiometry. Three γ″ phase variants are observed with crystallographic orientation relationships of [001]γ″//γ and (001)γ″//{100}γ. The lenticular γ″ particles with small volume fraction (7%) causes a significant yield strength increase (670 MPa) and ductility retention (40%), resulting in excellent yield strength-ductility combination. The excellent strengthening effect of the γ″ phase is attributed to both ordering strengthening and coherency strengthening. The present study proposes a new design strategy of precipitates and developed a superior reinforcing phase for HEAs. These findings will not only promote the development of precipitation-hardened HEAs but deepen the fundamentals of precipitates design for other complex concentrated alloys as well.Graphical abstractImage 1
       
  • 25Zr+alloys:+A+comparative+study+of+deformation+mechanisms”+[Acta+Mater.+161+(2018)+420–430]&rft.title=Acta+Materialia&rft.issn=1359-6454&rft.date=&rft.volume=">Corrigendum to “Conventional vs harmonic-structured β-Ti-25Nb25Zr
           alloys: A comparative study of deformation mechanisms” [Acta Mater. 161
           (2018) 420–430]
    • Abstract: Publication date: Available online 4 February 2019Source: Acta MaterialiaAuthor(s): F. Mompiou, D. Tingaud, Y. Chang, B. Gault, G. Dirras
       
  • Experimental study of asymmetrical tilt boundaries in WC-Co alloys
    • Abstract: Publication date: Available online 1 February 2019Source: Acta MaterialiaAuthor(s): Maxime Pellan, Sabine Lay, Jean-Michel Missiaen The rotation angle distribution of grain boundaries with [2 -1 -1 0] misorientation axis in WC-Co alloys was investigated by electron backscatter diffraction for a range of cobalt contents. The effect of carbon content and sintering time was also studied. Preferred misorientations correspond to low-index boundary habit planes involving the basal or prismatic plane for at least one grain. The 26.2° [2 -1 -1 0] boundary was imaged by high resolution transmission electron microscopy. The joining of the mismatched planes at the boundary is accommodated by dislocations which are interpreted with reference to the 29.4° [2 -1 -1 0] boundary. The dislocation spacing and direction found experimentally are in agreement with calculated values obtained by the 02-lattice approach taking into account the misorientation and parametric misfit. The periodicity and characteristics of the dislocations indicate an optimization of the grain boundary structure and energy.Graphical abstractImage 1
       
  • Particle deformation and microstructure evolution during cold spray of
           individual Al-Cu alloy powder particles
    • Abstract: Publication date: Available online 1 February 2019Source: Acta MaterialiaAuthor(s): Tian Liu, Jeremy D. Leazer, Luke N. Brewer This paper examines the single particle impact process for a series of Al-Cu alloy powder particles with 2-5 wt% copper, performed using a low-pressure cold spray system with helium as the carrier gas. The cold spray deposition process is fundamentally controlled by the deformation processes, which occur during single particle impacts. Single particle depositions on steel substrates were produced for each composition over a range of gas temperatures (225-325 ⁰C). Cross sections from single, impacted particles were produced using focused ion beam methods. The deformation microstructure in individual cold sprayed particles was studied using precession electron diffraction (PED) and transmission Kikuchi diffraction (TKD) techniques. For Al-Cu alloy particles, the amount of deformation estimated using a compression ratio did not show a significant difference with varying copper alloy additions. The single particles experienced large deformation upon impact, and ultrafine grains were formed at the particle/substrate interface via dynamic recrystallization. The particles were bonded to the steel substrate via a thin amorphous layer at the interface.Graphical abstractImage 1
       
  • On the effect of Re addition on microstructural evolution of a CoNi-based
           superalloy
    • Abstract: Publication date: Available online 1 February 2019Source: Acta MaterialiaAuthor(s): P. Pandey, A.K. Sawant, B. Nithin, Z. Peng, S.K. Makineni, B. Gault, K. Chattopadhyay In this study, the effect of rhenium (Re) addition on microstructural evolution of a new low-density Co-Ni-Al-Mo-Nb based superalloy is presented. Addition of Re significantly influences the γ′ precipitate morphology, the γ/γ′ lattice misfit and the γ/γ′ microstructural stability during long term aging. An addition of 2 at.% Re to a Co-30Ni-10Al-5Mo-2Nb (all in at.%) alloy, aged at 900 °C for 50 h, reduces the γ/γ′ lattice misfit by ∼ 40% (from +0.32% to +0.19%, measured at room temperature) and hence alters the γ′ morphology from cuboidal to round-cornered cuboidal precipitates. The composition profiles across the γ/γ′ interface by atom probe tomography (APT) reveals Re partitions to the γ phase (KRe=0.34) and also results in the partitioning reversal of Mo to the γ phase (KMo=0.90) from the γ′ precipitate. An inhomogeneous distribution of Gibbsian interfacial excess for the solute Re (ΓRe, ranging from 0.8 to 9.6 atom.nm−2) has been observed at the γ/γ′ interface. A coarsening study at 900 °C (up to 1000 h) suggests that the coarsening of γ′ precipitates occurs solely by an evaporation–condensation (EC) mechanism. This is contrary to that observed in the Co-30Ni-10Al-5Mo-2Nb alloy as well as in some of the Ni-Al based and high mass density Co-Al-W based superalloys, where γ′ precipitates coarsen by coagulation/coalescence mechanism with extensive alignment of γ′ along directions as a sign of microstructural instability. The γ′ coarsening rate exponent (Kr) and γ/γ′ interfacial energy are estimated to be 1.41 × 10−27 m3/s and 8.4 mJ/m2, which are comparable and lower than Co-Al-W based superalloys.Graphical abstractImage 1
       
  • In situ X-ray tomography densification of firn: the role of mechanics and
           diffusion processes
    • Abstract: Publication date: Available online 31 January 2019Source: Acta MaterialiaAuthor(s): Alexis Burr, Pierre Lhuissier, Christophe L. Martin, Armelle Philip One of the most efficient proxy methods for paleoclimatology consists of obtaining data previously preserved within polar ice cores. Models for past climate reconstruction are based in particular on the characterization of entrapped gases in ice closed pores. Improving the temporal accuracy of these models requires a better understanding of firn densification mechanisms. In particular, the interplay between viscoplastic deformation and diffusion processes for pore closure is not well understood. In this work, we describe the first in situ laboratory densification experiments on polar firn retrieved from Antarctica with live characterization by X-ray tomography. Our in situ tests allow for the first time to approximately access the process of pore closure in ice, which takes thousands of years to occur in Antarctica, from visualizations and quantitative analyses of short time laboratory experiments. The parameters of pore separation and closure and the microstructural changes that accompany them are monitored. We show that densification of polar firn and pore closure could be replicated at higher strain rate and warmer temperature. Experiments allow the viscoplastic part of the firn deformation to be decoupled from the diffusion mechanisms that occur at high temperature. Our results show that density alone is not sufficient to predict the close-off density at which gases get entrapped. More generally, the method laid out here may find useful application in the domain of high temperature powder compaction, for which pore closure and grain growth are significant process parameters.Graphical abstractImage 1
       
  • Shock induced damage and fracture in SiC at elevated temperature and high
           strain rate
    • Abstract: Publication date: 1 April 2019Source: Acta Materialia, Volume 167Author(s): Wanghui Li, Eric N. Hahn, Xiaohu Yao, Timothy C. Germann, Xiaoqing Zhang Large-scale molecular dynamics simulations are used to investigate shock-induced damage and fracture in 3CSiC single crystals at an elevated initial temperature of 2000 K and a high tensile strain rate of ∼1010 s−1. Three crystal orientations have been evaluated: [001], [110] and [111]. A comprehensive comparison has been made between cases at 2000 K and at 300 K to address the effects of high temperature on the mechanical performance of SiC under shock loading. Results show that for shock compression, the high temperature decreases the longitude elastic wave speeds as well as the shock stresses. The shock-induced plasticity is mainly in the form of deformation twinning at 300 K, but twinning is absent at 2000 K. The high temperature decreases the structural phase transition threshold pressure in SiC from ∼90 GPa at 300 K (for all three orientations) to ∼75 GPa in [001], ∼57 GPa in [110] and ∼64 GPa in [111] at 2000 K, with corresponding particle velocities of 2.75 km/s, 2.0 km/s, and 2.25 km/s, respectively, in agreement with trends observed in recent experiments. The spall fracture behavior reveals that high temperature reduces the spall strength with an average spall strength of ∼20.7 GPa in [001], ∼21.4 GPa in [110] and ∼22.5 GPa in [111] at 2000 K in the classical spall regime, which are about 33% lower than strengths measured at 300 K. However, in the micro-spall regime the spall strengths are very similar at both temperatures. The corresponding thresholds of particle velocity to trigger spall decrease at elevated temperature except for [001] loading, as well as the thresholds for generating overdriven phase transition waves.Graphical abstractImage 1
       
  • Deformation-driven bidirectional transformation promotes bulk
           nanostructure formation in a metastable interstitial high entropy alloy
    • Abstract: Publication date: 1 April 2019Source: Acta Materialia, Volume 167Author(s): Jing Su, Xiaoxiang Wu, Dierk Raabe, Zhiming Li We investigate the mechanisms of deformation-driven forward and reverse (bidirectional) martensitic transformation and the associated nanostructure formation in a metastable carbon-doped high entropy alloy (HEA) upon cold rolling. At thickness reductions below 14%, forward hexagonal-close packed (HCP) martensitic transformation prevails in the single face-centered cubic (FCC) matrix. Surprisingly, at the intersections of two crossing HCP lamellae, deformation-induced reverse transformation from the HCP martensite back to the FCC phase occurs. At higher thickness reductions around 26%–34%, multiple deformation kink bands develop, mainly on the pyramidal habit planes of the HCP martensite, among which reverted FCC phase is also observed resulting in a dual-phase nano-laminated structure. The deformation-induced reverted FCC phase regions exhibit a twin stacking sequence relative to the prior FCC matrix, which is related to the underlying dislocation reactions and rearrangement of the partial dislocations. At 67% thickness reduction, the deformation bands develop further into micro-shear bands consisting of nanosized (sub)grains. For rendering the dual-phase nanostructure back to single-phase FCC, 400 °C/10 min tempering is applied on a 34% cold-rolled specimen. The resulting nanostructure is characterized by nano-(sub)grains and nano-twins. It exhibits an excellent strength-ductility synergy (ultimate tensile strength 1.05 GPa at 35% total elongation) due to the improved work hardening enabled by both, FCC-HCP martensitic transformation in confined regions and mechanical twinning. With this, we show that bulk nanostructured alloys with bidirectional transformation can be designed by tuning the materials’ phase stability to their thermodynamic limits with the aim to trigger sequential athermal forward and reverse transformation under load.Graphical abstractImage 1
       
  • Electrical properties of proton-conducting BaCe0.8Y0.2O3-δ and the
           effects of bromine addition
    • Abstract: Publication date: 1 April 2019Source: Acta Materialia, Volume 167Author(s): Ángel Triviño-Peláez, Domingo Pérez-Coll, Glenn C. Mather A detailed analysis of the electrical properties of the proton-conducting BaCe0.8Y0.2O3-δ phase (BCY20) and the effects of Br doping has been undertaken. Members of the system of nominal stoichiometry BaCe0.8Y0.2O2.9-(x/2)±δBrx (x = 0, 0.05, 0.1 and 0.2) were synthesised by a citrate-nitrate process and high-temperature annealing up to 1500 °C. X-ray diffraction revealed the formation of single-phase material with a monoclinically distorted perovskite structure (space group I2/m) with greater distortion for Br-synthesised phase. Significant contrasts in electrical behaviour and stability between BCY20 and Br-synthesised series members were also found. However, chemical analysis by total X-ray fluorescence spectroscopy indicated that only trace amounts of Br remain after synthesis, indicating that Br addition influences stoichiometry but not directly the physicochemical properties. Higher conductivity is observed for the Br-synthesised compositions in wet and dry oxidising conditions in the temperature range 300–900 °C, reaching a value of 5.8 S m−1 at 800 °C for the x = 0.2 member in wet O2 (pH2O ∼ 0.022 atm). Determination of partial-conductivity components indicated that, in humid conditions and low temperature, the conductivity of 20% Br-synthesised material is principally protonic (tH = 0.91 at 600 °C and pO2 = 0.2 atm) and superior to that of BCY20. Mixed oxide-ionic-electronic conductivity dominates at high pO2 (1 atm) and oxide-ionic conductivity at low pO2 (∼10−4 atm) in the temperature range 600–900 °C for both BCY and Br-doped samples. Stability was found to be poorer in CO2 for the Br-synthesised phases as determined by thermogravimetry and prolonged conductivity measurements.Graphical abstractBr addition on synthesis of BCY samples improves the ionic transport numbers in both wet and dry conditions.Image 1
       
  • High-throughput solid solution strengthening characterization in high
           entropy alloys
    • Abstract: Publication date: 1 April 2019Source: Acta Materialia, Volume 167Author(s): Francisco Gil Coury, Paul Wilson, Kester D. Clarke, Michael J. Kaufman, Amy J. Clarke While some high entropy alloys (HEAs) have been shown to display remarkable combinations of properties, exploration of the extensive multicomponent space by conventional methods is experimentally intractable. Thus, identifying and developing high-throughput screening methods is paramount to alloy design. Here, an experimental methodology is developed for rapid yield strength estimations of single-phase HEAs, which involves the production and testing of a compositionally-graded sample made by a diffusion-multiple approach. The sample is analyzed by a combination of nanoindentation and microstructural characterization, where the nanohardness results are analyzed by different conversion equations to determine yield strength. The values estimated by nanohardness agree with bulk tensile properties for a total of 8 compositions. Both are compared to a solid solution strengthening model, again yielding a good correlation. The experimental and simulation results indicate that, in this system, the strength is maximized when the atomic size mismatch is maximized. Furthermore, it is necessary to consider the strain hardening of these alloys to accurately estimate their strength by nanoindentation. A pathway is presented here. This work shows that high-throughput methodologies for predicting and measuring properties are promising for designing new HEAs with desirable combinations of properties.Graphical abstractImage 1
       
  • Three-dimensional observations of grain volume changes during annealing of
           polycrystalline Ni
    • Abstract: Publication date: 1 April 2019Source: Acta Materialia, Volume 167Author(s): Aditi Bhattacharya, Yu-Feng Shen, Christopher M. Hefferan, Shiu Fai Li, Jonathan Lind, Robert M. Suter, Gregory S. Rohrer The orientations, locations, and sizes of approximately 2500 grains in a Ni polycrystal were measured at six points in time during an interrupted annealing experiment by synchrotron x-ray based, near field high-energy diffraction microscopy. The volume changes were compared to the geometric characteristics of the grains in both the original microstructures and microstructures in which adjacent twin related domains were merged. Neither the size of a grain nor the number of its nearest neighbors correlates strongly to a grain's volume change over the time scale of this experiment. However, the difference between the number of neighbors a grain has, F, and the average number of neighbors of the neighboring grains have, FNN, is correlated to the volume change. A grain with more (fewer) neighbors than FNN usually grows (shrinks). The correlation between the volume change and F - FNN is obvious only if adjacent twin related domains are merged. The correct sign of the volume change is predicted by F - FNN about two thirds of the time. The results show that the volume change for a given grain is better predicted by comparing a grain's characteristics to its neighbor's characteristics than to the characteristics of the entire ensemble of grains.Graphical abstractImage 1
       
  • The role of elastic and plastic anisotropy in intergranular spall failure
    • Abstract: Publication date: Available online 31 January 2019Source: Acta MaterialiaAuthor(s): Thao Nguyen, Darby J. Luscher, Justin W. Wilkerson Recent mesoscale experimental observations of dynamic ductile failure [1, 2] have demonstrated a strong relationship between grain boundary (GB) misorientation and the likelihood of failure initiation along said GB. This correlation has been attributed to inherent GB weakness of particular misorientation. Here we discuss the role played by mechanics, i.e. elastic and plastic anisotropy, on the experimental observation [1, 2]. We make use of a recently developed framework for modeling dislocation-based crystal plasticity and ductile failure of single crystals under dynamic loading (CPD-FE) [3]. Polycrystals are studied at the mesoscale level through the explicit resolution of individual grains, i.e. resolving each individual grain’s size, shape, and orientation. In our simulations, failure naturally localizes along the GBs with no necessity for ad hoc rules governing damage nucleation. We carry out a few thousand mesoscale calculations, systematically varying the misorientation angles of the GB in the computational microstructure. Despite the fact that we neglect the possibility of variations in inherent GB weakness, our simulations agree favorably with the experimental observations, implying that stress concentration generated by elastic and plastic anisotropies across GBs is a dominant governing factor in this phenomenon. Lastly, we find that misorientation angle is an insufficient GB descriptor to predict the likelihood of intergranular spall failure, which is better understood through the consideration of additional GB degrees of freedom.Graphical abstractImage 1
       
  • Grain Boundary Shear Coupling is Not a Grain Boundary Property
    • Abstract: Publication date: Available online 30 January 2019Source: Acta MaterialiaAuthor(s): Kongtao Chen, Jian Han, Spencer L. Thomas, David J. Srolovitz Shear coupling implies that all grain boundary (GB) migration necessarily creates mechanical stresses/strains and is a key component to the evolution of all polycrystalline microstructures. We present MD simulation data and theoretical analyses that demonstrate the GB shear coupling is not an intrinsic GB property, but rather strongly depends on the type and magnitude of the driving force for migration and temperature. We resolve this apparent paradox by proposing a microscopic theory for GB migration that is based upon a statistical ensemble of line defects (disconnections) that are constrained to lie in the GB. Comparison with the MD results for several GBs provides quantitative validation of the theory of shear coupling factor as a function of stress, chemical potential jump and temperature.
       
  • Deformation and degradation of superelastic NiTi under multiaxial loading
    • Abstract: Publication date: Available online 30 January 2019Source: Acta MaterialiaAuthor(s): Wei-Neng Hsu, Efthymios Polatidis, Miroslav Šmíd, Steven Van Petegem, Nicola Casati, Helena Van Swygenhoven The degradation of the superelastic properties of a commercial NiTi alloy is studied during uniaxial, biaxial and load-path-change cycling performed in-situ with synchrotron X-ray diffraction. Careful examination of the diffraction pattern during uniaxial loading shows the R-phase as a transition between the austenite and the B19’ martensite. Degradation of the superelasticity is found to depend strongly on the loading and unloading path followed, and it is discussed in terms the B19’ martensitic variant selection, the accumulation of dislocations, and the residual R-Phase and B19’ martensite. Cycling biaxially leads to faster degradation than uniaxially due to a larger accumulation of dislocations. If the deformation cycle contains a load path change, dislocation accumulation increases further and more martensite is retained.Graphical abstractImage 1
       
  • Catalyst-assisted epitaxial growth of ferromagnetic TiO2/TiN
           nanowires
    • Abstract: Publication date: Available online 30 January 2019Source: Acta MaterialiaAuthor(s): A. Moatti, R. Sachan, D. Kumar, J. Narayan We report a novel method of growth for single-crystalline TiO2/TiN nanowires through oxidation of epitaxial TiN nanowires on Si-SiO2 (amorphous) and c-sapphire (crystalline) as practical substrates. We propose that the laser ablated Ti and N diffuse into molten Au to form TiN nanodots where the growth rate of nanowires is directly proportional to the laser ablation flux due to high diffusion in molten Au. The TiN nanowires were grown by Pulsed Laser Deposition method using Au as a catalyst. The TiN nanowires were then oxidized to create TiO2/TiN core-shell nanowires. The growth of TiO2 (rutile) occurs by domain matching epitaxy paradigm in such a way that (002) planes of the TiO2 match with (200) plane of TiN, where the TiO2 thickness can be tuned by adjustment of oxidation time and temperature. This design provides a core-shell structure of TiO2/TiN nanowires integrated with silicon and sapphire substrates. The Rutile TiO2 nanowires show ferromagnetic behavior, while the as-grown TiN exhibits diamagnetic behavior. The SEM, TEM, and EBSD are used to characterize the microstructure and atomic alignments of TiO2 nanowires. The simple method of oxidation combined with tunable magnetic properties provides benefits to many smart applications where the magnetic field can be used as an external stimulation.Graphical abstractImage 1
       
  • On the structure of defects in the Fe7Mo6
           μ-Phase
    • Abstract: Publication date: Available online 30 January 2019Source: Acta MaterialiaAuthor(s): S. Schröders, S. Sandlöbes, B. Berkels, S. Korte-Kerzel Topologically close packed phases, among them the μ-phase studied here, are commonly considered as being hard and brittle due to their close packed and complex structure. Nanoindentation enables plastic deformation and therefore investigation of the structure of mobile defects in the μ-phase, which, in contrast to grown-in defects, has not been examined yet. High resolution transmission electron microscopy (HR-TEM) performed on samples deformed by nanoindentation revealed stacking faults which are likely induced by plastic deformation. These defects were compared to theoretically possible stacking faults within the μ-phase building blocks, and in particular Laves phase layers. The experimentally observed stacking faults were found resulting from synchroshear assumed to be associated with deformation in the Laves-phase building blocks.Graphical abstractImage 1
       
  • Directional and oscillating residual stress on the mesoscale in additively
           manufactured Ti-6Al-4V
    • Abstract: Publication date: Available online 30 January 2019Source: Acta MaterialiaAuthor(s): M. Strantza, B. Vrancken, M.B. Prime, C. Truman, M. Rombouts, D.W. Brown, P. Guillaume, D. Van Hemelrijck In additive manufacturing of metals as compared to conventional processing, the directional track-by-track and layer-by-layer nature of the fabrication process can lead to residual stresses that locally are both directionally and spatially heterogeneous. Much of the existing literature has focused either on the macroscale residual stress inside the entire part, or on the microscale residual stresses that are created around a scan vector, thereby neglecting the intermediate length-scale of the different layers. The objective of this research is to investigate such mesoscale residual stress distributions across several layers in Ti-6Al-4V components produced by laser metal deposition process. The incremental centre hole drilling and incremental slitting methods provide measurements with excellent spatial resolution within and across the layer length scale. In this work, the two methods complemented each other to quantify both mesoscale and directional variations of residual stress that correlate with the deposition pattern. Our findings also provide strong evidence that an oscillatory residual stress variation persists even after thermal cycling that occurs during deposition of subsequent layers.Graphical abstractImage 1
       
  • Microstructure and residual elastic strain at graphite nodules in ductile
           cast iron analyzed by synchrotron X-ray microdiffraction
    • Abstract: Publication date: Available online 30 January 2019Source: Acta MaterialiaAuthor(s): Y.B. Zhang, T. Andriollo, S. Fæster, R. Barabash, R. Xu, N. Tiedje, J. Thorborg, J. Hattel, D. Juul Jensen, N. Hansen The microstructure and residual elastic strain at graphite nodules (GNs) in ductile cast iron produced using either a fast or slow cooling rate have been characterized using synchrotron 3D X-ray Laue microdiffraction. The results show that thermal stress is introduced during cooling and that part of this stress is relaxed by plastic deformation of the polycrystalline ferrite matrix. It is found that the plastic deformation is accommodated by the formation of dislocations and dislocation boundaries, which are organized in a cell structure. The dislocation density quantified based on the microstructure is most pronounced at the GN/matrix interface around small GNs in the fast cooled sample. Residual elastic strain is also present, which is mainly compressive with a maximum of 6.0-9.9 × 10-4 near the GNs. Gradients of plastic deformation and elastic strain field around the GNs are observed. The results document for the first time that both the elastic strain field and the plastic strain field averaged over the grains around the GNs is approximately scaling with GN size and not affected by the cooling rate. The experimental data are compared with simulations by a finite element method, and agreement and disagreement are discussed in detail.Graphical abstractImage 1
       
  • The role of lattice defects, element partitioning and intrinsic heat
           effects on the microstructure in selective laser melted Ti-6Al-4V
    • Abstract: Publication date: Available online 30 January 2019Source: Acta MaterialiaAuthor(s): Jan Haubrich, Joachim Gussone, Pere Barriobero-Vila, Philipp Kürnsteiner, Eric A. Jägle, Dierk Raabe, Norbert Schell, Guillermo Requena The microstructure and phase composition in selective laser melted (SLM) Ti-6Al-4V plays a key role for its mechanical performance. The microstructure evolution in SLM Ti-6Al-4V was studied in the as-built condition and after sub-transus heat treatments between 400 °C and 800 °C focusing on elemental partitioning and the role of lattice defects on precipitation of the β phase. With SLM parameters corresponding to low volume energy density (EV = 77 J/mm3) the as-built microstructure consisted of acicular martensite and showed a higher density of lattice defects than that synthesized under high EV = 145 J/mm3 condition. High energy X-ray diffraction indicated the presence of ∼2 wt% β-phase at this high EV. Moreover, atom-probe tomography revealed enrichments in β-stabilizers at one- and two-dimensional lattice defects. These fine enriched one-dimensional columnar and two-dimensional features are identified as precursors of β-phase, revealing the role of lattice defects for β-precipitation. Upon annealing at 400 °C and 530 °C, β-films began to fragment into β−platelets and nanoparticles, whereas annealing at 800 °C led to a coarse-lamellar α/β-microstructure. Moreover, α2-Ti3Al was found in the 400 °C annealed condition. In line with the microstructure changes, Vickers hardness increased upon annealing at temperatures up to 530 °C and dropped when coarsening occurred at higher temperatures. Substantial element partitioning occurred during thermally driven martensite decomposition, which was significantly stronger for Fe than for V.Graphical abstractImage 1
       
  • Response of 14YWT alloys under neutron irradiation: A complementary study
           on microstructure and mechanical properties
    • Abstract: Publication date: Available online 29 January 2019Source: Acta MaterialiaAuthor(s): E. Aydogan, J.S. Weaver, U. Carvajal-Nunez, M.M. Schneider, J.G. Gigax, D.L. Krumwiede, P. Hosemann, T.A. Saleh, N.A. Mara, D.T. Hoelzer, B. Hilton, S.A. Maloy Nanostructured ferritic alloys (NFAs) having sub-micron grain size with a high density of nano-oxides (NOs) (size of ∼2-3 nm) are one of the best candidates for structural components in Generation IV nuclear systems. In this study, 14YWT NFA cladding tubes were irradiated in BOR60 reactor up to 7 dpa at 360-370 °C. Detailed microstructural analysis has been conducted using bright field transmission electron microscopy, bright field scanning transmission electron microscopy, energy filtered transmission electron microscopy, energy dispersive X-ray spectroscopy, electron energy loss spectroscopy and transmission Kikuchi diffraction techniques. This revealed cavities, and type dislocation loops, and α′ precipitates forming after irradiation with relationships between cavities and NOs, and α′ precipitates and NOs. Cavities mostly form on the NOs; whereas, α′ precipitates form between the NOs where the point defect concentration is high. Moreover, α′ precipitates are distributed homogenously on and around the dislocation loops which is consistent with the observation that there is no Cr segregation on dislocation loops. Grain boundaries were found to be mostly depleted in Cr; however, the characteristics of each grain boundary determines the Cr behavior and the α′ denuded zone around the grain boundaries. Mechanical properties of the irradiated tubes have been determined by using both low force and high force nanoindentation techniques, resulting in 1.03±0.33 GPa and 0.82±0.20 GPa hardening, respectively. Dispersed barrier hardening calculations and nanoindentation measurements are in good agreement. In this study, 14YWT NFA has been systematically studied after neutron irradiation to better understand its superior performance: low α′ concentration, low swelling and low radiation-induced hardening.Graphical abstractImage 1
       
  • 9R phase enabled superior radiation stability of nanotwinned Cu alloys via
           in situ radiation at elevated temperature
    • Abstract: Publication date: Available online 29 January 2019Source: Acta MaterialiaAuthor(s): Cuncai Fan, Dongyue Xie, Jin Li, Zhongxia Shang, Youxing Chen, Sichuang Xue, Jian Wang, Meimei Li, Anter El-Azab, Haiyan Wang, Xinghang Zhang Nanotwinned metals exhibit outstanding radiation tolerance as twin boundaries effectively engage, transport and eliminate radiation-induced defects. However, radiation-induced detwinning may reduce the radiation tolerance associated with twin boundaries, especially at elevated temperatures. Here we show, via in-situ Kr ion irradiation inside a transmission electron microscope, that 3 at. % Fe in epitaxial nanotwinned Cu (Cu97Fe3) significantly improves the thermal and radiation stability of nanotwins during radiation up to 5 displacements-per-atom at 200 oC. Such enhanced stability of nanotwins is attributed to a diffuse 9R phase resulted from the dissociation of incoherent twin boundaries in nanotwinned Cu97Fe3. The mechanisms for the enhanced stability of twin boundaries in irradiated nanotwinned alloys are discussed. The stabilization of nanotwins opens up opportunity for the application of nanotwinned alloys for aggressive radiation environments.Graphical abstractImage 1
       
  • Atomic-scale structural characterization of grain boundaries in epitaxial
           Ge/Si microcrystals by HAADF-STEM
    • Abstract: Publication date: Available online 29 January 2019Source: Acta MaterialiaAuthor(s): Yadira Arroyo Rojas Dasilva, Rolf Erni, Fabio Isa, Giovanni Isella, Hans von Känel, Pierangelo Gröning, Marta D. Rossell The atomic structure of grain boundaries in Ge micro-crystals grown on Si pillars for the fabrication of a monolithically integrated X-ray detector was studied by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Three different boundaries are found in Ge: coherent twin boundaries Σ3{111}, incoherent twin boundaries Σ3{112}, Σ9{122} and Σ27{552} grain boundaries. They are described using the structural unit models containing single columns. Remarkably, we find for the first time a Σ3{112} incoherent twin boundary exhibiting two different atomic structures; one symmetric and one asymmetric. Their co-occurrence is explained by the presence of a small step in the boundary plane and the introduction of dislocations. Likewise, the atomic structure of junctions formed by the interaction of twin boundaries which result in Σ9{122} and Σ27{552} grain boundaries is also revealed. Geometrical phase analysis is applied to map the strain fields at two triple junctions and to uncover the position of the dislocations.Graphical abstractImage 1
       
  • Correlative effect of critical parameters for η recrystallization texture
           development in rolled Fe81Ga19 sheet: modeling and experiment
    • Abstract: Publication date: Available online 29 January 2019Source: Acta MaterialiaAuthor(s): Q. Fu, Y.H. Sha, F. Zhang, C. Esling, L. Zuo Strong η (∥RD, rolling direction) recrystallization texture is successfully developed through thickness in Fe81Ga19 sheets with fine-grained microstructure by primary recrystallization process. An excellent combination of magnetic and mechanical properties is achieved with magnetostriction coefficient of 150 ppm, yield strength of 608 MPa and elongation of 11.7%. Quantitative macro/micro-texture analysis indicates that the enhanced η texture derives from number and size advantages established by preferred nucleation and efficient growth. The highly attractive phenomenon is that deformed grains with α (∥RD) texture can be extensively consumed by η recrystallized grains in addition to deformed grains with γ (∥ND, normal direction) texture. A physical model is proposed that well describes the relationship between strong η texture development in competition with γ and α recrystallization textures, where several critical parameters characteristic of deformation texture and microstructure are included. The correlative effects of these parameters are figured out based on the modeling and experimental data. The present work can provide an effective method for recrystallization texture design and control in body-centered cubic alloy sheets.Graphical abstractImage 1
       
  • Nitriding and martensitic phase transformation of the Copper and Boron
           doped Iron Nitride magnet
    • Abstract: Publication date: Available online 24 January 2019Source: Acta MaterialiaAuthor(s): Md Mehedi, Yanfeng Jiang, Bin Ma, Jian-Ping Wang We have investigated two solid-state phase transformations-nitriding of the ternary alloy FeCuB ribbons and martensitic phase transformation in the FeCuBN ribbons using x-ray diffraction, Auger Electron spectroscopy, and scanning electron microscopy. We also studied the diffusion kinetics of nitrogen in FeCuB ribbons and found the activation energy of N to diffuse in the FeCuB matrix and the diffusion coefficient of N at different temperatures. We investigated the evolution of the microstructure during nitriding process and found a layered growth of iron nitride in the FeCuB matrix. Finally, the martensitic phase transformation of the FeCuBN ribbons was also optimized, and the optimizing parameters for the martensitic phase transformation of FeCuB ribbons were reported.Graphical abstractImage 1
       
  • Role of external magnetic fields in determining the thermodynamic
           properties of iron carbides in steel
    • Abstract: Publication date: Available online 24 January 2019Source: Acta MaterialiaAuthor(s): T.P. Hou, Z.H. Li, K.M. Wu, H.F. Lin, Y. Li, G.H. Zhang, W.M. Liu The experimental results indicated that a 12 Tesla magnetic field effectively promoted the precipitation of metastable χ-Fe5C2 iron carbides in low temperature tempered steel. A straightforward approach was proposed to analyze the thermodynamic mechanisms of iron carbides considering the combined effects of self-magnetism and external-field-induced magnetism. Our calculations revealed that the self-magnetic structure characteristics originated from the trigonal prism, which was composed of a lattice of six iron atoms surrounding a carbon atom positioned in the center of the framework. The external-field-induced magnetic exchange coupling increased the long and short-range magnetic order, which was inherently connected with changes in the Curie temperature, thereby altering the peak of magnetic entropy and enthalpy. The external-field-induced magnetism determined magnetic Gibbs free energy change, and further influenced on the precipitation stability of the iron carbides, which agreed well with the experimental results. These findings effectively illuminate the key magnetism problem for the Tokamak of the well-known ITER project for fusion energy and other Fe-based alloys and compounds in extreme conditions.Graphical abstractImage 1
       
  • Structure and mechanical behavior of ultrafine-grained aluminum-iron alloy
           stabilized by nanoscaled intermetallic particles
    • Abstract: Publication date: Available online 23 January 2019Source: Acta MaterialiaAuthor(s): Amandine Duchaussoy, Xavier Sauvage, Kaveh Edalati, Zenji Horita, Gilles Renou, Alexis Deschamps, Frédéric De Geuser Ultrafine-grained aluminum alloys offer interesting multifunctional properties with a combination of high strength, low electrical resistivity, and low density. However, due to thermally induced grain coarsening, they typically suffer from an intrinsic poor thermal stability. To overcome this drawback, an Al-2%Fe alloy has been selected because of the low solubility of Fe in Al and their highly positive enthalpy of mixing leading to the formation of stable intermetallic particles. The two-phase alloy has been processed by severe plastic deformation to achieve simultaneously submicrometer Al grains and a uniform distribution of nanoscaled intermetallic particles. The influence of the level of deformation on the microstructure has been investigated thanks to transmission electron microscopy and atom probe tomography and it is shown that for the highest strain a partial dissolution of the metastable Al6Fe particle occurred leading to the formation of a Fe super saturated solid solution. The thermal stability, and especially the precipitation of particles from the ultrafine-grained solid solution and the way they pin grain boundaries has been investigated both from static annealing and in-situ transmission electron microscopy experiments. The correlation between microstructural features and microhardness has been established to identify the various strengthening contributions. Finally, it is shown that ultrafine grained high purity Al with less than 0.01 at. % Fe in solid solution could preserve a grain size only 300nm after 1h at 250°C.Graphical abstractImage 1
       
  • Precipitation during high temperature aging of Al−Cu alloys: a
           multiscale analysis based on first principles calculations
    • Abstract: Publication date: Available online 22 January 2019Source: Acta MaterialiaAuthor(s): H. Liu, I. Papadimitriou, F.X. Lin, J. LLorca Precipitation during high temperature aging of Al−Cu alloys is analyzed by means of the integration of classical nucleation theory and phase-field simulations into a multiscale modelling approach based on well-established thermodynamics principles. In particular, thermal stability of θ'', θ' and θ precipitates was assessed from first principles calculations of the Helmholtz free energy while homogeneous and heterogeneous nucleation of θ'' and θ' was analysed using classical nucleation theory. Precipitate growth was finally computed by means of mesoscopic phase-field model. The model parameters that determine quantitatively the driving forces for each transformation were obtained by means of first principles calculations and computational thermodynamics. The predictions of the models were in good agreement with experimental results and provided a comprehensive understanding of the precipitation pathway in Al−Cu alloys. It is envisaged that the strategy presented in this investigation can be used in the future to design optimum microstructures based on the information of the different energy contributions obtained from first principles calculations.Graphical abstractImage 1
       
  • Correlation between the microstructure and magnetic configuration in
           coarse-grain inhibited hot-deformed Nd–Fe–B magnets
    • Abstract: Publication date: Available online 21 January 2019Source: Acta MaterialiaAuthor(s): Zexuan Wang, Ke Pei, Jijun Zhang, Renjie Chen, Weixing Xia, Jinzhi Wang, Ming Li, Aru Yan Stress-induced alignment of Nd2Fe14B nanograins along (001) planes represents a practical process with which to create uniaxial high-anisotropy Nd–Fe–B magnetic materials. The residual non-oriented coarse-grain defect at ribbons’ surfaces not only deteriorates the [001]-oriented structure but also facilitates the low-field nucleation of reversed magnetic domains. Although the local structural defect has proved to be controllable via doping refractory ceramic nanomaterials, the natures of coarse-grain suppression and corresponding magnetic configuration variation remain poorly known. Herein, we comprehensively discuss the correlation between structures and magnetic configurations to address the role of WC dopants in the kinetics of stress-induced deformation system. The statistical data suggest that the reinforced stress caused by WC nanoparticles effectively suppresses the size of Nd2Fe14B grains and induces their anisotropic growth. Simultaneously, the evolved (006) polar figures further confirm that the crystallographic orientation of interfacial Nd2Fe14B grains is also improved by reinforced stress. The analysis for magnetic configurations around dopants confirms that the coarse-grain inhibited structure reduces the low-field nucleation probability of reversed magnetic domains. Relying on their combined effect, the coercivity is increased by about 12%. These observations are further verified by the micromagnetic simulation computations. All these above findings may enrich the knowledge of stress-induced deformation mechanism and deepen the fundamental understanding of the correlation between structure and magnetic features.Graphical abstractImage 1
       
 
 
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