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Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 243  
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3163 journals]
  • Elucidating the contribution of mobile hydrogen-deformation interactions
           to hydrogen-induced intergranular cracking in polycrystalline nickel
    • Abstract: Publication date: Available online 20 July 2018Source: Acta MaterialiaAuthor(s): Zachary D. Harris, Samantha K. Lawrence, Douglas L. Medlin, Gael Guetard, James T. Burns, Brian P. Somerday Uniaxial mechanical testing conducted at room temperature (RT) and 77 K on hydrogen-exposed nickel was coupled with targeted microscopy to evaluate the influence of deformation temperature, and therefore mobile hydrogen (H)-deformation interactions, on intergranular cracking in nickel. Results from interrupted tensile tests conducted at cryogenic temperatures (77 K), where mobile H-deformation interactions are effectively precluded, and RT, where mobile H-deformation interactions are active, indicate that mobile H-deformation interactions are not an intrinsic requirement for hydrogen-induced intergranular fracture. Moreover, an evaluation of the true strain for intergranular microcrack initiation for testing conducted at RT and 77 K suggests that H which is segregated to grain boundaries prior to the onset of straining dominates the H-induced fracture process for the prescribed H concentration of 4000 appm. Finally, recent experiments suggesting that H-induced fracture is predominately driven by mobile H-deformation interactions, as well as the increased susceptibility of coherent twin boundaries to H-induced crack initiation, are re-examined in light of these new results.Graphical abstractImage 1
  • Tracing the three-dimensional nanochemistry of phase separation in an
           inverse Ni-based superalloy
    • Abstract: Publication date: Available online 20 July 2018Source: Acta MaterialiaAuthor(s): F. Vogel, S. Ngai, K. Fricke, M. McKechnie, N. Wanderka, T. Hentrich, J. Banhart, G.B. Thompson Tailoring the properties of intermetallic alloys and phases, which resemble the foundation of a vast variety of high-temperature materials like nickel-based superalloys, is a challenge for improving this class of materials that requires comprehensive understanding of their three-dimensional (3D) nanochemistry. Here we use high-resolution microscopy techniques to reveal the microstructural and 3D nanochemical evolution of disordered γ particles from nanoscale nickel-rich heterogeneities (clusters) to γ spheres and then γ plates in an inverse Ni78Al13Ti9 alloy having an ordered (L12) γ' matrix. The γ particles require aging to achieve a thermodynamically stable morphology and composition, determined by reducing the Gibbs free energy. The fundamental analogy between γ particles in a γ' matrix of an inverse alloy and γ particles in γ' precipitates of a hierarchical Ni86.1Al8.5Ti5.4 alloy is established. Our results demonstrate that this analogy can be harnessed as a novel approach for improving the properties of nickel-based superalloys.Graphical abstractImage 1
  • γ γ ' +microstructures+of+Ni-based+superalloys&rft.title=Acta+Materialia&rft.issn=1359-6454&">Microstructure and property based statistically equivalent RVEs for
           intragranular γ − γ ' microstructures of Ni-based superalloys
    • Abstract: Publication date: Available online 20 July 2018Source: Acta MaterialiaAuthor(s): M. Pinz, G. Weber, W.C. Lenthe, M.D. Uchic, T.M. Pollock, S. Ghosh This paper develops statistically equivalent RVEs or SERVEs for intragranular microstructures of Ni-based superalloys, characterized by γ-γ' phase distribution. The SERVE represents an optimal computational domain to be used for micromechanical simulations for effective properties or response variables in the microstructure. The SERVE is further classified as a microstructure-based SERVE or M-SERVE or property-based SERVE or P-SERVE, depending on whether the statistics of morphological characteristics or convergence of chosen material properties are its determinants. Starting from FIB-SEM data for the superalloy René 88DT, the paper systematically develops a host of algorithms for generating validated statistically equivalent virtual microstructures, from which the M-SERVE is estimated from convergence of selected morphological and spatial distributions. Subsequently the P-SERVEs are established for global properties like yield strength and hardening rate, and local variables including dislocation density and the maximum resolved shear stress. Spatially-averaged quantities are found to converge quicker than the local distributions for both M-SERVE and P-SERVE.Graphical abstractImage 1
  • Role of atomic-scale chemical heterogeneities in improving the plasticity
           of Cu-Zr-Ag bulk amorphous alloys
    • Abstract: Publication date: Available online 19 July 2018Source: Acta MaterialiaAuthor(s): Hong-Kyu Kim, Jae-Pyoung Ahn, Byeong-Joo Lee, Kyoung-Won Park, Jae-Chul Lee The addition of minor elements to binary amorphous alloys often results in simultaneous improvement in plasticity and strength. This is particularly the case for alloy systems that promote atomic-scale compositional separation during quenching. Considering that plasticity and strength are two contrasting properties originating from different atomic-scale structures characterized by short-range orders (SROs), this experimental observation at first seems contradictory when viewed from an SRO perspective. We performed comparative studies on (Cu0.5Zr0.5)100-xAgx amorphous alloys using experiments and molecular dynamics (MD) simulations to elucidate how these two mutually exclusive properties can be realized at the same time. MD simulations showed that while the addition of Ag promotes the formation of stable icosahedral orders responsible for improved strength, it also encourages the formation of weak/unstable Ag-bearing polyhedra that mitigate strain localization. In this study, the mechanistic origin of the enhanced plasticity is assessed by introducing a new descriptor that can quantitatively evaluate the characteristics of Ag-bearing polyhedra in terms of their size, softness, population, and spatial uniformity.Graphical abstractImage 1
  • Fatigue life and mechanistic modeling of interior micro-defect induced
           cracking in high cycle and very high cycle regimes
    • Abstract: Publication date: Available online 19 July 2018Source: Acta MaterialiaAuthor(s): Ming-Liang Zhu, Long Jin, Fu-Zhen Xuan Axially loaded push-pull cyclic tests of a precipitation-hardened stainless steel with different sampling orientations were conducted in high cycle and very high cycle fatigue regimes. Results showed apparent fatigue anisotropy with non-metallic inclusions dominating crack initiation behavior. A fatigue lifing model was developed by combining size, location and shape of inclusions into a new form of Z parameter to rationalize the orientation effect. Using a multi-scale and full-field approach, the inclusion-induced interior cracking mechanisms were found to be associated with inclusion-microstructure interaction resulted plasticity. Micro-hardness at the cracking site was the lowest on the fracture surface, and surrounding microstructures showed formation of small grains with clear interfaces. The fine granular area was characteristic of several nano-scale fine grains formed in terms of dislocation cell structures by martensitic laths breakdown. The coalescence of interfaces or micro-crackings finally became interior early fatigue cracks. The mechanistic modeling of “fragmentation of martensitic laths and formation of dislocation cells” revealed a microstructure-dependent crack initiation and stage I growth for interior fatigue cracking. All these inform the significance of combining metallurgical and processing factors in designing against fatigue of engineering materials.Graphical abstractImage 1
  • Thermodynamics and kinetics of core-shell versus appendage
           co-precipitation morphologies: an example in the Fe-Cu-Mn-Ni-Si system
    • Abstract: Publication date: Available online 19 July 2018Source: Acta MaterialiaAuthor(s): Shipeng Shu, Peter B. Wells, Nathan Almirall, G. Robert Odette, Dane D. Morgan What determines precipitate morphologies in co-precipitating alloy systems? We focus on alloys of two precipitating phases, with the fast-precipitating phase acting as heterogeneous nucleation sites for a second phase manifesting slower kinetics. Kinetic lattice Monte Carlo simulations show that the interplay between interfacial and ordering energies, plus active diffusion paths, strongly affect the selection of core-shell verses appendage morphologies. We study a FeCuMnNiSi alloy using the combination of atom probe tomography and simulations, and show that the ordering energy reduction of the MnNiSi phase heterogeneously nucleated on a pre-existing copper-rich precipitate exceeds the energy penalty of a predominantly Fe/Cu interface, leading to initial appendage, rather than core-shell, formation. Diffusion of Mn, Ni and Si around and through the Cu core towards the ordered phase results in subsequent appendage growth. We further show that in cases with higher primary precipitate interface energies and/or suppressed ordering, the core-shell morphology is favored.Graphical abstractImage 1
  • Hydrogen embrittlement of the equi-molar FeNiCoCr alloy
    • Abstract: Publication date: Available online 19 July 2018Source: Acta MaterialiaAuthor(s): Kelly E. Nygren, Shuai Wang, Kaila M. Bertsch, Hongbin Bei, Akihide Nagao, Ian M. Robertson The influence of hydrogen on the mechanical response of an FCC equi-molar solid-solution alloy, FeNiCoCr, was investigated. Hydrogen caused a reduction in ductility in terms of total elongation of only a few percent in the FeNiCoCr alloy, but the local reduction in area after necking was reduced, and the fracture mode transitioned from transgranular ductile microvoid coalescence to predominantly intergranular. This result demonstrates unequivocally that this equi-molar alloy is susceptible to hydrogen embrittlement. Specifically, hydrogen reduced the elongation of the grains in the loading direction, increased the out-of-plane distortion of grains on the specimen surface, increased the orientation deviations with respect to the mean of individual grains, and induced a more advanced microstructural state in the form of dislocation tangles and dislocation cells. Hydrogen transport by dislocations caused a redistribution of the hydrogen increasing the hydrogen concentration on the grain boundary. Hydrogen accommodated in dislocation cell walls and on dislocations, effectively locked the microstructure in a specific configuration that inhibited strain transfer across grain boundaries. Consequently, an incompatibility constraint across the grain boundary was introduced. Together these effects explain why the hydrogen-charged alloy did not exhibit necking and transitioned to a grain boundary failure mode.Graphical abstractImage 1
  • Cross-slip of long dislocations in FCC solid solutions
    • Abstract: Publication date: Available online 18 July 2018Source: Acta MaterialiaAuthor(s): Wolfram Georg Nöhring, W.A. Curtin Cross-slip of screw dislocations is a dislocation process involved in dislocation structuring, work hardening, and fatigue. Cross-slip nucleation in FCC solid solution alloys has recently been shown to be strongly influenced by local fluctuations in spatial arrangement of solutes, leading to a statistical distribution of cross-slip nucleation barriers. For cross-slip to be effective macroscopically, however, small cross-slip nuclei (∼40b) must expand across the entire length of typical dislocation segments (102–103b). Here, a model is developed to compute the relevant activation energy distribution for cross-slip in a random FCC alloy over arbitrary lengths and under non-zero Escaig and Schmid stresses. The model considers cross-slip as a random walk of successive flips of adjacent 1b segments, with each flip having an energy consisting of a deterministic contribution due to constriction formation and stress effects, plus a stochastic contribution. The corresponding distribution is computed analytically from solute-dislocation and solute-solute binding energies. At zero stress, the probability of high activation energies increases with dislocation length. However, at stresses of just a few MPa, these barriers are eliminated and lower barriers are dominant. For increasing segment length, the effective energy barrier decreases according to a weak-link scaling relationship and good analytic predictions can be made using only known material properties. Overall, these results show that the effective cross-slip barrier in a random alloy is significantly lower than estimates based on average elastic and stacking fault properties of the alloy.Graphical abstractImage 1
  • { 10 1 ¯ 2 } +twin-dominated+deformation+of+Mg+pillars:+Twinning+mechanism,+size+effects+and+rate+dependency&rft.title=Acta+Materialia&rft.issn=1359-6454&">In-situ TEM observation of { 10 1 ¯ 2 } twin-dominated deformation of
           Mg pillars: Twinning mechanism, size effects and rate dependency
    • Abstract: Publication date: Available online 18 July 2018Source: Acta MaterialiaAuthor(s): Jiwon Jeong, Markus Alfreider, Ruth Konetschnik, Daniel Kiener, Sang Ho Oh To investigate the mechanism of {101¯2} twinning in magnesium (Mg) single crystal and its influence on mechanical size effects and strain rate sensitivity, in-situ microcompression of Mg [21¯1¯0]pillars of various sizes from 0.5 μm to 4 μm was carried out in a scanning electron microscope (SEM) and also in a transmission electron microscope (TEM) with covering the strain rates from 10−4 to 10−2 s−1. The in-situ observations directly showed that the pile-up of prismatic dislocations acts as local stress concentration for the twin nucleation. Preceding the twin nucleation, the lead dislocation in the pile-up cross-slips to the basal plane and dissociates into partial dislocations, one of which trails a stacking fault (SF) behind. A twin nucleus of a finite size formed at the junction between prismatic dislocations and basal SFs and subsequently propagated rapidly across the pillar. The present in-situ observations reveal that not only the dislocation pile-up but also the dissociation reaction of dislocations play critical roles in the nucleation of {101¯2} twins. Furthermore, the {101¯2} twinning exhibits a relatively strong size effect in terms of the twin nucleation stress (size exponent, n = 0.7). This pronounced size effect may arise from the fact that the precursor to twin nucleation, namely dislocation pile-up and junction formation, depends more strongly on the crystal size than the ordinary dislocation source operation. Moreover, a noticeable effect of the strain rate is that a higher rate (10−2 s−1) promotes the activation of basal slip within {101¯2} twin. While the twin nucleation occurs more easily at a high strain rate, here the twin growth rate cannot cope with the applied strain rate, so that strain energy accumulation increases with applied strain. When the twin grows to reach the required twin thickness for basal slip, the basal slip promptly activates within the twinned region to release the accumulated strain energy and plastic deformation swiftly catches up with the applied strain rate.Graphical abstractImage 1
  • On the Bulk Glass Formation in the Ternary Pd-Ni-S System
    • Abstract: Publication date: Available online 18 July 2018Source: Acta MaterialiaAuthor(s): Alexander Kuball, Benedikt Bochtler, Oliver Gross, Victor Pacheco, Moritz Stolpe, Simon Hechler, Ralf Busch We report on the formation of bulk metallic glasses in the ternary Pd-Ni-S system. In a large compositional range, glass formation is observed by copper mold casting with a glass forming ability of up to 2 mm in diameter for the composition Pd37Ni37S26. The best compromise of thermal stability upon heating from the as-cast state and glass forming ability was found for Pd31Ni42S27, having a critical diameter of 1.5 mm and an extension of the supercooled liquid region of 27.2 K (ΔTx = Tx – Tg). Differential scanning calorimetry and X-ray diffraction experiments were conducted in order to study the influence of the composition on the glass forming ability and thermal stability. The primary precipitating crystalline phases Ni3S2 and Pd4S are identified by in-situ high energy synchrotron X-ray scattering experiments upon heating from the glassy state as well as upon cooling from the equilibrium liquid. Finally, the origin of the bulk glass formation in this novel system is discussed regarding thermodynamics and kinetics and compared to current models for the prediction of the glass forming ability. Furthermore, the mechanical properties are investigated and discussed with respect to the rather fragile kinetic behavior. All in all, we gain new insights in the process of glass formation in this novel alloying system and give conclusions about the determining contributions for the glass forming ability and glass forming range.Graphical abstractImage 1
  • Clustering kinetics during natural ageing of Al-Cu based alloys with (Mg,
           Li) additions
    • Abstract: Publication date: Available online 18 July 2018Source: Acta MaterialiaAuthor(s): R. Ivanov, A. Deschamps, F. De Geuser Room temperature solute clustering in aluminium alloys, or natural ageing, despite its industrial relevance, is still subject to debate, mostly due to its experimentally challenging nature. To better understand the complex multi-constituents’ interactions at play, we have studied ternary and quaternary subsystems based on the Al-Cu alloys, namely Al-Cu-Mg, Al-Cu-Li and Al-Cu-Li-Mg. We used a recently introduced correlative technique using small-angle neutrons and X-ray scattering (SANS and SAXS) to extract the chemically resolved kinetics of room temperature clustering in these alloys, which we completed with DSC and micro-hardness measurements. The comparison of the clustering behaviours of each subsystem allowed us to highlight the paramount role of Mg as a trigger for diffusion and clustering. Indeed, while a strong natural ageing was observed in the Al-Cu-Mg alloy, virtually none was shown for Al-Cu-Li. A very slight addition of Mg (0.4%) to this system, however, drastically changed the situation to a rapid formation of essentially Cu-rich hardening clusters, Mg only joining them later in the reaction. This diffusion enabling effect of Mg is discussed in terms of diffusion mechanism and complex interactions with the quenched-in vacancies.Graphical abstractImage 1
  • Direct electrical switching of ferroelectric vortices by a sweeping biased
    • Abstract: Publication date: Available online 17 July 2018Source: Acta MaterialiaAuthor(s): L.L. Ma, Ye Ji, W.J. Chen, J.Y. Liu, Y.L. Liu, Biao Wang, Yue Zheng The precise manipulation of ferroelectric vortices is one of the most important issues concerning its potential applications in functional electronic devices such as non-volatile memory. Despite investigations focusing on the switching of toroidization of vortex domain structure performed in recent years, there is still a lack of a simple and general method to realize vortex switching. In this work, we propose a direct electrical method to switch ferroelectric vortices by sweeping a biased tip, which is easy-to-operate in practice and is demonstrated to be feasible for various ferroelectric structures. It is the electric field gradient generated by the tip bias in conjunction with the time-reversal asymmetric tip-sweeping operation that induces the vortex switching. The influencing factors of this method, e.g., field profile, tip size, tip bias, tip displacement and surface screening conditions, etc., are systematically studied. As an implementation, we put forward a nanowire vortex memory system in which the information stored by vortex chirality can be successfully manipulated by the tip-sweeping method. The effects of temperature and nanowire structure on the feasibility of vortex switching are analyzed. Our findings provide an efficient control strategy on ferroelectric vortices and suggest broad device opportunities exploiting vortex domain structures.
  • Grain refinement in a metastable beta Ti alloy deformed to large strains
           at high strain rates
    • Abstract: Publication date: Available online 17 July 2018Source: Acta MaterialiaAuthor(s): A. Zafari, K. Xia Shear punching to large plastic strains up to 40 was conducted on a metastable β Ti alloy at various speeds, creating severe plastic deformation at high shear rates (γ˙) up to 200 s–1. Grain refinement in the resulting shear bands (SBs), which experienced even faster deformation due to strain localisation, was studied using extensive TEM. Significant grain refinement occurred at γ˙ < 40 s–1, producing β grains of ∼5–10 nm, considerably smaller than those obtained by high pressure torsion (HPT) to a much higher strain of 240 at a slow shear rate of 0.5 s–1. The applied shear strain rate of 2 s–1 was found to be an optimum for attaining a pure nano-β grain structure thanks to maximum dislocation activity and complete α" to β reverse transformation. However, some stress induced α" remained as γ˙ deviated from this value and much coarser β grains formed at γ˙> 40 s–1. The effects of strain rates on grain refinement and the minimum achievable grain sizes at high strain rates are discussed based on classical dislocation dynamics.Graphical abstractImage 1
  • Controls on microstructural features during solidification of colloidal
    • Abstract: Publication date: Available online 17 July 2018Source: Acta MaterialiaAuthor(s): Jiaxue You, Zhijun Wang, M. Grae Worster We present a mathematical model of the directional freezing of colloidal suspensions. Key ingredients of the model are the disjoining forces between the colloidal particles and the solidified suspending fluid, flow of the suspending fluid towards the solidification front through an accumulating layer of particles, and flow through microscopic films of unfrozen liquid separating particles from the freezing front. Our model predicts three different modes of solidification leading to different microstructures: dendritic formations; laddered structures of ice spears and lenses; a frozen fringe, from which transverse ice lenses can form. It explains why different researchers have reported the existence of ice lensing with and without the pre-existence of frozen fringes. Our quantitative predictions are encapsulated within a universal, dimensionless phase diagram showing which microstructure is to be expected under which operating conditions, and we show that these predictions are consistent with previous experimental studies as well as new experiments that we present here.Graphical abstractImage 1
  • BCC-FCC interfacial effects on plasticity and strengthening mechanisms in
           high entropy alloys
    • Abstract: Publication date: Available online 17 July 2018Source: Acta MaterialiaAuthor(s): Indranil Basu, Václav Ocelík, Jeff ThM. De Hosson Al0.7CoCrFeNi high entropy alloy (HEA) with a microstructure comprising strain free face-centered cubic (FCC) grains and strongly deformed sub-structured body centered cubic (BCC) grains was subjected to correlative nanoindentation testing, orientation imaging microscopy and local residual stress analysis. Depending on the geometry of BCC-FCC interface, certain boundaries indicated appearance of additional yield excursions apart from the typically observed elastic to plastic displacement burst. The role of interfacial strengthening mechanisms is quantified for small scale deformation across BCC-FCC interphase boundaries. An overall interfacial strengthening of the order of 4GPa was estimated for BCC-FCC interfaces in HEAs. The influence of image forces due to the presence of a BCC-FCC interface is quantified and correlated to the observed local stress and hardness gradients in both the BCC and FCC grains.Graphical abstractImage 1
  • A free energy landscape perspective on the nature of collective diffusion
           in amorphous solids
    • Abstract: Publication date: Available online 17 July 2018Source: Acta MaterialiaAuthor(s): Yun-Jiang Wang, Jun-Ping Du, Shuhei Shinzato, Lan-Hong Dai, Shigenobu Ogata The nature of collective diffusion in amorphous solids is in strong contrast with diffusion in crystals. However, the atomic-scale mechanism and kinetics of such collective diffusion remains elusive. Here the free energy landscape of collective diffusion triggered by single atom hopping in a prototypical Cu50Zr50 metallic glass is explored with well-tempered metadynamics which significantly expands the observation timescale of diffusion at atomic-scale. We clarify an experimentally suggested collective atomic diffusion mechanism in the deep glassy state. The collective nature is strongly temperature-dependent. It evolves from string-like motion with only several atoms to be large size collective diffusion at high temperature, which can promote the atomic transport upon glass transition temperature. We also clarify the apparent diffusivity is dominated by the highest free energy barrier of atomic diffusion among widely distributed free energy barriers due to the dynamic heterogeneity of metallic glass, which suggests the sequential nature of diffusion is a proper assumption to the metallic glasses with dynamic heterogeneity. The temperature and pressure dependence of diffusion free energy landscape are further quantified with activation entropy, (19.6±2.5)kB, and activation volume, (7.9±3.4)Å3, which agree quantitatively with experiments. Laboratory timescale simulations of atomic diffusion brings physical insights into the unique atomic motion mechanism in non-crystalline materials.Graphical abstractThe collection diffusion mechanism and the related free energy surface of amorphous solids is explored with accelerated molecular dynamics which significantly expands the observation time of diffusion process with atomic resolution. The diffusion patterns are recognized over very wide temperature range from deep glassy state to glass transition that has never been achieved via the conventional atomistic simulations. The string-like motion and its temperature dependence are discussed in terms of free energy landscape.Image 1
  • Measurements of Plastic Localization by Heaviside-Digital Image
    • Abstract: Publication date: Available online 12 July 2018Source: Acta MaterialiaAuthor(s): F. Bourdin, J.C. Stinville, M.P. Echlin, P.G. Callahan, W.C. Lenthe, C.J. Torbet, D. Texier, F. Bridier, J. Cormier, P. Villechaise, T.M. Pollock, V. Valle In polycrystalline metallic materials, quantitative and statistical assessment of the plasticity in relation to the microstructure is necessary to understand the deformation processes during mechanical loading. Plastic deformation often localizes into physical slip bands at the sub-grain scale. Detrimental microstructural configurations that result in the formation and evolution of slip bands during loading require advanced strain mapping techniques for the identification of these atomically sharp discontinuities. A new discontinuity-tolerant DIC method, Heaviside-DIC, has been developed to account for discontinuities in the displacement field. Displacement fields have been measured at the scale of the physical slip bands over large areas in nickel-based superalloys by high resolution scanning electron microscopy digital image correlation (SEM DIC). However, conventional DIC methods cannot quantitatively measure plastic localization in the presence of discontinuous kinematic fields such as those produced by slip bands. The Heaviside-DIC technique can autonomously detect discontinuities, providing information about their location, inclination, and identify slip systems (in combination with orientation mapping). Using Heaviside-DIC, discontinuities are physically evaluated as sharp shear-localization events, allowing for the quantitative measure of strain amplitude nearby the discontinuities. Measurements using the new Heaviside-DIC technique are compared to conventional DIC methods for identical materials and imaging conditions.Graphical abstractImage 1
  • A comprehensive first-principles study of solute elements in dilute Ni
           alloys: Diffusion coefficients and their implications to tailor creep rate
    • Abstract: Publication date: Available online 12 July 2018Source: Acta MaterialiaAuthor(s): Chelsey Z. Hargather, Shun-Li Shang, Zi-Kui Liu Diffusion regulates a vast number of materials properties and phenomena such as creep, the focus of the present work. However, a deep understanding of the effect of how each alloying element in a Ni-base superalloy affects properties such as diffusion and creep is far from complete. Here, we report temperature-dependent dilute solute diffusion coefficients and their implications to tailor the creep rate for 26 transition metal solute elements, X's, in fcc Ni using first-principles calculations. Calculations are performed using the five-frequency model for dilute solute diffusion and the nudged elastic band method within the local density approximation. Thermodynamic properties at finite temperatures for all configurations are calculated using the quasi-harmonic Debye model. In general, the fastest diffusing solute elements in Ni are found at the left side of the periodic table and the slowest diffusing solute elements are found in group VIIB. In particular, the present work indicates that the diffusivity of the dilute solute elements is correlated to the compressibility of each solute element on the respective Ni X supercell, and not as strongly to the ionic radius of the solute elements, as previously suggested. Finally, results from the diffusivity study are combined with the previous results of elastic constants and stacking fault energies, and hence, a relative creep rate ratio for these 26 solute elements is modeled. It is shown that in most cases, slower diffusing solute elements provide the most creep resistance. This is true even at higher temperatures, due mainly to the solute's strong bonding with Ni atoms in the host lattice.
  • Influence of deformation induced nanoscale twinning and FCC-HCP
           transformation on hardening and texture development in medium-entropy
           CrCoNi alloy
    • Abstract: Publication date: Available online 12 July 2018Source: Acta MaterialiaAuthor(s): C.E. Slone, S. Chakraborty, J. Miao, E.P. George, M.J. Mills, S.R. Niezgoda Texture evolution during room-temperature tensile testing of recrystallized equimolar CrCoNi was studied using electron backscatter diffraction and electron channeling contrast imaging on specimens from interrupted tests. Dominant deformation mechanisms included slip at low strains and deformation twinning at larger strains, which were accompanied by the development of a strong texture parallel to the tensile axis. Highly deformed material also contained nanotwin/hcp lamellae, which have previously been hypothesized to act as potent barriers for non-coplanar dislocations. To examine this hypothesis, mean-field modeling was performed using the viscoplastic self-consistent framework with varying ratios for hardening by slip and twinning. In the optimal model, twinning produced approximately three times as much non-coplanar hardening as slip, which is larger than previous observations in other twinning-induced plasticity materials that do not form twin/hcp lamellae. Additional full-field elasto-viscoplastic simulations were performed using the fast Fourier transform (EVP-FFT) method to examine intragranular rotation and the effect of initial grain orientation on the deformation mode. Grains with initial orientations near had the greatest propensity for deformation twinning while grains near were more likely to deform by slip even at large strains. Excellent quantitative agreement was obtained between the experiments and EVP-FFT model.Graphical abstractImage 1
  • Phase stability of γ′-Ni2Cr and α-Cr in the
           Ni-Cr binary
    • Abstract: Publication date: 15 September 2018Source: Acta Materialia, Volume 157Author(s): Cody Miller, Robert Field, Michael Kaufman The stability of the γ′-Ni2Cr long-range order (LRO) phase present on the Ni-Cr binary phase diagram was examined in a Ni-55Cr (wt. %) alloy. It is shown that the γ′-Ni2Cr phase is metastable, and that the two-phase γ-Ni + α-Cr phase field extends to room temperature. The as-cast alloy consisted of a typical dendritic microstructure, with Cr-rich primary α-Cr dendrites and a Cr-lean, γ-Ni interdendritic. Annealing the as-cast alloy at 500 °C for 1000 h (γ′-Ni2Cr + α-Cr phase field) resulted in pockets of α-Cr and γ-Ni + γ′-Ni2Cr at the dendritic/interdendritic interface, with little change to the interdendritic γ-Ni microstructure. Annealing at 900 °C for 4 h (γ-Ni + α-Cr phase field) resulted in the interdendritic precipitation of α-Cr, both as a continuous and discontinuous precipitate. Annealing at 900 °C for 4 h followed by 500 °C for 1000 h resulted in the interdendritic precipitation of α-Cr at 900 °C, but no precipitation of γ′-Ni2Cr during subsequent annealing at 500 °C. This behavior suggests that the γ′-Ni2Cr phase is not the equilibrium phase, but rather a metastable transition phase. Furthermore, it is speculated that this phase in more complex alloys (i.e. Alloy 690) is also metastable, and its metastability should be considered in applications involving long-term, high temperature exposures.Graphical abstractImage 1
  • Separating macro- (Type I) and micro- (Type II+III) residual stresses by
           ring-core FIB-DIC milling and eigenstrain modelling of a plastically bent
           titanium alloy bar
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Joris Everaerts, Enrico Salvati, Fatih Uzun, León Romano Brandt, Hongjia Zhang, Alexander M. Korsunsky A novel approach to separating macroscopic (Type I) from microscopic (Type II + III) residual stress is presented, based on Focused Ion Beam – Digital Image Correlation (FIB-DIC) ring-core stress evaluation and eigenstrain modelling. This approach was applied to study the residual stresses for a titanium alloy bar following plastic four-point bending. It was found that electrochemical polishing is a surface preparation technique that is very well suited for FIB-DIC ring-core measurements, in the sense that it removes the influence of prior sample grinding and polishing, leads to a stress profile that satisfies force and moment equilibrium, and thus enables the evaluation of absolute values of total residual stress. The obtained relief strain profile across the bar width is asymmetric, highlighting the difference in the alloy's response to tension and compression. Total experimental residual stress values were calculated using (i) the assumption of material elastic isotropy, with an average Young's modulus, and (ii) under the assumption of elastic anisotropy, taking into account the crystallographic orientation of each investigated grain. Based on the measured relief strain values, the eigenstrain distribution in the bar was reconstructed and used to obtain the macroscopic (Type I) residual stress profile. The differences in the residual stress between the eigenstrain reconstruction values and the individual experimental results were ascribed to the local microscopic (Type II + III) residual stresses. This conclusion was substantiated by revealing the correlation between the residual stress values in individual grains in the elastic zone and their respective Young's moduli in the loading direction, as well as the correlation between the residual stress values in grains located in the plastic zone and their respective Schmid factors for basal slip.Graphical abstractImage 1
  • Structural changes and their effect on Li-ion conductivity upon quenching
           of La(1-x)/3Li x NbO3 solid electrolytes
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Xiaobing Hu, Craig A.J. Fisher, Shunsuke Kobayashi, Yumi H. Ikuhara, Yasuyuki Fujiwara, Keigo Hoshikawa, Hiroki Moriwake, Keiichi Kohama, Hideki Iba, Yuichi Ikuhara La(1-x)/3LixNbO3 (LLNbO) solid solutions constitute a promising family of electrolyte materials for use in all-solid-state lithium-ion batteries (ASSLIBs) because of their good electrochemical stability in contact with Li metal. Even in monocrystalline form, however, their Li-ion conductivities are insufficient for practical use. Post-synthesis heat treatment is a commonly applied technique for modifying nano- and microstructures, and hence properties of materials, although the effect of thermal treatment on A-site deficient layered perovskites is not well understood. Here we combine high temperature in situ X-ray diffraction, atomic-resolution scanning transmission electron microscopy, and molecular dynamics simulations to examine the effect of quenching on LLNbO with Li contents x ≈ 0.05 (Li-poor) and x ≈ 0.1 (Li-rich). Quenching results in a number of nanostructural changes, including weakening of the modulated structure by disordering of La atoms and vacancies within A1 layers, and movement of some La and Li atoms from A1 layers into A2 layers. Rumpling of Nb-O-Nb layers in the [001]p direction also becomes less pronounced and domain boundaries disappear as a result of suppression of NbO6 octahedral tilting. In the case of the Li-poor sample, the Li-ion conductivity decreased by about 66% after quenching, while that of the Li-rich sample increased by about 20%. Thus, despite success in displacing some cations from A1 to A2 layers, the combined effects of quenching failed to increase the Li-ion conductivities to useful levels. More effective means of increasing charge carrier concentrations while decreasing migration barrier energies in LLNbO need to be found if it is to be competitive as a solid electrolyte in ASSLIBs.Graphical abstractImage 1
  • Strengthening mechanisms and Hall-Petch stress of ultrafine grained
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Tianlin Huang, Linfei Shuai, Aneela Wakeel, Guilin Wu, Niels Hansen, Xiaoxu Huang An ultrafine grained Al-0.3 wt %Cu has been produced by cold rolling to a thickness reduction of 98% (εvM = 4.5). The deformed structure is a typical lamellar structure with a boundary spacing of 200 nm as characterized by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Coarsening of the deformed structure to recrystallization is achieved by heat treatment in the range of 100–300 °C. Good thermal stability has been observed up to 175 °C with some segregation of Cu to the boundaries as observed by 3D atom probe characterization. Tensile tests have shown a flow stress (0.2% offset) of 198 MPa with continuous flow with no yield drop and Lüders elongation. To quantify the contribution of boundary strengthening to the flow stress, dislocation strengthening and solid solution hardening have been calculated and subtracted from the flow stress. It has been found that boundary strengthening can be expressed by a Hall-Petch relationship and that the constants in this equation are in very good agreement with previous observation of recrystallized pure polycrystalline aluminium with a grain size in the tens of micrometer range. Thereby the Hall-Petch relationship of aluminium can be extended an order of magnitude from the micrometer to the sub-micrometer range, which is of both scientific and technical importance.Graphical abstractImage 1
  • Slip transmission of high angle grain boundaries in body-centered cubic
           metals: Micropillar compression of pure Ta single and bi-crystals
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Jordan S. Weaver, Nan Li, Nathan A. Mara, David R. Jones, Hansohl Cho, Curt A. Bronkhorst, Saryu J. Fensin, George T. Gray Here we seek to probe and understand the mechanical behavior of grain boundaries in pure Ta as a model bcc material using micropillar compression experiments. Three high angle grain boundaries are chosen with varying crystal orientations. Multiple bi-crystal pillars are prepared containing a single, nearly vertical grain boundary in the approximate center of the pillar and compared against their single crystalline pillar counterparts. The main phenomenon of interest was slip transmission or strain transfer in the bi-crystals which was considered to occur when slip traces aligned across the grain boundary. This occurred in two of the three bi-crystals. These observations were compared against two slip transmission factors, m'=cos(ψ)cos(κ) and LRB=cos(θ)cos(κ), where ψ,θ,andκ are the angles between the slip vectors, slip plane normals, and the intersection of the slip planes with the grain boundary from the slip systems on either side of the grain boundary. Additionally, transmission was compared against the stress-strain response and overall deformation (i.e., sheared boundary) of the bi-crystal. The transmission factors exhibited a consistent behavior with slip transmission occurring for high transmission factors and not occurring for low transmission factors. For example, slip transmission occurred for m'≥ 0.85 and did not occur for m'≤ 0.46. The engineering stress-strain response and overall deformation behavior did not show correlations with the presence or absence of slip transmission. High angle boundaries in bcc metals are shown to represent a diverse set of responses in bi-crystalline micropillar compression experiments.Graphical abstractImage 1
  • Crystal plasticity FEM study of twinning and slip in a Mg single crystal
           by Erichsen test
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): C.S. Hyun, M.S. Kim, S.-H. Choi, K.S. Shin The twinning behavior and deformation mechanisms of a pure magnesium (Mg) single crystal were investigated via Erichsen test at room temperature (RT). In order to establish the unique twinning behaviors according to the position of a deformed specimen, microtexture analyses were performed on two cross-sections of a deformed specimen via the electron backscatter diffraction (EBSD) technique. The EBSD results revealed that thin twin bands with different types of twin variants were developed throughout the deformed specimen. The crystal plasticity finite element method (CPFEM), in relation to both the crystallographic slip and deformation twinning, was used to explain the heterogeneous evolution of the twin bands throughout the deformed specimen during the Erichsen test at RT. CPFEM results such as strain components, relative activity of deformation modes, and accumulated volume fraction of twin variants can effectively explain the experimentally observed heterogeneity of twin bands.Graphical abstractImage 1
  • In-situ high-energy X-ray characterization of neutron irradiated HT-UPS
           stainless steel under tensile deformation
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Chi Xu, Xuan Zhang, Yiren Chen, Meimei Li, Jun-Sang Park, Peter Kenesei, Jonathan Almer, Yong Yang The tensile deformation behavior of a high-temperature, ultrafine-precipitate strengthened (HT-UPS) stainless steel was characterized in-situ with high-energy X-ray diffraction at 20 and 400 °C. The HT-UPS samples were neutron irradiated to 3 dpa at 400 °C. Significant irradiation hardening and ductility loss were observed at both temperatures. Lattice strain evolutions of the irradiated samples showed a strong linear response up to near the onset of the macroscopic yield, in contrast to the unirradiated HT-UPS which showed a pronounced non-linear behavior well below the macroscopic yield. While the room-temperature diffraction elastic moduli in the longitudinal direction increased after irradiation, the 400 °C moduli were similar before and after irradiation. The evolution of the {200} lattice strain parallel to the loading axis (ε{200}L) showed unique characteristics: in the plastic regime, the evolution of ε{200}L after yield is temperature-dependent in the unirradiated specimens but temperature-independent in the irradiated specimens; and the value of ε{200}L at the yield is an irradiation-sensitive, temperature-independent parameter. The evolution of ε{200}L corresponds well with the dislocation density evolution, and is an effective probe of the deformation-induced long-range internal stresses in the HT-UPS steel.Graphical abstractImage 1
  • Segregation assisted grain boundary precipitation in a model Al-Zn-Mg-Cu
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Huan Zhao, Frédéric De Geuser, Alisson Kwiatkowski da Silva, Agnieszka Szczepaniak, Baptiste Gault, Dirk Ponge, Dierk Raabe Understanding the composition evolution of grain boundaries and grain boundary precipitation at near-atomic scale in aluminum alloys is crucial to tailor mechanical properties and to increase resistance to corrosion and stress corrosion cracking. Here, we elucidate the sequence of precipitation on grain boundaries in comparison to the bulk in a model Al-Zn-Mg-Cu alloy. We investigate the material from the solution heat treated state (475 °C), through the very early stages of aging to the peak aged state at 120 °C and further into the overaged regime at 180 °C. The process starts with solute enrichment on grain boundaries due to equilibrium segregation accompanied by solute depletion in their vicinity, the formation of Guinier–Preston (GP) zones in the solute-enriched grain boundary regions, and GP zones growth and transformation. The equilibrium segregation of solutes to grain boundaries during aging accelerates this sequence compared to the bulk. Analysis of the ∼10 nm wide precipitate-free zones (PFZs) adjacent to the solute-enriched grain boundaries shows that the depletion zones are determined by (i) interface equilibrium segregation; (ii) formation and coarsening of the grain boundary precipitates and (iii) the diffusion range of solutes in the matrix. In addition, we quantify the difference in kinetics between grain boundary and bulk precipitation. The precipitation kinetics, as observed in terms of volume fraction, average radius, and number density, is almost identical next to the depletion zone in the bulk and far inside the bulk grain remote from any grain boundary influence. This observation shows that the region influenced by the grain boundaries does not extend beyond the PFZs.Graphical abstractImage 1
  • Evolution of the nanoporous microstructure of sintered Ag at high
           temperature using in-situ X-ray nanotomography
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): X. Milhet, A. Nait-Ali, D. Tandiang, Y.-J. Liu, D. Van Campen, V. Caccuri, M. Legros Silver pastes sintering is a potential candidate for die bonding in power electronic modules. The joints, obtained by sintering, exhibit a significant pore fraction, thus reducing the density of the material compared to bulk silver. The mechanical properties (Young's modulus, yield strength and ultimate tensile stress) are then drastically altered. However, while careful analysis of the nanoporous structure has been reported in 2D, little is known about its quantitative spatial evolution during thermal aging and more specifically during temperature jumps endured by the assembly during operation. Here, high temperature evolutions of the 3D nanoporous structure have been observed in-situ using a heater fitted into the beamline 6-2c at SLAC-SSRL. Segmentation of the porosity and subsequent statistical analysis of the tomographic dataset reveal a complex evolution of the porous nanostructure including growth, separation and coalescence at a constant density. Such an analysis provides insight into the microstructural evolution of sintered nanoporous Ag joints in-service.Graphical abstractImage 1
  • Correlating the five parameter grain boundary character distribution and
           the intergranular corrosion behaviour of a stainless steel using 3D
           orientation microscopy based on mechanical polishing serial sectioning
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): D. An, T.A. Griffiths, P. Konijnenberg, S. Mandal, Z. Wang, S. Zaefferer Using 3D-orientation microscopy consisting of electron backscatter diffraction (EBSD) and serial sectioning by mechanical polishing we investigated the correlation between grain boundary (GB) character expressed by its 5-rotational parameters and the GB precipitation and intergranular corrosion (IC) behaviour on an AISI 304L stainless steel. From the measurements, a volume of approximately 4.8 × 106 μm3 containing 451 grains and approximately 1500 GBs was reconstructed. Dominance of {111} planes for Σ3n (n = 1, 2, 3) GBs was found. In general, the IC behaviour of random high angle grain boundaries (RHAGBs) and low-Σ coincidence site lattice (CSL) boundaries (Σ ≤ 29) does not show a large difference. The corrosion behaviour of low angle grain boundaries (LAGBs) is strongly dependent on the misorientation angles, while the crystallographic GB plane becomes dominant for the behaviour of low-Σ CSL GBs and RHAGBs. It was found that the corrosion resistance is related on the atom packing density (APD) of the GB plane. High corrosion resistance appears for GBs with high APD values (in particular for the (111) planes with a maximum APD value).Graphical abstractImage 1
  • Synchrotron tomographic quantification of the influence of Zn
           concentration on dendritic growth in Mg-Zn alloys
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Sansan Shuai, Enyu Guo, Jiang Wang, A.B. Phillion, Tao Jing, Zhongming Ren, Peter D. Lee Dendritic microstructural evolution during the solidification of Mg-Zn alloys was investigated as a function of Zn concentration using in situ synchrotron X-ray tomography. We reveal that increasing Zn content from 25 wt% to 50 wt% causes a Dendrite Orientation Transition (DOT) from a six-fold snow-flake structure to a hyper-branched morphology and then back to a six-fold structure. This transition was attributed to changes in the anisotropy of the solid-liquid interfacial energy caused by the increase in Zn concentration. Further, doublon, triplon and quadruplon tip splitting mechanisms were shown to be active in the Mg-38 wt%Zn alloy, creating a hyper-branched structure. Using the synchrotron tomography datasets, we quantify, for the first time, the evolution of grain structures during the solidification of these alloys, including dendrite tip velocity in the mushy zone, solid fraction, and specific surface area. The results are also compared to existing models. The results demonstrate the complexity in dendritic pattern formation in hcp systems, providing critical input data for the microstructural models used for integrated computational materials engineering of Mg alloys.Graphical abstractImage 1
  • Analysis of grain topology and volumetric growth rate relation in
           three-dimensional normal grain growth
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Vishal Yadav, Nele Moelans The volumetric growth rate of individual grains as a function of their number of sides, as well as the average normalized volumetric growth rate for a given topological class are studied based on large-scale phase-field simulations of normal grain growth. For all cases, where different initial grain size distributions are considered, the same averaged normalized volumetric growth rate for a given topological class was obtained with the averaged normalized volumetric growth rate approximately zero for grains with Fo≈15 sides. The normalized average grain size as a function of topological class becomes constant within the transient regime and the relation in the transient and steady-state regime is similar and independent of initial grain structure. The relation does however not match Mullins' analytical model [W. Mullins, Acta Metall. 37 (1989) 2979–2984]. The effect of the average number of sides of the neighbouring grains on the average volumetric growth rate for a given topological class is analyzed, as well. Moreover, tracking of the evolution of four grains suggests that the normalized volumetric growth rate of a grain is strongly linked with the number of sides of the grain at the given time.Graphical abstractImage 1
  • Tailoring the formation of textured oxide films via primary and secondary
           nucleation of oxide islands
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Hanlei Zhang, Langli Luo, Xiaobo Chen, Guangwen Zhou Complex hierarchical architectures of nano/microcrystals are of great interests for tuning material properties at the micro- and meso-scale. Unfortunately, the fundamental mechanisms underlying the hierarchical organization of nanocrystals, especially the preferred crystallographic directions and their inter-transitions, remain largely elusive. Herein, we demonstrate the tunability of the crystallographic orientations in textured oxide films through a two-stage nucleation process of oxide nanoislands in metal oxidation. Using in situ environmental transmission electron microscopy, we manipulate the oxidation of copper to result in a dually-structured architecture of Cu2O nanocrystalline films via primary nucleation of cube-on-cube epitaxy of oxide islands, and secondary nucleation of oxide islands on side facets of the primary islands. This dual-nucleation mechanism of oxide islands may find broader applicability in making hierarchical architectures of other nanocrystalline metal oxides.Graphical abstractImage 1
  • Micromechanistic study of textured multiphase polycrystals for resisting
           cold dwell fatigue
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Zhen Zhang A micromechanical study has been conducted on low temperature dwell fatigue resistance in multiphase polycrystalline titanium alloys. The origin of the observed peak in strain rate sensitivity (SRS) over temperature has been explained by the transition from high-stress/low-temperature to low-stress/high-temperature thermally activated dislocation escape. The SRS peak is found to depend considerably on the rate sensitive slip systems in hexagonal close packed (HCP) α phase and body centered cubic (BCC) β phase in Ti alloys. This motivates the study of structural rate dependence, using the SRS peak, in commercially important textured multiphase Ti alloys.It is found that the SRS peak is dependent on texture and phase morphology in multiphase titanium alloys, which is different from some conventional binary alloys. A computational investigation of crystallographic texture shows that stronger rate sensitivity results from polycrystals with higher fractions of grains well-orientated for basal slip activation. This has also been demonstrated in independent experimental studies. Basketweave structures with multiple α variants have been shown to give the lowest SRSs over those with less variant and colony structures. In addition, the SRS peak, for representative textures and morphologies, has been found to be closely related to the creep behaviour in cold dwell fatigue. This understanding is important in microstructural design of titanium alloys for resisting cold dwell fatigue.Graphical abstractImage 1
  • Interpretation of hydrogen-assisted fatigue crack propagation in BCC iron
           based on dislocation structure evolution around the crack wake
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Domas Birenis, Yuhei Ogawa, Hisao Matsunaga, Osamu Takakuwa, Junichiro Yamabe, Øystein Prytz, Annett Thøgersen A new model for hydrogen-assisted fatigue crack growth (HAFCG) in BCC iron under a gaseous hydrogen environment has been established based on various methods of observation, i.e., electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM), to elucidate the precise mechanism of HAFCG. The FCG in gaseous hydrogen showed two distinguishing regimes corresponding to the unaccelerated regime at a relatively low stress intensity factor range, ΔK, and the accelerated regime at a relatively high ΔK. The fracture surface in the unaccelerated regime was covered by ductile transgranular and intergranular features, while mainly quasi-cleavage features were observed in the accelerated regime. The EBSD and ECCI results demonstrated considerably lower amounts of plastic deformation, i.e., less plasticity, around the crack path in the accelerated regime. The TEM results confirmed that the dislocation structure immediately beneath the crack in the accelerated regime showed significantly lower development and that the fracture surface in the quasi-cleavage regions was parallel to the {100} plane. These observations suggest that the HAFCG in pure iron may be attributed to “less plasticity” rather than “localized plasticity” around the crack tip.Graphical abstractImage 1
  • Revealing the novel fracture mechanism of the interfaces of TiB2/Fe
           composite from a first principles investigation
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Y.F. Li, B. Xiao, G.L. Wang, L. Sun, Q.L. Zheng, Z.W. Liu, Y.M. Gao We investigate the atomic structures, chemical bonding, stability and fracture mechanism of B- and Ti-terminated incoherent TiB2 (0001)/Fe (111) and semi-coherent TiB2 (0001)/Fe (100) interfaces using first-principles calculations. It is found that all Ti-terminated interfaces (Ti-HCP, Ti-MT and Ti-OT) as well as B-HCP type TiB2 (0001)/Fe (100) interface are non-diffusive type. Meanwhile, B-HCP, B-MT and B-OT configurations of TiB2 (0001)/Fe (111) interface are diffusive type due to the formation of additional FexB intermetallic compound at the original Fe/TiB2 interface. The calculated works of adhesion and interfacial energies indicate that Ti-HCP and B-HCP are the most stable structures for both incoherent and semi-coherent interfaces. We find that the magnitude of interfacial elastic energy is comparable to that of the chemical energy for the semi-coherent TiB2 (0001)/Fe (100) interface. The electronic structures of TiB2/Fe interfaces reveal the formation of Fe-B, Ti-B and Fe-Ti bonds at or next to the interface. Using Griffith's theory, it is predicted that the mechanical failure of TiB2/Fe composite would initiate at the interface between TiB2 and Fe. The first principles tensile experiment performed on all Ti-HCP interfaces agrees with the prediction. In the case of B-HCP interfaces, due to the formation of either FexB diffusive layer or strong covalent Fe-B bonds, the mechanical failure eventually occurs in the Fe slab rather than that predicted by Griffith's theory. We also find the formation of diffusive FexB layer could significantly suppress the local magnetic moment of Fe atom at TiB2/Fe interface due the formation of strong covalent Fe-B bond.Graphical abstractBonds length and relevant bond angle with respect to x-y plane as a function of strain during the tensile test for B-terminated HCP type interface. The mid-point charge density values of interfacial bonds are shown on the right.Image 1
  • Discrete Dislocation Dynamics simulations of dislocation transport during
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): J. Gagel, D. Weygand, P. Gumbsch Plastic deformation largely determines the properties of tribological contacts. To modify such contacts, it is thus necessary to understand the underlying mechanisms of dislocation motion and multiplication. In a sliding contact, dislocations do not only multiply, but also follow the moving tip — and are therefore ”transported” by the tip. Three dimensional Discrete Dislocation Dynamics simulations of sliding with a spherical tip are conducted to analyse dislocation transport in an fcc crystal, at stresses below those typically required to nucleate dislocations at a free surface. Sliding direction, glide plane orientation and Burgers vector orientation determine whether a dislocation can be transported or not, and how far the transport leads. A model is proposed to identify the glide systems, on which dislocations are transported.Graphical abstractImage 1
  • From metallic glasses to nanocrystals: Molecular dynamics simulations on
           the crossover from glass-like to grain-boundary-mediated deformation
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Tobias Brink, Karsten Albe Nanocrystalline metals contain a large fraction of high-energy grain boundaries, which may be considered as glassy phases. Consequently, with decreasing grain size, a crossover in the deformation behaviour of nanocrystals to that of metallic glasses has been proposed. Here, we study this crossover using molecular dynamics simulations on bulk glasses, glass–crystal nanocomposites, and nanocrystals of Cu64Zr36 with varying crystalline volume fractions induced by long-time thermal annealing. We find that the grain boundary phase behaves like a metallic glass under constraint from the abutting crystallites. The transition from glass-like to grain-boundary-mediated plasticity can be classified into three regimes: (1) For low crystalline volume fractions, the system resembles a glass–crystal composite and plastic flow is localised in the amorphous phase; (2) with increasing crystalline volume fraction, clusters of crystallites become jammed and the mechanical response depends critically on the relaxation state of the glassy grain boundaries; (3) at grain sizes ≥10nm, the system is jammed completely, prohibiting pure grain-boundary plasticity and instead leading to co-deformation. We observe an inverse Hall–Petch effect only in the second regime when the grain boundary is not deeply relaxed. Experimental results with different grain boundary states are therefore not directly comparable in this regime.Graphical abstractImage 1
  • Measurement of anisotropic coefficients of thermal expansion of SAC305
           solder using surface strains of single grain with arbitrary orientation
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Bulong Wu, Yu-Hsiang Yang, Bongtae Han, Joshua Schumacher The anisotropic coefficient of thermal expansions (CTEs) of SAC305 grain are measured using a full-field in-plane displacement measurement technique. Theoretical relationships among (1) the transversely-isotropic CTEs, (2) the surface strains of a specimen containing a single grain with arbitrary orientation, and (3) the direction of a grain orientation are derived first. Cube shape specimens that contain a single SAC305 grain are fabricated by controlling cooling rates. Thermally-induced displacements fields with a sub-micron resolution are documented on two perpendicular surfaces of the specimen as a function temperature, and the engineering strains are calculated from the displacement fields. Two directional CTE values are determined from the theoretical relationships. The direction of the c-axis is also obtained during CTE calculations. The validity of the measurement is corroborated by comparing the c-axis direction obtained from the experiment with a grain orientation measured by the electron backscatter diffraction (EBSD) method.Graphical abstractImage 1
  • Femtosecond laser rejuvenation of nanocrystalline metals
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Glenn H. Balbus, McLean P. Echlin, Charlette M. Grigorian, Timothy J. Rupert, Tresa M. Pollock, Daniel S. Gianola Nanocrystalline metals are distinct from traditional engineering materials due to their high concentration of grain boundaries and corresponding structural disorder at grain boundaries. The effect of local disorder in nanocrystalline materials manifests in ways reminiscent of fully amorphous materials, such as mesoscale shear localization and pressure-dependent yielding, owing to the high concentration of grain boundaries and their predominance in governing plasticity. Relaxation processes in nanocrystalline materials that facilitate reconfigurations of grain boundaries and lower their energy, such as low temperature annealing, have been shown to enhance mechanical strength. However, processes that raise the energy of a nanocrystalline metal have not been observed, limiting the tunability of properties and the prospect for suppressing shear localization. Here, we use femtosecond laser processing as a unique non-equilibrium process that can generate complex stress states due to ultrafast electronic excitation and subsequent relaxation events. Experiments on nanocrystalline Al-O and Cu-Zr alloys indicate that sub-ablation femtosecond laser pulses cause up to an 87% reduction in hardness with no change in grain size, which can be ascribed to grain boundary-mediated processes. Parallels between our results and rejuvenation processes in glassy systems will be discussed in the context of controlling metastable structural configurations through novel processing routes.Graphical abstractImage 1
  • On the first direct observation of de-twinning in a twinning-induced
           plasticity steel
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Scott J. McCormack, Wei Wen, Elena V. Pereloma, Carlos N. Tomé, Azdiar A. Gazder, Ahmed A. Saleh Electron back-scattering diffraction was used to track the microstructure evolution of a fully annealed Fe-24Mn-3Al-2Si-1Ni-0.06C TWinning Induced Plasticity (TWIP) steel during interrupted reverse (tension-compression) loading. Direct observation of the same selected area revealed that all deformation twins formed during forward tension loading (0.128 true strain) were removed upon subsequent reverse compression loading (0.031 true strain). Consequently, the present study provides the first unambiguous experimental evidence of de-twinning during the reverse loading of a polycrystalline TWIP steel. The reverse loading behaviour was simulated by a dislocation-based hardening model embedded in the Visco-Plastic Self-Consistent (VPSC) polycrystal framework taking into account the accumulation and annihilation of dislocations and back-stress effects. The model has been extended to account for the processes of twinning and de-twinning, as well as the twin barrier effect under load reversal. A new formulation based on the changes in the dislocation mean free path is proposed to track twin lamellae generation/annihilation throughout deformation along with its associated effect on hardening.Graphical abstractImage 1
  • Nano-precipitates evolution and their effects on mechanical properties of
           17-4 precipitation-hardening stainless steel
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Zemin Wang, Hui Li, Qin Shen, Wenqing Liu, Zhanyong Wang Precipitates play significant roles in materials design but in-depth understanding of the evolution of nano-scale precipitates in complex martensitic steels is still limited. In this study, the evolution of nano-precipitates and their effects on the mechanical properties of 17-4 precipitation hardening stainless steel (17-4 PH SS) aged at 450 °C were investigated by high-resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT). The results from APT revealed that the abundant nucleation of Cu-rich clusters was responsible for the initial significant hardening effect during aging. Core-shell structured Cu-rich precipitates (CRPs) were formed at peak aging condition. As aging was prolonged, co-precipitation of Ni, Mn, Si and Nb-rich precipitates (NMSN) and CRPs occurred at the expense of core-shell CRPs precursors with untwinned 9R structure. Ni, Si and (Mn, Nb) atoms were preferentially segregated at the (009)9R plane of twinned or W-shaped twinned CRPs, with atom ratio of 16:7:6. Furthermore, as CRPs became coarser, Cr atoms were rejected from CRPs into the matrix enabling nucleation of Cr-rich α′domains. The density and size of Cr-rich α′ domains increased as aging was prolonged to 200 h. Finally, the individual contributions of CRPs and Cr-rich α′ domains were calculated. Overall, these findings indicated the evolution of multiple precipitates and their interactive effects, which can be helpful for the design and fabrication of next generation PH steels for wide applications.Graphical abstractImage 1
  • Microstructure of a Dy-free Nd-Fe-B sintered magnet with 2 T
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): X.D. Xu, T.T. Sasaki, J.N. Li, Z.J. Dong, H. Sepehri-Amin, T.H. Kim, T. Ohkubo, T. Schrefl, K. Hono We investigated the microstructure of the Dy-free high coercivity sintered magnet that was developed by optimizing the chemical composition of Nd-Fe-B sintered magnet with 0.1 at.%Ga doping in order to understand the reasons for the high coercivity of μ0Hc = 2.0 T and the improved squareness of 0.95 compared to a recently developed 0.5 at. %Ga doped magnet. The as-sintered 0.1Ga sample shows a relatively high coercivity of 1.5 T owing to the finer grain size of ∼3.3 μm and the higher Al concentration up to 1.3 at.% compared to post-sinter annealed 0.5Ga magnet that has a grain size of ∼5.5 μm and Al concentration of 0.9 at. %. The post-sinter annealing leads to a substantial coercivity increase from 1.5 to 2.0 T due to the formation of a thick and crystalline intergranular grain boundary (GB) phase containing 30-60 at.% of Nd. The trace Ga addition of 0.1 at.% improved the wettability of the liquids and facilitated the formation of thick GB phase during the post-sinter annealing. The good squareness is mainly attributed to the ferromagnetic nature of the intergranular GB phase as well as the strong crystal alignment to the easy axis. Finite element micromagnetic simulations of the demagnetization processes of the models incorporating experimentally determined GB thickness and chemistry well explain the simultaneous achievement of the high coercivity and good squareness observed in this magnet in contrast to the very poor squareness observed in the post-sinter annealed 0.5Ga sample.Graphical abstractImage 1
  • Coercivity improvement of hot-deformed Nd–Fe–B magnets by
           stress-induced Pr–Cu eutectic diffusion
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Zexuan Wang, Jijun Zhang, Jinzhi Wang, Jinyun Ju, Renjie Chen, Xu Tang, Wenzong Yin, Don Lee, Aru Yan Low-melting eutectic diffusion has emerged as a practical approach to enhance the coercivity of hot-deformed Nd–Fe–B magnets. Herein, we report a high-efficient stress-induced eutectic diffusion method to develop high-coercivity hot-deformed Nd–Fe–B magnets. In this method, fast Pr–Cu eutectic diffusion is accomplished with the hot-pressed and hot-deformed processes simultaneously. The coercivity of diffusion-processed magnets is substantially increased by about 33% with a slight remanence loss. The applied stress effectively controls the distribution of Pr–Cu eutectic and relieves the remanence loss. Microstructure analysis confirms that the Pr–Cu concentration gradient constructed in hot-pressed precursors effectively suppresses the near-surface grain growth during the hot-deformation process and reduces the lateral/longitudinal ratio of platelet-shaped grains. The formation of thick and continuous Pr-rich grain boundaries weakens the exchange coupling between neighboring grains, offering more “pinning” sites for domain wall displacement. Partially Pr-substitution improves the locally magnetically “hardening” of 2:14:1 main phases. The optimized structural and magnetic characterizations hinder the nucleation and propagation of reversed magnetic domains at low magnetic fields, which is beneficial to coercivity enhancement. These findings clarify the characteristics of the stress-induced diffusion method and give a new insight into the structural modulation for Nd–Fe–B materials.Graphical abstractImage 1
  • Cu / Nb &rft.title=Acta+Materialia&rft.issn=1359-6454&">Effects of interface shear strength during failure of semicoherent
           Metal–Metal nanolaminates: An example of accumulative roll-bonded
           Cu / Nb
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): I. Radchenko, H.P. Anwarali, S.K. Tippabhotla, A.S. Budiman Plastic deformation mechanisms in metal–metal nanolayer composites (nanolaminates) have been studied extensively during the last decade. It has been observed that, for the case of metal–metal nanolaminates with a semicoherent interface, such as Cu/Nb, low interface shear strength increases the interface barrier to dislocation crossing, which improves nanolaminate plasticity. In this study, we use Cu (63 nm)/Nb(63 nm) accumulative roll-bonded nanolaminates, which have a large anisotropy of the interface shear strength between rolling and transverse directions (RD and TD, respectively), to study the effect of interface shear strength on the failure in metal–metal nanolaminates with a semicoherent interface during in situ clamped beam bending. Further, finite element analysis is used to understand the observed behavior. The results show a substantial difference between the fracture behaviors along the RD and TD owing to differences in the interface shear strength and grain size. For the RD beams, the slip bands originate from the Nb layers at the notch/crack tip followed by crack propagation along these bands. For the TD beams, the crack propagation is inhibited by interface shear. We suggest that shear bands form subsequently through the beam and lead to the final beam failure. However, under the assumption of the presence of the grain boundaries near the stress concentration zone, the interface shear in the TD beams could be inhibited. In this case, the crack growth can be attributed to the formation of microcracks at grain boundaries beside the main crack.Graphical abstractImage 1
  • On conventional versus direct ageing of Alloy 718
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): F. Theska, A. Stanojevic, B. Oberwinkler, S.P. Ringer, S. Primig Alloy 718 is a Ni-based superalloy commonly used for parts of aircraft engines and power plants where very high demands on high-temperature yield strength, creep and corrosion resistance must be met. Polycrystalline parts such as turbine discs are industrially manufactured via hot-forging followed by solution annealing and dual ageing to form nanoscale γ′ and γ" precipitates. However, through the alternative process of ‘direct ageing’, increased yield strength contributions of ∼10% can be achieved while maintaining sufficient creep resistance. In addition to this so-called ‘direct ageing effect’, the omission of solution annealing between forging and ageing is economically attractive. Therefore, to date, direct ageing is widely implemented in the production of forged aerospace parts. However, the detailed mechanisms behind the direct ageing effect yet remain unclear. We present a correlative microscopy approach to identify the detailed microstructural evolution during conventional versus direct ageing. Our results confirm higher dislocation densities, lower δ-phase volume fractions and the absence of selected γ-matrix grain growth, as suggested in previous research. However, a key finding reported here via the use of atom probe microscopy is a remarkable structuring within and between the nanoscale γ′ and γ" precipitation. Not only do we find an increase in both the volume fraction and size of the γ" precipitates after direct ageing, we report a prevalence of stacked co-precipitation in various sequences depending on the nucleation condition. The findings are summarized in a microstructural model for conventional and direct ageing.Graphical abstractImage 1
  • α / ω +thermal+stability+in+shocked+Zr:+A+coupling+between+dislocation+removal+and+phase+transformation&rft.title=Acta+Materialia&rft.issn=1359-6454&">Modeling the α / ω thermal stability in shocked Zr: A coupling between
           dislocation removal and phase transformation
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): T.S.E. Low, S.R. Niezgoda Under high pressure, Zr undergoes a transformation from its ambient equilibrium hexagonal close packed α phase to a simple hexagonal ω phase. Subsequent unloading to ambient conditions does not see a full reversal to the α phase, but rather a retainment of significant ω. Previously, the thermal stability of the ω phase was investigated via in-situ synchrotron X-ray diffraction analysis of the isothermal annealing of Zr samples shocked to 8 and 10.5 GPa at temperatures 443, 463, 483, and 503 K [25]. The phase volume fractions were tracked quantitatively and the dislocation densities were tracked semi-quantitatively. Trends included a rapid initial (transient) transformation rate from ω→α followed by a plateau to a new metastable state with lesser retained ω (asymptotic). A significant reduction in dislocation densities in the ω phase was observed prior to initiation of an earnest reverse transformation, leading to the hypothesis that the ω→α transformation from is being hindered by defects in the ω phase. As a continuation of this work, we present a temperature dependent model that couples the removal of dislocations in the ω phase and the reverse transformation via a barrier energy that is associated with the free energy of remaining dislocations. The reduction of dislocations in the ω phase occurs as a sum of glide and climb controlled processes, both of which dictate the transient and asymptotic behavior of the annealing process respectively.Graphical abstractImage 1
  • ε -Al60Sm11+phase+during+devitrification+of+Al-10.2 at.%+Sm+glasses&rft.title=Acta+Materialia&rft.issn=1359-6454&">Spatially-correlated site occupancy in the nonstoichiometric meta-stable
           ε -Al60Sm11 phase during devitrification of Al-10.2 at.% Sm glasses
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Lin Yang, Feng Zhang, Fan-Qiang Meng, Lin Zhou, Yang Sun, Xin Zhao, Zhuo Ye, Matthew J. Kramer, Cai-Zhuang Wang, Kai-Ming Ho A metastable ε-Al60Sm11 phase appears during the initial devitrification of as-quenched Al-10.2 at.% Sm glasses. The ε phase is nonstoichiometric in nature since Al occupation is observed on the 16f Sm lattice sites. Scanning transmission electron microscopic images reveal profound spatial correlation of Sm content on these sites, which cannot be explained by the “average crystal” description from Rietveld analysis of diffraction data. Thermodynamically favorable configurations, established by Monte Carlo (MC) simulations based on a cluster-expansion model, also give qualitatively different correlation functions from experimental observations. On the other hand, molecular dynamics simulations of the growth of ε-Al60Sm11 in undercooled liquid show that when the diffusion range of Sm is limited to ∼4 Å, the correlation function of the as-grown crystal structure agrees well with that of the scanning transmission electronic microscopy (STEM) images. Our results show that kinetic effects, especially the limited diffusivity of Sm atoms plays the fundamental role in determining the nonstoichiometric site occupancies of the ε-Al60Sm11 phase during the crystallization process.Graphical abstractImage 1
  • Microstructural and mechanical characterization of an equiatomic YGdTbDyHo
           high entropy alloy with hexagonal close-packed structure
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): R. Soler, A. Evirgen, M. Yao, C. Kirchlechner, F. Stein, M. Feuerbacher, D. Raabe, G. Dehm The microstructural and mechanical characterization of an equiatomic YGdTbDyHo high entropy alloy with hexagonal close-packed structure was performed. The phase state and chemical homogeneity of the solid solution were analysed with respect to crystal structure, phase stability, and oxide formation. It was found that Y-rich precipitates form at grain boundaries and that the alloy is prone to oxidation, leading to a homogeneous distribution of ∼10 nm-sized oxides in the grain interiors. The plastic response at the sub-grain level was studied in terms of the activated slip systems, critical resolved shear stresses (CRSS), and strain hardening using micropillar compression tests. We observe plastic slip on the basal system, with a CRSS of 196 ± 14.7 MPa. Particle strengthening and strength dependence on sample size are discussed on the basis of dislocation particle interaction and mechanical size effects.Graphical abstractImage 1
  • Three-dimensional grain growth in pure iron. Part I. statistics on the
           grain level
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Jin Zhang, Yubin Zhang, Wolfgang Ludwig, David Rowenhorst, Peter W. Voorhees, Henning F. Poulsen Grain evolution in pure iron is determined in three dimensions using diffraction contrast tomography at a synchrotron source. During annealing for 75 min at 800∘C, the evolution of initially 1327 grains is quantified as a function of 15 time-steps. A comprehensive statistical analysis is provided based on the equivalent radius, the number of faces and the mean width parameters of the grains. We introduce analytical relations between these parameters, validate them, and discuss their physical meaning. While the sample is fully recrystallized, the growth is found not to be self-similar, as evidenced in changes in the distributions of normalized grain size and number of faces per grain. More importantly, a strong decrease in the slope of the growth rate over the mean width of grain faces is observed, indicating a slowdown of grain growth. The data is used to determine the applicability of the isotropic MacPherson-Srolovitz theory to an anisotropic material such as iron. Geometrical properties that are averaged over the entire grain ensemble are well described by the model, but the properties and evolution of the individual grains exhibit substantial scatter.Graphical abstractImage 1
  • Sulfur – induced embrittlement in high-purity, polycrystalline
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Thorsten Meiners, Zirong Peng, Baptiste Gault, Christian H. Liebscher, Gerhard Dehm Tensile tests were carried out in high-purity, polycrystalline copper alloys with three concentrations of sulfur impurities (14, 27 and 7920 at ppm) at temperatures between 20 °C and 400 °C. The ductility drops with increasing sulfur concentration and temperature while the ultimate tensile strength increases. The alloys exhibit a grain size of several millimeters and contain mostly random grain boundaries (GBs). The microstructure and composition is investigated by transmission electron microscopy (TEM) and atom probe tomography (APT). The microstructure of the samples with sulfur contents of 14 and 27 ppm consists of globular grains and neither of the microanalytical techniques employed reveals the formation of Cu-sulfides or sulfur segregation to GBs. Even after annealing at 500 °C, no sulfide formation or sulfur segregation to GBs was detected. In the alloy with a sulfur content of 7920 ppm, a dendritic structure is observed and in the interdendritic region monoclinic Cu2S precipitates with a size range from 5 nm to several μm are observed at GBs and also within the grains. The influence of S on the ductility is discussed considering the TEM and APT results.Graphical abstractImage 1
  • Al2Cu+eutectic+alloy&rft.title=Acta+Materialia&rft.issn=1359-6454&">Plasticity of laser-processed nanoscale AlAl2Cu eutectic alloy
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): S.J. Wang, G. Liu, D.Y. Xie, Q. Lei, B.P. Ramakrishnan, J. Mazumder, J. Wang, A. Misra Deformation behavior of as-cast and laser re-melted AlAl2Cu eutectic alloys subjected to room temperature rolling was investigated by transmission electron microscopy. For as-cast alloys, with inter-lamellar spacing of a few micrometers, Al2Cu layers exhibit brittle behavior but plastic co-deformation was observed in rolled, laser processed nanoscale alloys. The nanoscale Al2Cu lamellae, constrained by nanoscale Al, deform via defects not previously reported in monolithic Al2Cu intermetallic: localized shear on {011}Al2Cu planes and shear-induced faults on {121}Al2Cu planes. Based on crystallographic analysis of slip continuity across the interface, the unexpected plasticity mechanisms are ascribed to the slip continuity across interface between α-Al and θ-Al2Cu layers associated with the orientation relationship in AlAl2Cu eutectics. The difference in shear mechanisms on the two planes is attributed to low energy faulted structures associated with shear on {011}Al2Cu and {121}Al2Cu planes, as examined by first-principles density functional theory calculations. These findings demonstrated that nanoscale microstructures promote plastic co-deformation in metallic composites with soft and hard phases.Graphical abstractImage 1
  • Temperature-insensitive electric-field-induced strain and enhanced
           piezoelectric properties of textured (K,Na)NbO3-based lead-free
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Haibo Zhang, Yiwei Zhu, Pengyuan Fan, Mohsin Ali Marwat, Weigang Ma, Kai Liu, Hongming Liu, Bing Xie, Ke Wang, Jurij Koruza Texturing is an effective method to improve the piezoelectric properties of lead-free piezoceramics. In the present study, textured (K,Na)NbO3-based piezoceramics with a Lotgering factor of 89.7% were successfully fabricated by the templated grain growth (TGG) method. Compared with its non-textured counterparts, great enhancements of the piezoelectric coefficient (50% improvement) and electromechanical coupling factor are observed in the textured samples. More importantly, electric-field-induced strain is also remarkably improved, as evident by a 17% higher large-signal piezoelectric coefficient (346 pm/V) of the textured sample. A relatively low strain deviation of 2% is observed between room temperature and 150 °C, and even up to 175 °C, 86% of their room temperature large-signal piezoelectric coefficient value is maintained. This temperature stability of textured (K,Na)NbO3-based piezoceramics is of great significance for actuators with a stable response over a broad temperature range.Graphical abstractBy texturing, a great enhancement of piezoelectric properties and temperature stability was achieved in KNN-based lead-free piezoceramics, which was ascribed to the wide temperature range of tetragonal phase and the stable ferroelectric domains. These favorable properties show its promising application for high-temperature actuators.Image 1
  • A novel approach to investigate delta phase precipitation in cold-rolled
           718 alloys
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Zi Kang Low, Thibaut Chaise, Didier Bardel, Sophie Cazottes, Philippe Chaudet, Michel Perez, Daniel Nelias This paper proposes a numerical alternative to the lengthy experimental approaches typically employed to characterize delta phase precipitation in 718 alloys. A high-throughput experimental case study is first performed on cold-rolled alloy 718 of known composition and initial microstructure. Direct resistance heating is used to generate highly heterogeneous thermal fields, which enables investigation of a wide range of temperatures with relatively few experiments. Through a coupled finite element and Kampmann-Wagner numerical model, topographies of delta phase characteristics resulting from the complex heat treatments are generated. The predicted precipitation state shows good agreement with scanning electron microscopy observations of the heat-treated samples, demonstrating the validity of the proposed numerical approach.Graphical abstractImage 1
  • Ab initio study of energetics and structures of heterophase interfaces:
           From coherent to semicoherent interfaces
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): Song Lu, John Ågren, Levente Vitos Density functional theory calculations have been performed to study the structures and energetics of coherent and semicoherent TiC/Fe interfaces. A systematic method for determining the interfacial energy of the semicoherent interface with misfit dislocation network has been developed. The obtained interfacial energies are used to evaluate the aspect ratio for the plate-like precipitate and a quantitative agreement with the experimental results is reached. Based on the obtained interfacial energies and atomic structure details, we propose two scenarios for heterogeneous nucleation on an edge dislocation, shedding light on the thermodynamics of precipitate nucleation and growth. The present method can be easily applied to any heterophase interfaces between metals and oxides/carbides/nitrides.Graphical abstractImage 1
  • ( 10 1 ¯ 2 ) +twin+boundary+in+titanium&rft.title=Acta+Materialia&rft.issn=1359-6454&">First-principles prediction of oxygen diffusivity near the
           ( 10 1 ¯ 2 ) twin boundary in titanium
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): M.S. Hooshmand, C. Niu, D.R. Trinkle, M. Ghazisaeidi We study the diffusivity of oxygen interstitials around a (101¯2) twin boundary in Titanium. First, we identify all possible stable interstitial sites around the twin boundary and compute the corresponding site energies and transition energy barriers for jumps between these sites, using density functional theory. We show that the site energies and the barriers are consistently lower than in bulk, suggesting the higher tendency of oxygen to segregate to the twin boundary region. Using the site and transition energies and an exact solution to the master equation, we then compute the diffusivity of oxygen in the presence of the twin boundary and find enhanced diffusivity around the boundary in all directions. Enhanced diffusivity towards the boundary determines the feasibility of oxygen segregation to favorable sites at the boundary, while increased diffusivity in the boundary plane provides a path for fast diffusion of oxygen. This result reveals the underlying mechanism governing the slow growth of (101¯2) twin by pinning at the segregated oxygen interstitials.Graphical abstractImage 1
  • Shape-memory characterization of NiTi microtubes fabricated through
           interdiffusion of Ti-Coated Ni wires
    • Abstract: Publication date: 1 September 2018Source: Acta Materialia, Volume 156Author(s): A.E. Paz y Puente, D.C. Dunand Near-equiatomic NiTi microtubes were fabricated using an additive alloying method consisting of two steps: (i) depositing a Ti-rich coating onto ductile, pure Ni wires (50 μm in diameter) via pack cementation, resulting in a Ni core coated with concentric NiTi2, NiTi and Ni3Ti shells, and (ii) homogenizing the coated wires to near-equiatomic NiTi composition via interdiffusion between core and shells, while concomitantly creating Kirkendall pores. Because of the spatial confinement and radial symmetry of the interdiffusing core/shell structure, the Kirkendall pores coalesce near the center of the wire and form a continuous longitudinal channel, thus creating a microtube. Both the mechanical and thermal response of the NiTi microtubes were characterized in this study using a combination of dynamic mechanical analysis and differential scanning calorimetry, respectively, in conjunction with conventional metallography and X-ray tomographic microscopy. Due to slight compositional variations, both shape-memory and superelastic behaviors were observed within the same microtube, which achieved a total tensile strain of ∼8% before failure: the largest contribution to the strain recovery was the thermal shape memory effect showing near complete strain recovery occurring during multiple cycles. A second microtube exhibited only superelastic behavior, achieving a maximum, recoverable strain of 2.5% at 110 MPa, likely limited by the presence of a remaining Ni3Ti core as a result of under-titanization. Finite-element analysis of elastic stresses in a wire segment modeled from actual tomography data illustrates the extent of stress concentrations due to inner and outer tube surface roughness. The stress concentrations are responsible for a 65% increase in the top 1% average von Mises stress, which may further affect the shape-memory behavior of the tubes.Graphical abstractImage 1
  • High thermoelectric performance in Cu-doped Bi2Te3 with carrier-type
    • Abstract: Publication date: Available online 11 July 2018Source: Acta MaterialiaAuthor(s): Hsin-Jay Wu, Wan-Ting Yen For decades, the Bismuth-telluride (Bi2Te3) has been intensively studied as the thermoelectric (TE) cooler. Still, new ideas and emerging results are put forward for raising the conversion efficiency. Herein, we re-visit the Cu-doped Bi2Te3, and report the high zT values nearing the room temperature, for both the p-type (Cu2Te)0.01(Bi2Te3)0.99/Cu0.01Bi1.99Te3 (zT∼1.2 at 300 K) as well as the n-type (Cu2Te)0.09(Bi2Te3)0.91 (zT∼1.09 at 363 K), respectively. Given that the phase boundary mapping is essential for the optimization of high-efficiency TE materials, the isothermal section of ternary Bi-Cu-Te at 523 K is constructed, by collecting the phase equilibria information of various thermally-equilibrated alloys; it further guides the alloying directions for (Cu2Te)x (Bi2Te3)1-x and CuyBi2-yTe3, respectively. Small modulation in the stoichiometry leads to the carrier type transition. As a consequence, the lamellae composed of Bi2Te3 and Cu7Te5 precipitate along the grain boundary of the n-type (Cu2Te)0.09(Bi2Te3)0.91, resulting in the reduced κ, due to the stronger interfacial phonon scattering, and the higher PF, owing to the higher amounts of Te vacancy (VTe). On the contrary, the promising p-type (Cu2Te)0.01(Bi2Te3)0.99/Cu0.01Bi1.99Te3 features the single-phase Bi2Te3, whereas the soluble Cu introduces extra holes and might therefore promote the p-type conduction.Graphical abstractImage 1
  • Novel insight into the chemical analysis of light elements in oxycarbides
    • Abstract: Publication date: Available online 11 July 2018Source: Acta MaterialiaAuthor(s): F. Réjasse, O. Rapaud, J. Léchelle, G. Trolliard, H. Khodja, O. Masson, G. Martin, A. Maître Pure powders of TiCxO(1-x) solid solution were synthesized through the carbothermal route. The chemical analysis of the light elements in the as-obtained TiCxO(1-x) oxycarbides powders were performed by Instrumented Gas Analysis (IGA). The cell parameters of the samples were determined with an accuracy of about 2% by means of X-ray powder diffraction and the internal standard method. As a result, a model correlating the cell parameters to the chemical composition was established. These reference TiCxO(1-x) oxycarbides powders were then sintered in order to obtain pellets of dense ceramics. After having determined that the sintering process does not change the chemical composition of the starting powder, chemical analysis of the different samples of the solid solution were successfully undertaken by Ion Beam Analysis techniques (IBA). The Nuclear Reaction Analysis (NRA) method - that was used to analyse light elements with very high sensitivity - was coupled with Rutherford Back Scattering (RBS) analysis in order to accurately determine the metallic over light elements ratio and to determine the stoichiometry of the phase on massive samples. Exhaustive simulations of the NRA spectra were performed and demonstrated that discrete compositions of the TiCxO(1-x) can be efficiently measured locally for bulk samples. Compared to IGA results, the relative amounts of carbon and oxygen of bulk materials were determined with a bias lower than 5%. This protocol being implemented for the TiCxO(1-x) system was then tested on HfCxO(1-x)with the same success.Graphical abstractImage 1
  • Unveiling the formation of basal texture variations based on twinning and
           dynamic recrystallization in AZ31 magnesium alloy during extrusion
    • Abstract: Publication date: Available online 11 July 2018Source: Acta MaterialiaAuthor(s): M.G. Jiang, C. Xu, H. Yan, G.H. Fan, T. Nakata, C.S. Lao, R.S. Chen, S. Kamado, E.H. Han, B.H. Lu Commercial Mg extrusions usually develop various types of basal textures, affecting the formability and mechanical properties. The mechanisms for the formation of basal texture variations were systematically investigated for the first time based on the twinning and dynamic recrystallization (DRX) during extrusion using electron back-scatter diffraction (EBSD) characterization. The results indicate that {101¯2} extension twins with various variants played a key role in the formation of [101¯0] fiber texture, while twin-induced DRX mechanism associated with {101¯1} twins made a limited contribution to overall texture evolution. The microstructure developed consecutively as a result of continuous DRX (CDRX) and discontinuous DRX (DDRX) after twinning with different nucleation of new orientations. In the unDRXed region, new DRXed grains with 30° [0001] GBs preferentially nucleated via CDRX, forming preferred selection of [21¯1¯0] fiber orientation. Consequently, [101¯0] fiber texture became weakened with [21¯1¯0] fiber component strengthening, progressively leading to the development of [101¯0]-[21¯1¯0] double fiber texture. In the DRXed region, DDRX further occurred along the serrated grain boundaries and at the triple junctions of the parent grains by bulging, forming fresh DRXed grains without preferred orientation selection. Thus, the [101¯0]-[21¯1¯0] double fiber texture became randomized and gradually transformed into non-fiber texture at the late stage of extrusion. Besides, typical nucleation sites for DRX and subsequent grain growth were also discussed in this study. These findings unveil that basal texture variations are attributed to the twinning and DRX mechanisms during extrusion, which leads to new insight to design new wrought Mg alloys with high performance by grain refinement and texture modification.Graphical abstractImage 1
  • Irregular bending growth of free-standing Al microwire by electromigration
    • Abstract: Publication date: Available online 11 July 2018Source: Acta MaterialiaAuthor(s): Yasuhiro Kimura Electromigration (EM) can generate free-standing micro- and nanowires that are occasionally irregularly bent to kinked and curved or collapsed shapes. The mechanism behind irregular bending was discussed based on electron microscopy data. Al microwires with the bamboo structure or single crystals were kinked at the grain boundaries (GBs) or at locations without GBs, respectively. Dislocation network and clusters of oxidation products were found in the wires. The kinking mechanism was attributed to the changing growth direction owing to GBs and the different growth rate of the outer circumference of the wire owing to the friction between the clusters and the TiN passivation. No buckling owing to gravity was observed. The high growth rate of Al microwires would be accompanied by low resistance in the discharge through a hole, which is attributed to the uniform friction along the periphery of the hole. In addition, the low wire diameter minimized the differences in growth rate. Hence, the increase in the growth rate of the wire by increasing the input current and substrate temperature and the decrease in the wire diameter favor wire straightness.Graphical abstractImage 1
  • Origin of large electrostrain in Sn4+ doped
           Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics
    • Abstract: Publication date: Available online 11 July 2018Source: Acta MaterialiaAuthor(s): Zhen Liu, Ruihao Yuan, Dezhen Xue, Wenwu Cao, Turab Lookman A relatively large bipolar strain of 0.23% was recently reported in 3% Sn4+ doped (BaTi0.8Zr0.2)O3-x (Ba0.7Ca0.3)TiO3 (BZT-xBCT) ceramics under a field of 20 kV/cm [1]. This strain is 53% higher than that in BZT-50BCT (0.15% at 20 kV/cm). To systematically study the mechanism and origin of the large electrostrain enhancement in Sn4+ doped BZT-53.3BCT, we develop a parameterized Landau free energy model and perform phase field simulations. Our results indicate that the softening of the elastic modulus C′=C11−C12 accompanied by the reduction of anisotropy energy due to the 3% Sn4+ doping is primarily responsible for the large electrostrain enhancement.Graphical abstractImage 1
  • Crystal orientation relationships in ternary eutectic
    • Abstract: Publication date: Available online 11 July 2018Source: Acta MaterialiaAuthor(s): Philipp Steinmetz, Anne Dennstedt, Melis Şerefoğlu, Irmak Sargin, Amber Genau, Ulrike Hecht The microstructure of ternary eutectic Al-Al2Cu-Ag2Al arranges in several patterns of three solid phases during directional solidification. One key question for understanding the behavior of this system is if and how the patterns depend on crystal orientation relationships (ORs) between the solid phases. In order to study the correlation between the ORs and the evolving patterns for different process conditions, electron backscatter diffraction (EBSD) is performed on samples of directionally solidified ternary eutectic Al-Al2Cu-Ag2Al which have been processed with different solidification velocities and temperature gradients. The results show that characteristic ORs occur, influencing the type of the evolving pattern, the alignment of the phases and the degree of order. For one specific OR the pattern was observed to change in response to an imposed increase in the growth velocity even though the OR was retained. Based on the obtained EBSD results, an explanation for the observed behavior is proposed. For the other ORs, specific microstructures were observed for each of them. The outcomes demonstrate that knowledge about crystal ORs is essential to improve the understanding of the pattern formation in complex eutectic alloys.Graphical abstractImage 1
  • Locating Si atoms in Si-Doped Boron Carbide: a Route to Understand
           Amorphization Mitigation Mechanism
    • Abstract: Publication date: Available online 11 July 2018Source: Acta MaterialiaAuthor(s): Atta U. Khan, Anthony M. Etzold, Xiaokun Yang, Vladislav Domnich, Kelvin Y. Xie, Chawon Hwang, Kristopher D. Behler, Mingwei Chen, Qi An, Jerry C. LaSalvia, Kevin J. Hemker, William A. Goddard, Richard A. Haber The well-documented formation of amorphous bands in boron carbide (B4C) under contact loading has been identified in the literature as one of the possible mechanisms for its catastrophic failure. To mitigate amorphization, Si-doping was suggested by an earlier computational work, which was further substantiated by an experimental study. However, there have been discrepancies between theoretical and experimental studies, about Si replacing atom/s in B12 icosahedra or the C-B-C chain. Dense single phase Si-doped boron carbide is produced through a conventional scalable route. A powder mixture of SiB6, B4C, and amorphous boron is reactively sintered, yielding a dense Si-doped boron carbide material. A combined analysis of Rietveld refinement on XRD pattern coupled with electron density difference Fourier maps and DFT simulations were performed in order to investigate the location of Si atoms in boron carbide lattice. Si atoms occupy an interstitial position, between the icosahedra and the chain. These Si atoms are bonded to the chain end C atoms and result in a kinked chain. Additionally, these Si atoms are also bonded to the neighboring equatorial B atom of the icosahedra, which is already bonded to the C atom of the chain, forming a bridge like geometry. Owing to this bonding, Si is anticipated to stabilize the icosahedra through electron donation, which is expected to help in mitigating stress-induced amorphization. Possible supercell structures are suggested along with the most plausible structure for Si-doped boron carbide.Graphical abstractImage 1
  • Increasing the creep resistance of Fe-Ni-Al-Cr superalloys via Ti
           additions by optimizing the B2/L21 ratio in composite nano-precipitates
    • Abstract: Publication date: Available online 11 July 2018Source: Acta MaterialiaAuthor(s): Sung-Il Baik, Shao-Yu Wang, Peter K. Liaw, David C. Dunand The Fe-10Cr-10Ni-6.5Al-3.4Mo-0.25Zr-0.005B (wt.%) ferritic FBB8 superalloy shows good creep resistance due to the presence of B2-NiAl precipitates, created upon aging. When titanium is added to the alloy, L21-Ni2TiAl sub-precipitates are developed within the B2-NiAl main precipitates. The microstructural evolutions of these B2/L21 composite precipitates - radius, number density, volume fraction, edge-edge distance, and B2/L21 phase fraction - are studied here for Ti additions spanning up to 4 wt%. As Ti increases from 0 to 3.5 wt%, the alloy strength rises due to an increase of the L21 sub-precipitate volume fraction within the B2 precipitate, which enhances their lattice misfit with the matrix up to ∼1.26%, and increases coherency strengthening. The alloy strength drops sharply for 4 wt% Ti, consistent with the precipitates losing (i) their composite structure (by becoming a fully L21 phase), (ii) their coherency with the matrix (and showing high dislocation density at their interfaces), and (iii) their coarsening behavior and increasing abruptly their size. Creep resistance follows a similar trend (raising from 0 to 3.5 wt% Ti and dropping sharply at 4 wt% Ti), this trend is consistent with the lattice misfit between the coherent B2/L21 precipitates and the matrix, increasing with the L21 fraction, thus, producing a stronger elastic stress field, which makes the climb bypass of the precipitates by the matrix dislocations more difficult.Graphical abstract(Left) Dark-field TEM micrograph of Fe-Ni-Al-Cr-Mo FBB8 ferritic alloy modified with 2.5%Ti aged at 700 °C, showing B2 precipitates (green) and Ti-rich L21 sub-precipitates (red). (Right) Plots of the L21 vol fraction within B2/L21 precipitates and creep threshold stress (σth) as a function of the Ti concentration in the FBB8 alloys.Image 1
  • Unique effect of carbon addition on development of deformation texture
           through changes in slip activation and twin deformation in heavily
           cold-rolled Fe-3% Si alloys
    • Abstract: Publication date: Available online 9 July 2018Source: Acta MaterialiaAuthor(s): M. Takenaka, N. Fujita, Y. Hayakawa, N. Tsuji The effect of carbon addition on the development of cold-rolling texture in heavily cold-rolled ultra-low carbon (ULC) 3% Si steel (
  • Prediction of tensile stiffness and strength of Ti-6Al-4V using
           instantiated volume elements and crystal plasticity
    • Abstract: Publication date: Available online 9 July 2018Source: Acta MaterialiaAuthor(s): Kamalika Chatterjee, McLean P. Echlin, Matthew Kasemer, Patrick G. Callahan, Tresa M. Pollock, Paul Dawson Based on simulations of tensile tests, the macroscopic stiffness and strength of the α phase of Ti-6Al-4V are estimated. Sets of virtual samples are instantiated using characterization data from three-dimensional electron back-scattered diffraction (EBSD) scans collected with the TriBeam microscope and local properties extracted from in situ loading high-energy x-ray diffraction (HEXD) experiments. The simulations use a crystal-scale finite element framework to compute the mechanical response of the virtual samples under tensile loading. From the simulation data, mechanical properties are extracted from volume elements ranging in size from a fraction of the gage section to its entire length. To detect macroscopic yield, a flood-fill algorithm is used to identify a zone of plastically deformed finite elements extending through a volume element. Trends in the estimated properties as functions of the volume element size are examined. The lower bound of volume element size necessary to replicate experimentally measured macroscale properties is indicated. The technique and results provide a guideline for estimating macroscale property values in components designed with size smaller than the lower bound due to constraint.Graphical abstractImage 1
  • Precipitation in Fe-Cu and Fe-Cu-Mn model alloys under irradiation: Dose
           rate effects
    • Abstract: Publication date: Available online 7 July 2018Source: Acta MaterialiaAuthor(s): Shipeng Shu, Nathan Almirall, Peter B. Wells, Takuya Yamamoto, G. Robert Odette, Dane D. Morgan Irradiation effects are studied using accelerated-damage experiments in test reactors and charged-particle accelerator facilities. Neutrons and ions create vacancy and self-interstitial defects, in dose unit of displacements per atom (dpa). It is important to understand the effects of irradiation dose rate on microstructural evolution processes, including irradiation enhanced precipitation, to properly interpret results from experiments using accelerated-damage experiments. Here we report on precipitation in Fe-Cu and Fe-Cu-Mn model alloys irradiated at dose rates ranging from ∼10−10 dpa/s (neutrons) to ∼10−5 dpa/s (70 MeV Fe ions). Atom probe tomography, small angle neutron scattering, and rate theory models show that precipitation is affected by both dose rate and alloy composition. Higher dose rates: 1) decrease in the efficiency of radiation enhanced diffusion, due to increased vacancy-self interstitial recombination; and, 2) increase the rate of ballistic mixing that partly dissolves the precipitate constituents. The key parameter is the rate of ballistic mixing relative to the rate that solutes are reacquired by diffusion. The Mn in the ternary alloy traps vacancies and enhances recombination, leading to very different precipitate evolution kinetics.Graphical abstractImage 1
  • Severe tuning of permanent magnet properties in gas-atomized MnAl powder
           by controlled nanostructuring and phase transformation
    • Abstract: Publication date: Available online 6 July 2018Source: Acta MaterialiaAuthor(s): J. Rial, P. Švec, E.M. Palmero, J. Camarero, P. Švec, A. Bollero Isotropic nanocrystalline MnAl particles have been synthesized by gas-atomization with permanent magnet properties tailored through a rapid-milling method followed by annealing at reduced temperatures. Unprecedented short milling times of 90 and 270 s have been used in the milling process. The study has allowed establishing a correlation between morphology, microstructure and magnetic properties, which has resulted in the possibility of tuning magnetization and coercivity by nanostructuring and phase transformation in a controlled manner. The choice of milling media (steel and tungsten carbide) with dissimilar densities determines the impact energy during the milling process and influences morphological and microstructural characteristics. As a consequence an increase in coercivity above 55% that of the optimized starting material while maintaining remanence has been achieved (steel media) by comparison with an extremely high-coercive powder (4.8 kOe) at expenses of magnetization (tungsten carbide). This has been possible by reduction of the crystalline size to the nanometer scale, inducement of efficient microstrain in the grains and coexistence of the ferromagnetic τ-MnAl phase, responsible of the magnetization, and the non-magnetic β-phase, playing a fundamental role in coercivity development. A common characteristic of both milling processes is the possibility of reducing the annealing temperature needed for achievement of optimized permanent magnet properties in about 75 °C, by comparison with the starting material. This decreased temperature, in combination with the extremely short processing times, makes of this route a promising one to be used in the production of isotropic nanocrystalline MnAl powder with tailored properties.Graphical abstractImage 1
  • Effects of cold and warm rolling on the shape memory response of
           Ni50Ti30Hf20 high-temperature shape memory alloy
    • Abstract: Publication date: Available online 6 July 2018Source: Acta MaterialiaAuthor(s): N. Babacan, M. Bilal, C. Hayrettin, J. Liu, O. Benafan, I. Karaman The present work reveals the effects of cold and warm rolling on the shape memory response and thermomechanical cyclic stability of Ni50Ti30Hf20 high-temperature shape memory alloy (HTSMA). Cold rolling without intermediate annealing was performed up to the maximum possible thickness reduction before surface cracks appeared. These samples were subsequently annealed at various temperatures. 550 °C was determined to be the optimum annealing temperature for 30 min durations based on the differential scanning calorimetry (DSC) and microhardness test results. Isobaric thermal cycling experiments were conducted under different constant tensile stresses. The effects of warm rolling at different temperatures and thickness reductions on the resulting recoverable transformation strains, residual strains, transformation temperatures, and thermal hysteresis were also evaluated. It was shown that the rolling led to an increase in the resistance against defect generation accompanying martensitic transformation in the present HTSMA, resulting in a significant improvement in dimensional stability during thermal cycling. The results revealed that 15% cold rolling followed by the 550 °C 30 min annealing condition showed the best actuation response, exhibiting comparable recoverable transformation levels to the hot extruded samples, with much lower residual strains. All warm rolled samples exhibited notable two-way shape memory effect (TWSME) with compressive two-way shape memory (TWSM) strains, pointing out the existence of compressive internal stress storage following warm rolling. While transformation temperatures of all rolled samples were lower than those of the starting hot extruded sample, thermal hysteresis was notably higher in the rolled samples. This was attributed to the increase in dislocation density and the change in the martensite microstructure with rolling. The present study demonstrates that NiTiHf HTSMAs that have attracted recent interest in high-temperature applications can be processed by using conventional rolling methods while preserving desired cyclic shape memory response.Graphical abstractImage 1
  • Oxygen Potential Transition in Mixed Conducting Oxide Electrolyte
    • Abstract: Publication date: Available online 6 July 2018Source: Acta MaterialiaAuthor(s): Yanhao Dong, I-Wei Chen It is generally assumed that oxygen potential in a thin oxide electrolyte follows a linear distribution between electrodes. Jacobsen and Mogensen have shown, however, that this is not the case for thin zirconia membranes in solid oxide electrochemical cells. Here we demonstrate that there is a ubiquitous oxygen potential transition rooted in the p-type/n-type transition of electronic conductivity inside mixed conducting oxides, and that the transition is extremely sensitive to electrode potential and current density. It is also remarkably sensitive to the conductivity ratio of electrons and holes, as well as their association with lattice oxygens and vacancies, which tends to increase the oxygen flow. Direct evidence of a sharp oxygen potential transition has been found in an equally sharp grain size transition in electrically loaded zirconia. More broadly speaking, the oxygen potential transition is akin to a first-order phase transition. Therefore, it will suffer interface instability, especially in high-current-density devices. These findings provide new opportunities to understand several disparate observations in the literature, from microstructural degradation and stress distribution in solid oxide fuel/electrolyzer cells, to field-assisted sintering, to conducting filaments in resistance memory, to dendrite formation in electrochemical cells.Graphical abstractSolutions of oxygen potential profiles in mixed conducting oxide electrolytes in electrolysis cells and fuel cells reveal an oxygen-potential transition, which explains the grain-size transition observed in zirconia ceramics.Image 1
  • Revealing the sequence of switching mechanisms in
           polycrystalline/ferroelectric/ferroelastic materials
    • Abstract: Publication date: Available online 6 July 2018Source: Acta MaterialiaAuthor(s): Jan Schultheiß, Lisha Liu, Hans Kungl, Michael Weber, Laltiha Kodumudi Venkataraman, Stefano Checchia, Dragan Damjanovic, John Elliot Daniels, Jurij Koruza Ferroelectric materials find application in numerous electronic devices and are continuously enabling the development of new technologies. Their versatility is intimately related to the unique property to switch the polarization with electric fields. However, the switching mechanisms in polycrystalline ferroelectric materials remain insufficiently understood. Here we reveal that switching in ferroelectric/ferroelastic materials consists of a sequence of individual events, separated into three regimes: rapid movement of non-180° domain walls, main switching phase with 180° and non-180° switching events, and creep-like non-180° domain wall movement. The determination of the mechanisms was enabled by a novel measurement approach, simultaneously tracking the time dynamics of switched polarization, macroscopic strain, and structural changes. Time-resolved in situ synchrotron diffraction allowed direct insight into the non-180° domain wall dynamics and lattice strains and gave evidence for strong time correlation of non-180° switching events in different grains of the polycrystalline material. The obtained results open new opportunities for targeted manipulation of individual switching events and tuning of material's functional properties.Graphical abstractImage 1
  • Influence of a nanotwinned, nanocrystalline microstructure on aging of a
           Ni-25Mo-8Cr superalloy
    • Abstract: Publication date: Available online 5 July 2018Source: Acta MaterialiaAuthor(s): Megan G. Emigh, Rebecca D. McAuliffe, Changqiang Chen, James C. Mabon, Timothy Weihs, Kevin J. Hemker, Daniel P. Shoemaker, Jessica A. Krogstad Precipitation pathways in many nickel-based superalloys are highly complex and in some cases proceed very slowly. This behavior has hampered their introduction into some novel application spaces ranging from microelectromechanical systems (MEMS) to protective coatings, wherein their high temperature strength and oxidation resistance would otherwise be highly desirable. In this investigation, the microstructure of Ni-25Mo-8Cr superalloy thin films prepared via direct current magnetron sputtering (DCMS) is shown to be key to manipulating and understanding phase transformations. Specifically, the columnar, highly textured, and nanotwinned grains are shown to facilitate precipitation of the equilibrium DOa (Ni3Mo) phase during a mild heat treatment. Conversely, elimination of the nanotwins through a two-step heat treatment leads to precipitation of metastable Ni2(Mo,Cr) (isomorphous with Pt2Mo and Ni2Cr) precipitates. The straightforward capacity to select one of several possible phase transformation pathways presents an opportunity to create precisely tuned microstructures specific to the desired application, while simultaneously providing unprecedented insight on these complex phase transformations.Graphical abstractImage 1
  • Combinatorial temperature resistance sensors for the analysis of phase
           transformations demonstrated for metallic glasses
    • Abstract: Publication date: Available online 5 July 2018Source: Acta MaterialiaAuthor(s): Haitao Zhang, Dongwoo Lee, Ye Shen, Yucong Miao, Jinhye Bae, Yanhui Liu, Jan Schroers, Yong Xiang, Joost J. Vlassak We describe a sensor for measuring the electrical resistance of a conducting thin-film material as a function of temperature and composition. The sensor has excellent sensitivity and can be used at temperatures as high as the melting temperature of the material of interest. The sensor is fabricated by applying a simple lift-off process to a thin film. By combining combinatorial sputtering to fabricate composition spreads with arrays of sensors, the phase transformation behavior of complex alloys can be mapped. We demonstrate this capabilities by using the sensor to determine the glass transition and crystallization temperatures of several PdSiCu-based metallic glasses. We found that in two glass-forming systems, PdCuSi and NiZr, the ratio of the resistance of the crystallized to as-deposited material is correlated with the glass-forming ability. The ability to readily determine glass forming ability, suggests that the sensor is a powerful tool for measuring the glass-forming ability in a high-throughput manner over large compositional spaces.Graphical abstractImage 1
  • Mean-field modelling of the intermetallic precipitate phases during heat
           treatment and additive manufacture of Inconel 718
    • Abstract: Publication date: Available online 5 July 2018Source: Acta MaterialiaAuthor(s): Magnus J. Anderson, Chinnapat Panwisawas, Yogesh Sovani, Richard P. Turner, Jeffery W. Brooks, Hector C. Basoalto A multi-phase, multi-component mean-field model has been developed for simulating the intermetallic precipitation kinetics in Inconel 718. The aim of this work is to develop predictive capability to aid in process optimisation and explore precipitation kinetics during additive manufacturing (AM). The model has been calibrated to available experimental data, and then applied to predict precipitation kinetics during typical solid solution treatment and aging operations, and during AM. It is shown that a Computer Coupling of Phase Diagrams and Thermochemistry (CALPHAD) based modelling approach provides a unified particle growth rate which can capture the growth, coarsening and dissolution of γ′, γ* and δ precipitates under relevant heat treatment conditions. To apply the model to AM, finite element simulations of a simple rectangular build have been carried out, using a property switching method to simulate the material deposition. The component level simulation provides the thermal fields to calculate precipitation kinetics during deposition, also allowing for the examination of the heat affected zone in the substrate. The modelling approach can capture the repeated nucleation and dissolution of precipitates that occurs during AM. The model shows good agreement with experimental data when applied to predicting precipitation kinetics during heat treatment.Graphical abstractImage 1
  • Recursive alloy Hamiltonian construction and its application to the
           Ni-Al-Cr system
    • Abstract: Publication date: Available online 4 July 2018Source: Acta MaterialiaAuthor(s): Jon Gabriel Goiri, Anton Van der Ven Navigating the high dimensional composition spaces of multi-principle element alloys, also referred to as high entropy alloys, will require new methods to construct first-principles alloy Hamiltonians. We introduce a recursive approach to parameterizing multi-component alloy Hamiltonians using interaction parameters from simpler subsystems as Bayesian informative priors. We applied this approach to perform a first-principles statistical mechanics study of the Ni-Al-Cr system. Ternary cluster expansions for the Ni-Al-Cr alloy were constructed by building on optimized Ni-Al and Ni-Cr binary cluster expansions. Monte Carlo simulations predict a sizable Cr solubility in the Ni-rich FCC based γ and γ′ phases. The L12 -ordered γ′ phase is predicted to dissolve Cr primarily on its Al-sublattice. We also identify a family of hierarchical long-period super structure orderings as groundstates in the Ni-Cr and Al-Cr binaries. The recursive approach to parameterizing alloy Hamiltonians opens the door to rigorous first-principles treatments of the elevated temperature thermodynamics of alloys in high dimensional composition spaces.Graphical abstractImage 1
  • Enhanced dispersoid precipitation and dispersion strengthening in an Al
           alloy by microalloying with Cd
    • Abstract: Publication date: Available online 3 July 2018Source: Acta MaterialiaAuthor(s): Feng Qian, Shenbao Jin, Gang Sha, Yanjun Li The dispersion hardening effect of Mn(Fe)-containing dispersoids in aluminium alloys has long been ignored since it is difficult to achieve a high number density of fine dispersoids with conventional alloying compositions. This work demonstrates a minor addition of Cd (0.05 at.%) can dramatically enhance the precipitation of α-Al(Mn,Fe)Si dispersoids and therefore the dispersion strengthening of AA3003 alloy. Similar to the 3003 base alloy, a peak hardness in the Cd-containing alloy was obtained after continuous heating to 450 °C. However, an improvement in yield strength by 25% was achieved by the Cd addition. Detailed transmission electron microscopy (TEM) and atom probe tomography (APT) investigations show that the Cd addition has changed the nucleation behaviour of α-Al(Mn,Fe)Si dispersoids from the conventional heterogeneous nucleation on dislocations to a more homogeneous manner. It is found that a high number density of Al-Cd nanoprecipitates formed during heating between 150 and 250 °C. These Al-Cd precipitates attracted Mn and Si atoms to form Mn,Si-rich clusters in/around them, which acted as the precursors for the later nucleation of α-Al(Mn,Fe)Si dispersoids at ∼300 °C. As a result, the number density of dispersoids formed in the Cd-containing alloy after heating to 350-450 °C is about twice as that in the base alloy subjected to the same heat treatment. This work proposes a new approach to enhance the nucleation of α-Al(Mn,Fe)Si dispersoids, which can help to further develop cheap Mn(Fe)-containing dispersoid-strengthened aluminium alloys for high-temperature applications.Graphical abstractImage 1
  • Probing Long-range Ordering in Nickel-base Alloys with Proton Irradiation
    • Abstract: Publication date: Available online 29 June 2018Source: Acta MaterialiaAuthor(s): Miao Song, Ying Yang, Mi Wang, Wenjun Kuang, Calvin R. Lear, Gary S. Was Twelve commercial-grade austenitic alloys based on the Ni-Cr-Mo-Fe quaternary system were irradiated using 2 MeV protons at 360 °C to a damage level of 2.5 displacements per atom (dpa). Long-range ordered (Pt2Mo type) precipitation under proton irradiation was observed, for the first time, in alloys C22, 625, 625 P, 625D, 725, and 690. No relevant short-range ordering was observed. These irradiation enhanced long-range ordered precipitates are coherent with the matrix despite their irregular shape. Of the potential influences on long-range ordering (Ni:Cr, and Ni:(Cr+Mo) ratios, Mo, and iron concentration), Fe content was the strongest by far. The volume fraction of LRO decreases with increasing Fe content by virtue of its role as a stabilizer of the disordered FCC phase, thus reducing the energy savings from ordering. The observed effects of Fe on long-range ordering show qualitative agreement with predictions from thermodynamic modeling. Although solid state diffusion kinetics dominate long-range ordering under purely thermal conditions, ordering under irradiation here (⁓ 2.5dpa) is controlled by the thermodynamic driving force. Proton irradiation thus offers a unique approach for studying the low temperature phase transformation in a thermodynamically favored, but kinetically constrained condition.Graphical abstractImage 1
  • Contribution of austenite-martensite transformation to deformability of
           advanced high strength steels: from atomistic mechanisms to
           microstructural response
    • Abstract: Publication date: Available online 21 June 2018Source: Acta MaterialiaAuthor(s): F. Maresca, V.G. Kouznetsova, M.G.D. Geers, W.A. Curtin Steels combining austenite (fcc) with lath martensite (bcc) in nanolaminate microstructures are tough, resistant to hydrogen-embrittlement, and inexpensive, making them attractive for many technological applications. Austenite provides plastic deformation while martensite provides strength, but the nanoscale processes that control plasticity in the austenite layers are not fully established. Recent atomistic simulations and crystallographic theory reveal a unified understanding of the structure and motion of the fcc austenite-bcc (lath) martensite interface in steels, with transformation strains up to ∼90% in Fe-C alloys. In this paper, the atomistic behavior is connected to the ductility of nanolaminate microstructures. First, the mechanical response of the atomistic fcc/bcc interface under shear loading is analyzed. The interface motion follows a Schmid-type law for resolved shear stresses in the transformation direction. Furthermore, the forward fcc-to-bcc transformation is spontaneous while the reverse bcc-to-fcc transformation requires high stress. The asymmetry correlates well with the Peierls stresses for fcc and bcc screw dislocations, respectively. Second, the atomistic results guide the formulation of a two-scale continuum model for the phase transformation. The multi-scale strategy adopted here accounts for the relevant nano-scale mechanisms and enables modeling the mechanical response of real martensite microstructures, up to the scale of tens of micrometers - which would be untractable with direct atomistic simulations. Multi-scale simulations show that forward transformation contributes significantly to the apparent plasticity in lath martensite. This reinforces recent work highlighting the importance of such nanoscale austenite films for achieving ductility and toughness in lath martensite. Overall, the present work demonstrates how atomistic insights can directly inform continuum models of microstructural deformation, and points toward directions for material control and optimization to achieve enhanced mechanical performance in these steels.Graphical abstractImage 1
  • Electron radiation-induced material diffusion and nanocrystallization in
           nanostructured amorphous CoFeB thin film
    • Abstract: Publication date: Available online 18 June 2018Source: Acta MaterialiaAuthor(s): Binghai Liu, Taiebeh Tahmasebi, Kenny Ong, Hanwei Teo, Zhiqiang Mo, Jeffrey Lam, Pik Kee Tan, Yuzhe Zhao, Zhili Dong, Dimitri Houssameddine, Jacob Wang, Junming Xue, Zhihong Mai Transmission electron microscopy (TEM) is widely used for physical characterization of CoFeB -based magnetic tunneling junctions (MTJ) with its atomic-scale resolution. However, highly energetic electron radiation during TEM analysis may cause phase and microstructure modification of CoFeB and its associated MTJ layers. It is the intention of this work to address the issues of the electron-beam sensitivity of CoFeB material. With in-situ TEM, we investigated the electron beam radiation-induced material diffusion and the nanocrystallization behaviors in nanostructured amorphous CowFexByOz/Co60Fe20B20/SiO2 thin films. It was found that electron radiation with different electron dose led to massive diffusion of Co, Fe, B and O atoms across the whole thin film layers, which directly resulted in the modification of the phase and composition of the thin film layers, i.e. the oxidation of Co, Fe, B with O diffusion and the formation of pure Si phase from SiO2. Two stages of material diffusion were observed. While Stage-I material diffusion proceeded with a high diffusion speed, Stage-II had a relatively low diffusion rate accompanying with the nanocrystallization at the bottom of the CoFeB layer. A detailed kinetic study by in-situ TEM revealed the electron-beam radiation induced massive diffusion was a non-thermal process, and the underlying driving force arose from radiation-enhanced diffusion (RED) effects. Nanocrystallization during Stage-II electron-radiation experiment showed unique phase transformation phenomena, repeated nanocrystallization, amorphization, and nanocrystallization processes in the sequence before a stable grain growth could be achieved. A detailed TEM analysis revealed that RED-enhanced B diffusion was responsible for such unique repeated phase transformation processes. B diffusion and the associated structure distortion and the local short-range re-ordering may also account for the phase transformation from fcc-CoxFe23-xB6 to B-rich orthorhombic- CoxFe3-xB phase.
  • The role of the interface stiffness tensor on grain boundary dynamics
    • Abstract: Publication date: Available online 18 June 2018Source: Acta MaterialiaAuthor(s): Fadi Abdeljawad, Stephen M. Foiles, Alex Moore, Adam R. Hinkle, Christopher Barr, Nathan M. Heckman, Khalid Hattar, Brad L. Boyce Grain boundary (GB) properties and associated anisotropies due to the boundary's geometric degrees of freedom (DOF) greatly influence many of the salient features of polycrystalline aggregates, and as a consequence the observable properties of the material. Through theoretical analysis and atomistic simulations, we show that when considering the GB plane normal DOF, the GB interface stiffness tensor plays a paramount role in a wide range of GB dynamical processes. As a demonstration, we examine the interface stiffness tensor of Σ3 GBs in nickel and show that the stiffness can be much larger in magnitude and more anisotropic than the GB energy itself. Moreover, it is found that a wide range of inclinations exhibit negative stiffness values, a signature of a structural instability. This is the first study to consider the complete spatial description of the GB stiffness tensor, where both angular variations describing the interface plane normal are explored. In broad terms, our results highlight the integral role that the stiffness tensor plays in GB interfacial phenomena, such as curvature-driven flow and faceting instabilities.Graphical abstractImage 1
  • Hierarchical microstructure design of a bimodal grained twinning-induced
           plasticity steel with excellent cryogenic mechanical properties
    • Abstract: Publication date: Available online 14 June 2018Source: Acta MaterialiaAuthor(s): Yu Li, Yufei Lu, Wei Li, Mahmoud Khedr, Huibin Liu, Xuejun Jin Combined nanoprecipitation and grain refinement were introduced in a bimodal grained (BG) twinning-induced plasticity (TWIP) high manganese steel to achieve high strength-ductility combinations. Hierarchical microstructural characteristics of heterogeneous grain size (0.2 μm - 4 μm) and precipitates (κ-carbides and Nb-rich carbides) distribution was obtained by a controlled thermo-mechanical treatment. Compared with the as-received states, the BG-TWIP steels showed a significant improvement in yield strength (YS) with little loss in plasticity whether at room temperature (RT) or liquid nitrogen temperature (LNT). The multiple strengthening contributions to YS mainly originate from the combination of solid solution and grain refinement strengthening. When deformed at RT, some deformation twins nucleated in the coarse grains (CG) of the BG-TWIP steels while only numerous stacking faults formed in the fine grains (FG) at a true strain of 0.14. The twin density remained nearly unchanged with progressive deformation and the dislocation strengthening dominated in the later deformation stage. In the early deformation stage at LNT, the twin amount of the BG-TWIP steel was still small. However, the volume fraction of twins increased greatly in both the FGs and CGs when deformed to a true strain of 0.26. The occurrence of high density of nano-twins in the later deformation stage at LNT not only contributes to a strength increment of 220 MPa, but also largely increases the geometrically necessary dislocations (GND) density to enhance the forest hardening effect, corresponding to a significant higher back stress hardening at large strains.Graphical abstractImage 1
  • Direct observation of stress-induced dislocations in protein crystals by
           synchrotron X-ray topography
    • Abstract: Publication date: Available online 14 June 2018Source: Acta MaterialiaAuthor(s): Ryo Suzuki, Masaru Tachibana, Haruhiko Koizumi, Kenichi Kojima Stress-induced dislocations in glucose isomerase (GI) crystals were investigated by synchrotron X-ray topography with simple indentation method. It is observed that the indentation gives rise to the deformation associated with strain and dislocations over the entire crystal. It is characterized that three kinds of mobile dislocations with Burgers vectors of 12111, 100 and 010 are at least introduced by the impact stresses due to the indentation. The shapes of the dislocations exhibit half loops and clusters of dislocation loops, as those in common inorganic crystals. It is suggested that the deformation in protein crystals also occurs by the dislocation mechanisms, as those in common inorganic crystals.Graphical abstractImage 1
  • Development of different methods and their efficiencies for the estimation
           of diffusion coefficients following the diffusion couple technique
    • Abstract: Publication date: Available online 25 April 2018Source: Acta MaterialiaAuthor(s): Varun A. Baheti, Aloke Paul The interdiffusion coefficients are estimated either following the Wagner's method expressed with respect to the composition (mol or atomic fraction) normalized variable after considering the molar volume variation or the den Broeder's method expressed with respect to the concentration (composition divided by the molar volume) normalized variable. On the other hand, the relations for estimation of the intrinsic diffusion coefficients of components as established by van Loo and integrated diffusion coefficients in a phase with narrow homogeneity range as established by Wagner are currently available with respect to the composition normalized variable only. In this study, we have first derived the relation proposed by den Broeder following the line of treatment proposed by Wagner. Further, the relations for estimation of the intrinsic diffusion coefficients of the components and integrated interdiffusion coefficient are established with respect to the concentration normalized variable, which were not available earlier. The veracity of these methods is examined based on the estimation of data in Ni–Pd, Ni–Al and Cu–Sn systems. Our analysis indicates that both the approaches are logically correct and there is small difference in the estimated data in these systems although a higher difference could be found in other systems. The integrated interdiffusion coefficients with respect to the concentration (or concentration normalized variable) can only be estimated considering the ideal molar volume variation. This might be drawback in certain practical systems.Graphical abstractImage 1
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