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Abstract: Solution and quench treatments are very important for the heat-treatable aluminum alloys because any change in these treatments will induce a trade-off in volume fraction between the constituents and precipitates and so will cause a change in the mechanical properties. In this paper, three types of solution + quench treatments were applied to two kinds of aluminum alloys, i.e., the Al–Cu–Mg alloy that contains disc/plate-shaped precipitates and the Al–Mg–Si alloy that contains rod/needle-shaped precipitates, to change the relative content between the constituents and the precipitates and to develop different coupling of the constituent and precipitates. The specimen treated with an enhanced solution or a stepped solution is found to exhibit a significant increase in yield strength, ductility, and fracture toughness. A multiscale model is presented to quantitatively estimate the coupled influence of the constituents and precipitates on the mechanical properties by combining with a strengthening model. The experimentally observed non-monotonic dependence of ductility on the trade-off in volume fraction between the constituents and precipitates is reasonably explained by using this multiscale model. In addition, the influence of stress triaxiality level on the ductility and fracture toughness is also calculated. All the calculations are in quite good agreement with the experimental results. [Copyright &y& Elsevier]

Body-centered cubic refractory metals, as a class of critical structural materials, are often restricted by poor low-temperature toughness, which is a common, undesirable phenomenon that continues to be the focus of intense research. Historically, rhenium (Re) has been considered the most promising alloying element in improving ductility and reducing the ductile-to-brittle transition (DBT) temperature of group-VI metals, known as the " Re effect". However, whether this is indeed possible remains uncertain so far. Here, in the modelled W-x Re (x = 0, 3, 5, 10 wt.%) alloys, we reveal that only in a limited temperature range of 50 °C∼200 °C does the Re softening occur, imparting a positive but small effect on fracture toughness. At higher temperatures (>300 °C), the dynamic interactions between Re solutes and dislocations produce profuse jogs and sessile loops that strongly hinder dislocation motion and cause hardening. The resultant difficulty in plastic flow significantly degrades the high-temperature (>300 °C) toughness of W- Re alloys, leading to an undesirable increase of the DBT temperature, which shifts higher with increasing Re content. Our discovery of the solution softening-to-hardening transition, including its dislocation mechanism, clarifies the intrinsic effect of Re on the toughness of recrystallized W alloy, providing essential knowledge for the compositional design of refractory alloys in general. [Display omitted] [ABSTRACT FROM AUTHOR]

Titanium (Ti) alloys, which are highly promising as structural materials in critical industrial applications, generally require high strength and ductility, particularly high fracture toughness. However, the conventional approach of enhancing strength through mechanical processing to induce dislocations often leads to a compromised ductility known as the strength-ductility trade-off. Here, we develop a new strategy of the nanocrystallized kinks to overcome this issue in Ti-11V metastable β -Ti alloys via combining industrially used cold forging (CF) and cold rolling (CR) processes. Deformation kinks are firstly activated by CF, and subsequently they are fragmented into nanograins during CR, architecting nanocrystallized kinks in the coarse-grained matrix. This unique microstructure effectively balances the strength-ductility conflict, endowing this Ti-V binary alloy with high yield strength ∼1200 MPa, appreciable ductility ∼17 % and high fracture toughness ∼ 52.0 MPa·m½, which is superior to numerous multielement engineering Ti alloys. This design strategy of nanocrystallized kinks can be extended to other engineering materials, e.g. , Mg and Zr alloys, for advanced performance at large industrial scales. [Display omitted] [ABSTRACT FROM AUTHOR]

Local chemical order (LCO) is energetically favorable in multi-principal element alloys (MPEAs) due to the chemical affinity difference among constituent species. Here we show that the increase of LCO is often accompanied by a decrease of lattice distortion in body-centered-cubic (BCC) MPEAs. This leads to a trade-off between LCO and lattice distortion in their effects on retarding the glide of edge and screw dislocations. With a random solid solution without LCO as a reference, the emergence of LCO at the expense of lattice distortion could weaken the local trapping force for dislocation glide and promote dislocation mobility. However, when the LCO becomes sufficiently large and robust, it acts to hinder dislocation glide. We found that the mobility of edge dislocation is more affected by local lattice distortion, while the kink-pair based screw dislocation motion is more governed by LCO. We further quantitatively evaluate the contribution of LCO and lattice distortion to the overall critical shear stress for dislocation glide using analytical models. For the present equiatomic NbTiZr MPEAs, the critical shear stress is dominated by lattice distortion when the LCO is still limited to chemical short-range order (CSRO). However, LCO could even surpass lattice distortion to become a major contributor to the critical shear stress once it grows to a sufficiently large size. Our work indicates that increasing the degree and extent of LCO may provide an effective means to regulate dislocation motion, shedding light on how to use LCOs to improve the strength of BCC MPEAs. [Display omitted] [ABSTRACT FROM AUTHOR]

An increasing number of studies on shape memory alloys (SMAs) have shown anomalous temperature dependence of superelasticity, i.e., the superelastic stress with temperature (d σ /d T) deviates from the behavior expected from the Clausius–Clapeyron relationship, especially showing a negative d σ /d T at low-temperatures. However, the underlying mechanism is still experimentally unclear. Here, we perform large-scale molecular dynamic (MD) simulations of NiTi-based SMAs to elucidate the relationship between the observed negative d σ /d T and fluctuation in chemical concentration. By comparing the superelastic behavior of Ni 50 Ti 47.2 Nb 2.8 SMAs, in which the solution Nb atoms are either disordered uniformly or nano-scale undulated, we find that the chemically disordered uniform can promote the d σ /d T deviation. It stems from that the disordered uniformed solid-solution atoms with relatively high concentrations of a homogenously lattice-level strain field at low temperatures can remarkably refine the martensite embryo over several lattice units. The phase transformation then undergoes a form of continuous transformation. In this sense, the formation of mature martensite is delayed upon loading, thus increasing the critical superelastic stress. Such a kinetic effect will be accentuated as the temperature decreases. Our findings highlight the importance of homogenization of heat treatment for the design of ultra-low temperature superelastic SMAs. [Display omitted] [ABSTRACT FROM AUTHOR]

The design of alloys has typically involved adaptive experimental synthesis and characterization guided by machine learning models fitted to available data. A bottleneck for sequential design, be it for self-driven or manual synthesis, by Bayesian Global Optimization (BGO) for example, is that the search space becomes intractable as the number of alloy elements and its compositions exceed a threshold. Here we investigate how reinforcement learning (RL) performs in the compositional design of alloys within a closed loop with manual synthesis and characterization. We demonstrate this strategy by designing a phase change multicomponent alloy (Ti 27.2 Ni 47 Hf 13.8 Zr 12) with the highest transformation enthalpy (ΔH) -37.1 J/g (-39.0 J/g with further calibration) within the TiNi-based family of alloys from a space of over 2 × 108 candidates, although the initial training is only on a compact dataset of 112 alloys. We show how the training efficiency is increased by employing acquisition functions containing uncertainties, such as expected improvement (EI), as the reward itself. Existing alloy data is often limited, however, if the agent is pretrained on experimental results prior to the training process, it can access regions of higher reward values more frequently. In addition, the experimental feedback enables the agent to gradually explore new regions with higher rewards, compositionally different from the initial dataset. Our approach directly applies to processing conditions where the actions would be performed in a given order. We also compare RL performance to BGO and the genetic algorithm on several test functions to gain insight on their relative strengths in materials design. [Display omitted] [ABSTRACT FROM AUTHOR]

The carbon partitioning and lengthening rate of bainitic ferrite (α b) are excellent experimental parameters to estimate our level of understanding of the mechanism of bainitic transformation from a continuum perspective and our ability to capture it in analytical expressions. For Fe-C alloys and relatively simple steels the classical Zener-Hillert theory captures the bainitic transformation rather well but mispredicts the level of carbon in solution in the bainite and overestimates the lengthening rates for transformations at lower temperatures. To address this issue, this paper presents a new thermo-kinetic model based on the Zener-Hillert theory and the Gibbs energy balance concept to simulate the lengthening behavior of α b in the Fe-C and low alloyed steels. The model incorporates the effect of the temperature dependent carbon diffusion within the migrating interface via a temperature dependent ferrite/austenite interfacial energy and a temperature dependent diffusion coefficient but does not impose local equilibrium across the interface. The good agreement between the model predictions and nine sets of published experiments indicates that both the carbon supersaturation in α b and the slower lengthening rate are caused by carbon diffusion within the migrating interface. It is found that the degree of carbon supersaturation in α b increases significantly with decreasing temperature. Consequently, the enhanced carbon solute drag effect, resulting from carbon diffusion within the interface, strongly retards the lengthening rates of α b at lower temperatures. Transformation strain is shown to have a modest effect on the lengthening rates but to lower the degree of carbon supersaturation. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: Nanostructured piezoelectric/magnetostrictive multiferroic composite films with a strong coupling between ferroelectric and ferromagnetic order parameters are attracting extensive attention. The coupling interaction in the nanostructured multiferroic film should be different from that in bulk composite due to large residual stress/strain in the film. In this paper, the magnetoelectric coefficient of the nanostructured BaTiO3–CoFe2O4 multiferroic films was studied by using the modified constitutive equations where the influence of residual stress and spontaneous polarization was considered. Our results show that the 1-3-type BaTiO3–CoFe2O4 composite films could exhibit a strong coupling between ferroelectric and ferromagnetic order parameters, while the 2-2-type nanostructured composite films were found to produce a much less coupling interaction. Magnetoelectric effect of the films is dependent on composition and residual stress in films, and temperature. The calculations are in broad agreement with the experimental results available. [Copyright &y& Elsevier]

Abnormal thermal expansion denotes intriguing negative and zero thermal expansion (NTE and ZTE). The integration of abnormal thermal expansion with low thermal conductivity is highly desirable for high-end devices. However, the coexistence of seemingly contradictory features presents a grand challenge for abnormal-thermal-expansion materials. Herein, a unique NTE, for the first time, is reported in Cd 2 Re 2 O 7 with a volumetric coefficient of thermal expansion, α V of -5.0 × 10−6/K (5 - 210 K). Moreover, a remarkable ZTE is attained in Cd 1.95 Ni 0.05 Re 2 O 7 with α V of 1.1 × 10−6/K (5 - 180 K). Concomitantly, the attained ultralow thermal conductivities of 0.38 and 0.56 W/mK at 300 K of the two compositions are also highly competitive, exceeding prevailing abnormal-thermal-expansion materials. Concurrently, Cd 2 Re 2 O 7 -based oxides demonstrate orders of magnitude higher electrical conductivity than representative NTE materials. By a combined investigation of synchrotron X-ray techniques, aberration-corrected scanning transmission electron microscope, and phonon calculations, the synergy of local structure distortion highlighted by ordered Re-Re dimer and soft acoustic phonons is integral to the simultaneous accomplishment of distinctive thermal expansion and thermal conductivity, which is totally distinct from predominant optical-phonon driven NTE. This integration of intriguing thermal properties effectively resolves thermal issues and permits excellent size and temperature stability. This work opens up an unusual strategy to delicately control thermal expansion and thermal conductivity simultaneously. [Display omitted] [ABSTRACT FROM AUTHOR]

Precipitates in alloys are traditionally considered as dislocation obstacles that often lead to high stress concentration and even microcracks owing to the strain incompatibility of semi-coherent interfaces, a cause of progressive strain localization and the origin of the strength-toughness conflict in engineering materials, such as titanium (Ti) alloys. Based on the metastability engineering, here we architect hierarchically ordered coherent interfaces for strength-toughness optimization through densely dispersed nanomartensites in a ductile Ti-Cr-Zr-Al alloy with ultrahigh specific-strength and superior fracture toughness. It is unveiled that these ordered coherent interfaces simultaneously serve as dislocation obstacles and sources, leading to a sustainable and self-hardening deformation mechanism via hierarchical nanomartensite-dislocation interactions for ultra-high strength and toughness of Ti alloys. These nanomartensites are thermally stable at elevated temperature less than 400 °C, above which tempering-induced ductile-to-brittle transition occurs due to the decomposition of hierarchically ordered nanomartensites and the spheroidization of prior β lamellae. The design strategy of hierarchically ordered coherent interfaces confers our cost-effective nanomartensite Ti alloys an unprecedented combination of strength, ductility and toughness, which provides a new pathway in the microstructural design for strong and ductile structural materials with superior fracture resistance. [Display omitted] [ABSTRACT FROM AUTHOR]

The intermittent plasticity of pure Mo microcrystals with diameters from 500 to 3500 nm was studied using micro-pillar compression methodology at temperatures ranging from 25 °C to 200 °C, in order to explore its dependence on external size and temperature. The technological background is the potential value of BCC metals in the manufacture of high-temperature components in modern fields requiring miniaturization, such as microelectronics. We analyzed plastic fluctuations in terms of wildness, avalanche size distribution, and burst peak velocity. This reveals coupled size and temperature effects on a mild-to-wild transition: the transition temperature decreases with decreasing sample diameter, while the transition diameter increases with rising temperature. In addition, strain-rate sensitivity tests were conducted, implying that mild plasticity is associated to screw dislocation motions controlled by thermally activated nucleation of kink-pairs, whereas wild plasticity is nearly athermal. This experimental observation is interpreted by a damping effect of the thermally activated motion of screw dislocations on the propagation of avalanches. From this, we propose a controlling parameter based on a simple dislocation source model, which unifies the coupled sample size and temperature effects on the mild-to-wild transition, and which could be of practical significance for microscale applications. [Display omitted] [ABSTRACT FROM AUTHOR]

We studied the torsion behavior of α-Fe nanowires seeded with twin boundaries (TBs) using molecular dynamics simulations. Twisting the wire generates topological defects in the twin walls, namely kinks inside the twin walls for small twist angles, and junctions between kinks for large twist angles. During twisting the kink motion is jerky and uncorrelated at small twist angles. The probability density function (PDF) of jerks strength follows approximately a Gaussian distribution, indicating a mild deformation mode. The kink dynamics transforms from mild to wild at larger twist angles when complex twin patterns with a high density of junctions are generated. The collective motion of kinks now shows avalanche behavior with the energy being power-law distributed. The wildness, which measures the proportion of strain energy relaxed through such avalanches, is correlated with the junction density, and controlled by the external length scale (wire diameter) as well as an internal length scale (twin boundary spacing). Good strain-stress recoverability is achieved when unloading the wire before the formation of complex twin patterns. We correlate the evolution of twin patterns with a statistical analysis of jerk dynamics, which identifies the unique mechanical properties governed by twin boundary motion in nanowires. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Vacancies strongly interact with twin boundaries and often change dramatically the properties of the ferroelastic material. However, the understanding of this behavior at an atomic-level is still deficient. Here we study vacancy diffusion processes across very large length- and time-scales using a combination of molecular dynamics and Monte Carlo simulations. We find that vacancies reduce their energy by residing at twin boundaries, kinks inside domain boundaries, and junctions between domain boundaries. Vacancies have the largest binding energy inside junctions and co-migrate with the motion of the junctions. For the weaker trapping inside twin boundaries, a "ghost line" may be generated because vacancies do not necessarily diffuse with moving boundaries and are left behind, leaving a trace of a previous position of the domain boundary. Needle twins act as channels for fast diffusion with almost one order of magnitude higher vacancy diffusivity than in bulk. The relative concentration of vacancies at twin boundaries (ρ Va) is a function of the average vacancy concentration (C Va) with ρ Va ∼ C Va α and α = 0.61, in contrast to that of immobile vacancies case with α = 0.4. The concentration of vacancies at twin boundaries is enriched ca. 5 times at low temperatures. With increasing temperature, the enrichment drops as the trapping potential at the twin boundaries decreases (thermal release). The distribution of energy-drop upon twin pattern evolution follows a power law. The exponent ε increases from ∼1.44 to 2.0 when the vacancy concentration increases. The power law exponent is the same in the athermal region, while Vogel-Fulcher behavior is found at high temperatures. Image 1 [ABSTRACT FROM AUTHOR]

Doping point defects into shape memory alloys (SMAs) influences their transformation behaviors and mechanical properties. We propose a general Landau free energy model to study doping effects, which only assumes that point defects produce local dilatational stresses coupled to the non-order parameter volumetric strain. Different dopants can be represented by their range of interaction and potency of dilatational stress. Time-dependent simulations based on our model successfully reproduce experimentally observed doping effects in SMAs, including the elevation or suppression of the transformation temperature, the modification of mechanical properties, the appearance of a cross-hatched tweed structure and the emergence of a frozen glassy state with local strain order. We predict that the temperature range for superelasticity will be enhanced in the crossover region between martensite and strain glass. In addition, an Elinvar effect appears most likely in alloys with dopants tending to increase the transformation temperature, which needs to be verified experimentally. Moreover, the two parameters in the Landau model, the interaction range and potency of the dilatational stress, inspire us to identify three material descriptors with which we can construct an empirical machine learning model. The model predicts the transformation temperature, and the slope of the change in transformation temperature as a function of doping composition, enabling an effective search for doped SMAs with targeted properties via machine learning. Image 1 [ABSTRACT FROM AUTHOR]

The calorimetric and dynamical glass transitions are uncoupled in high entropy metallic glasses (HEMGs). That is, the thermal glass-transition or vitrification temperature (T g) and α -relaxation temperature (T α) are uncorrelated as a function of alloy stoichiometry. The underlying mechanism that gives rise to this is not understood, nor is the connection to atomic structure. Here, molecular dynamic (MD) simulations of different HEMG model systems are performed to investigate the glass transition behavior. In addition to the high configurational entropy of mixing (for an ideal solution), we show that the effect of mismatch entropy plays a critical role in decoupling vitrification kinetics/ α -relaxation in HEMGs. Microscopically, the atomic size difference contributes significantly to the sluggishness of long-range diffusion and decreased dynamic heterogeneities. This gives rise to a depressed structural α -relaxation. More importantly, we find that HEMGs with the decoupling vitrification kinetics/ α -relaxation can show unique mechanical performance, such as better radiation tolerance and higher resistance to shear banding. Our results provide insights into the high entropy effect of glassy metallic materials, and suggest a means for tuning properties of HEMGs. [Display omitted] [ABSTRACT FROM AUTHOR]

High performance structural materials with combinations of excellent yet often mutually exclusive properties such as high yield strength, ductility and thermal stability are primarily accessed by synthesizing heterogeneous microstructures. Age-hardened Al alloys with hierarchical nanoprecipitates manifest a good combination of strength and ductility compared to fully amorphous Al alloys, however their thermal stability is in general low, stem from the fast-diffusion element-driven coarsening of nanoprecipitates at elevated temperature. Utilizing the sluggish-diffusion element Cr in the Al matrix, we propose a novel strategy of architecting symbiotically crystalline-amorphous nanostructure (CANS) endowed with self-assembled Cr segregation at crystalline/amorphous interfaces, to develop thermally stable, ultrastrong and ductile Al alloys. The symbiotic CANS-AlCr alloys with homogeneous twinning-induced plasticity of ∼15% strains at room temperature have ultrahigh compressive yield strength of ∼1.75 GPa and outstanding thermal stability up to ∼623 K, simultaneously superior to the parent nanocrystalline and amorphous Al alloys. The interfacial Cr segregation not only promotes twinning-induced plasticity of crystals but also triggers dynamic elemental partitioning between interfaces and amorphous nanolayers, which is critical to their thermodynamic and mechanical stabilization of symbiotic CANS-AlCr alloys. Our strategy advances efficient creation of hierarchical nanostructure and offers a facile route that is regulated across atomic and nanoscopic scales to achieve desirable materials with diverse-yet-precise performances. [Display omitted] [ABSTRACT FROM AUTHOR]

In conventional duplex titanium (Ti) alloys, the soft primary α-precipitates (α p) are beneficial to ductility with the loss of high yield strength, while the continuous α-precipitates at grain boundaries (GBs, α GBs) often accompanied with α precipitate-free zones (α-PFZs) renders the strain incompatibility for limited ductility. Here, to overcome this longstanding issue, we utilize interstitial (O and N) atoms to notably strengthen the α-phase, and simultaneously architect the multi-morphologic secondary α-precipitates (α s) so as to improve the strain compatibility nearby GBs, thus develop a Ti-4.1Al-2.5Zr-2.5Cr-6.8Mo (wt.%) alloy with ultra-high yield strength of ∼1580 MPa and good ductility of ∼8.2%. The phase boundaries between the hard α-precipitates and the soft β-matrix not only act as dislocation barriers for strengthening, but also as sustainable dislocation sources for ductilizing to achieve a good combination of strength and ductility in our Ti alloys. This strategy not only sheds light on the understanding of the strength-ductility synergy of duplex Ti alloys, but also offers an available pathway to design ultra-strong and ductile alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

The ductile-to-brittle transition is commonly observed in metastable β -titanium (Ti) alloys containing ω -precipitates, while the fundamental understanding on ω -embrittlement hitherto remains elusive. In this work, the prototypical Ti-20 wt% Mo metastable β -Ti alloy has been systematically investigated by coupling experiments and first-principles calculation to eliminate this puzzle. It is shown that the structural evolution of ω -phase controls the deformation mechanism transition of twinning-to-slip in Ti Mo alloy, being the origin of ductile-to-brittle transition of this alloy. The initial trigonal ω -structure continuously collapses to hexagonal ω -structure (structural collapse) whilst Mo-atoms are rejected out concurrently (stoichiometric varieties), both leading to hardening of ω -precipitates. This self-hardening of ω -precipitates was further rationalized in terms of the enhanced propensity for a covalent character of the atomic bond demonstrated by the electronic density of states (DOS) from first-principles calculation. Specifically, the self-hardening behavior of ω -precipitates promotes dislocation slip on isolated planes in lieu of correlative slip on successive planes inside ω1 -variant, while dislocations are completely blocked ahead ω2/ω3/ω4- variants. This in turn renders the transition from deformation twinning that contributes to great macro-plasticity to ordinary dislocation slip that contributes to localized deformation bands in the present Ti Mo alloy. Image 1 [ABSTRACT FROM AUTHOR]

The extreme miniaturization in modern technology calls for deeper insights into the non-conventional, fluctuation dominated mechanics of materials operating at microscale. For instance, both experiments and simulations show that sub-micron face-centered-cubic (FCC) crystals exhibit high yield strength accompanied by intermittent, power law distributed strain fluctuations. At macro-scales, the same bulk materials show bounded, uncorrelated fluctuations. Both anomalous strength and intermittency appear therefore as size effects: while the former is highly desirable, the latter is detrimental because stochastic dislocation avalanches interfere with forming processes and endanger structural stability. In this paper we quantify the coexistence of correlated and uncorrelated fluctuations in compressed Al alloys micro-pillars, demonstrate that the partition between the two is determined by sample size, and propose quantitative strategies allowing one to temper plastic intermittency by artificially tailored disorder. Our experimental results are rationalized using a theoretical framework that quantifies the competition between external (size related) and internal (disorder related) length scales. [ABSTRACT FROM AUTHOR]

The martensitic transformation serves as the basis for applications of shape memory alloys (SMAs). The ability to make rapid and accurate predictions of the transformation temperature of SMAs is therefore of much practical importance. In this study, we demonstrate that a statistical learning approach using three features or material descriptors related to the chemical bonding and atomic radii of the elements in the alloys, provides a means to predict transformation temperatures. Together with an adaptive design framework, we show that iteratively learning and improving the statistical model can accelerate the search for SMAs with targeted transformation temperatures. The possible mechanisms underlying the dependence of the transformation temperature on these features is discussed based on a Landau-type phenomenological model. [ABSTRACT FROM AUTHOR]

Short-range strain-ordered strain glass (STG) appears when the kinetic limitation (local stress field produced by random defects) is strong enough to suppress thermodynamic driven long-range strain-ordered martensitic transition. The point defects and precipitates have been proven to be effective to suppress the martensitic transition and trap the system in a frozen STG state. The dislocations can also introduce the random stress field down to atomic scale, however the effectiveness of which on the crossover from martensite to STG transition remains unknown. Here we report the first experimental finding of the dislocation induced STG transition in a Ti 50 Ni 45 Fe 5 martensitic alloy when the sample was cold compressed by over 20% plastically. Being different from the point defects and precipitates, the dislocations do not change the chemical composition of the matrix and consequently do not vary the transition temperature too much. Our finding verifies the effectiveness of local randomness on the crossover from martensite to STG transition and suggests that the global effect of defects is not a necessary condition for the occurrence of STG. The present finding may promote the application of STG alloy by assisting on the tuning of functional temperature range in STG alloy through adding the randomness of dislocations. [ABSTRACT FROM AUTHOR]

Unlike the general belief that either dislocations cut or by-pass the ω -precipitates contributes to the high strength of Ti alloys, here we uncovered a new strengthening mechanism of lattice disordering of ω -precipitates by investigating the deformation behavior of body-centered-cubic (bcc) single crystalline Ti-10V-2Fe-3Al (Ti1023) alloy micropillars. This unique ω -precipitate disordering strengthening mechanism emerges in-between the precipitate cutting and by-passing mechanisms, because the dislocations pile-up stress reaches the ideal strength of ω -phase. The ω -precipitates with four variants manifest the strong deformation anisotropies, as verified by both the crystallographic orientation analysis and energy landscapes from the first principle calculations. The combination of precipitates cutting of ω 1-variant and precipitates disordering of ω 2/ ω 3/ ω 4-variants leads to the formation of precipitate-free channel in bcc (146) β -oriented Ti1023 alloy micropillars. Due to the presence of ω -precipitates, the bcc Ti1023 alloy micropillars exhibit stable plastic flow, associated with weak-size dependent superior strength. These findings provide fundamental understanding of the deformation characteristic of precipitate-hardening alloys, and have important implications for the design of small-scale materials. [ABSTRACT FROM AUTHOR]

The role of grain boundary structures on the shock response of hexagonal-close-packed (hcp) metals is little understood. We use molecular dynamics simulations to investigate deformation mechanisms in shock compressed Ti bicrystals with three types of grain boundary (GB) microstructure. Our results show the shock response of phase Ti polycrystals are influenced by the GB characteristics, i.e., elastic shock wave induced inelastic deformation occurs on both sides of the { 10 1 ¯ 2 } coherent twin boundaries (CTBs) but only on one-side of the symmetric incoherent twin boundaries (ITB) or { 1 2 ¯ 10 } tilt grain boundaries regions. In particular, we find that the elastic shock wave can readily trigger the α→ω transformation at { 10 1 ¯ 2 } CTBs but not the other two GBs, and the α→ω transformation at CTBs leads to considerable wave attenuation (i.e., the elastic precursor decay). Combined with first principle calculations, we find that CTBs can facilitate the overcoming of the energy barrier for the α→ω transformation. Our findings have the potential to influence interface engineering and materials design under extreme conditions. [ABSTRACT FROM AUTHOR]

Interstitial solutes (e.g. O, N) often enhance strength; however, their poisoning effect leads to markedly decreased ductility and even embrittlement in titanium (Ti) alloys. Thus, using unavoidably interstitial O and N atoms to achieve low-cost, ductile Ti alloys with ultrahigh-specific-strength is significant for industrial applications. Here, taking the Ti-4.1Al-2.5Zr-2.5Cr-6.8Mo-0.17O-0.10N (wt.%) alloy as a model material, we successfully achieved an ultra-high yield strength of ∼1800 MPa in this low-cost Ti alloy by a hierarchically heterogeneous microstructure consisting of micron-scaled primary α, nano-scaled secondary α and ultrafine α-Widmanstätten nano-precipitates in the β-matrix. In particular, utilizing grain boundary engineering (GBE), the percolative nano-precipitates network directly precipitated from β-GBs, which not only strengthens GB cohesion, but also effectively blunts the crack tip and hinders crack propagation, rendering enhanced ductility. This strategy combining GBE and interstitial solutes opens an avenue to design ultra-strong and ductile Ti alloys with increased tolerance to interstitial impurities. [Display omitted] [ABSTRACT FROM AUTHOR]

Manipulating coherent twin boundaries (CTBs) opens an avenue to design strong nanostructured materials. However, below a critical TB spacing, these inherently defective CTBs decorated with kink-like steps (abbreviated by kinks) and intersected with grain boundaries (GBs) will suffer from the thermal/mechanical instability, leading to the degradation of material properties. Here, utilizing Cr-segregation at kinks and GBs via a minor (1 at.%) Cr-doping, we report the nanocrystalline-nanotwinned (NNT) Cu-Cr alloy manifests continuous strengthening reaching 1.2 GPa at extremely fine TB spacing of ∼ 2 nm, associated with excellent structural-mechanical stability after high-temperature (0.5 T m of Cu) annealing. The underlying mechanism mainly originates from the highly stabilized defective CTBs controlled by Cr-segregation at kinks and TB-GB junctions, which facilitates the plastic deformation mode transition: from detwinning dislocation nucleation to stacking faults (SFs) accumulation for ultrahigh strength. Under elevated temperature, the stabilized TBs inhibit GB motion and therefore result in enhanced thermal stability of NNT Cu-Cr alloys, which is quantitatively explained via a modified Zenner pinning model. Our findings not only deepen the understanding of deformation mechanisms in nanotwinned metals, but also provide a new perspective to design plainified Cu alloys with high performances. [Display omitted] Take nanocrystalline-nanotwinned Cu with minor Cr addition as an example, we synthesis a class of plainified, strong and stable Cu alloys and meanwhile demonstrate a novel alloy-designing concept, that the highly stabilized defective TBs controlled by grain boundary segregation can render ultrahigh strength and impart high thermal/mechanical stability to nanostructured metals. [ABSTRACT FROM AUTHOR]

Relationships between the three-dimensional (3D) grain structure and the local strain distribution within individual grains are investigated. Multimodal X-ray tomography, i.e. attenuation tomography (ACT) and diffraction contrast tomography (LabDCT), was applied, for the first time using a laboratory instrument, to non-destructively characterize the grain structure and the local plastic deformation behavior of a fully recrystallized Al-4mass%Cu alloy. The evolution of the internal strain distribution during tensile deformation was measured in-situ by means of a microstructural feature tracking (MFT) method based on ACT. By a correlation analysis of the microstructural parameters of 855 grains, it was investigated if and how the initial crystallographic orientations and sizes of the grains affect the local deformations. In addition, effects of grain boundaries were analyzed. Only weak correlations are found. It is suggested that specific interactions between neighboring grains, which depend on parameters such as grain shapes and orientation differences, are of critical importance for the development of the local strains. [Display omitted] [ABSTRACT FROM AUTHOR]

Improved thermal conductivity (TC) in UO 2 -based fuels is critically important for the security of nuclear reactors. Optimizing the microstructure of UO 2 pellets mixed with high thermally conducting materials such as BeO is an effective way to achieve this goal. In this paper, we investigate the relationship between the TC of the UO 2 /BeO composite and their corresponding microstructures by using a convolutional neural network (CNN). The dataset is created by the finite element method. Using the deep transfer learning technique, a well-established CNN (ResNet101) successfully predicts the TC based on the highly similar microstructures while the lightweight CNN fails. Meanwhile, the microstructural features impacting the TC of the UO 2 /BeO composite are determined by the ablation class activation map and subsequently verified. Moreover, a proposed microstructural feature contributes to the improved performance of the state-of-the-art machine learning model on our task, indicating its essential role in the TC of the UO 2 /BeO composite. This work demonstrates that the critical microstructural features that significantly affect the TC can be discovered by deep learning-based methods and may provide a guideline for enhanced TC of UO 2 /BeO fuel via careful design of its microstructure. : [Display omitted] [ABSTRACT FROM AUTHOR]

Hysteresis in martensitic transformations (MT) limits the usefulness of shape memory alloys (SMAs) in nanosized devices that require high sensitivity, high durability and high energy efficiency. Previous studies have shown that the MT is hindered in the surface region of nanosized SMAs, and therefore there is little hysteresis. However, we find that the hysteretic behavior in SMA nanofilms is not related to the MT suppression. Rather, the decrease in hysteresis is due to weaker spontaneous lattice distortion and spatial heterogeneity, leading to a more continuous phase transformation process. We demonstrate this by designing two classes of nano-sized SMAs, a free-standing Ni 62.5 Al 37.5 film in which the surface region promotes MT, and a multilayer of Fe–Ni 62.5 Al 37.5 –Fe in which the interface region suppresses MT. Both cases show a decrease in hysteresis with decreasing film thickness. Our findings suggest a method to reduce hysteresis in conventional bulk SMAs. [ABSTRACT FROM AUTHOR]

Shape memory alloys (SMAs) that exhibit superelasticity with large recoverable strain and small hysteresis are in demand for practical applications, although their synthesis remains a challenge. We introduce metastable engineering to dope conventional SMA solid-solution atoms of relatively high concentration with "weak" local lattice distortion to realize ultralow hysteretic superelasticity. Large-scale molecular dynamic (MD) simulations of NiTi-based SMAs are performed to demonstrate how the presence of 2 ∼ 4 at.% Nb dopants lead to a stress-induced transition from a metastable pretransitional state to a strain-glass state. This is facilitated by a macroscopically homogeneous and continuous phase transformation in the course of superelastic loading and unloading. This spinodal decomposition-like phase transformation process endows SMAs with anhysteretic superelasticity that is insensitive to loading direction and grain size (below 15 nm). These findings show promise of achieving ultralow hysteretic superelasticity with large recoverable strain for SMAs. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] Large-scale applications of elastocaloric cooling demand bulk materials showing both large adiabatic temperature change (Δ T a d ) and low-fatigue characteristics at room temperature. Using cold-rolling and aging treatment, we synthesize a bulk Ti 49.2 Ni 40.8 Cu 10 polycrystalline alloy that has microstructural features of nanocrystallinity and epitaxially related Ti(Ni,Cu) 2 nanoprecipitates. It exhibits a large Δ T a d of 13.8 K and a coefficient of performance of 13 at room temperature. Moreover, the degradation of Δ T a d is only 0.3 K after 450 tensile cycles. We attribute the favorable properties to the enhanced reversibility of martensitic transformation during stress cycling, aided by the internal epitaxy-generated stress at the interface between the Ti(Ni,Cu) 2 nanoprecipitates and matrix, together with grain refinement. The results indicate that the alloy offers a good balance of multiple objectives, holding promise for solid-state refrigeration applications. [ABSTRACT FROM AUTHOR]

Stress-driven grain boundary (GB) migration has been evident as a dominant mechanism accounting for plastic deformation in crystalline solids. Using molecular dynamics (MD) simulations on a Ti bicrystal model, we show that a uniaxial stress-driven coupling is associated with the recently observed 90° GB reorientation in shock simulations and nanopillar compression measurements. This is not consistent with the theory of shear-induced coupled GB migration. In situ atomic configuration analysis reveals that this GB motion is accompanied by the glide of two sets of parallel dislocation arrays, and the uniaxial stress-driven coupling is explained through a composite action of symmetrically distributed dislocations and deformation twins. In addition, the coupling factor is calculated from MD simulations over a wide range of temperatures. We find that the coupled motion can be thermally damped (i.e., not thermally activated), probably due to the absence of the collective action of interface dislocations. This uniaxial coupled mechanism is believed to apply to other hexagonal close-packed metals. [ABSTRACT FROM AUTHOR]

An open question in the kinetics of martensitic transformation is the microscopic mechanism responsible for the evolution of a new phase. We have analyzed data from shocked recovered samples that monitor the volume fraction of the ω phase in α-Zr as a function of time under isothermal conditions in the temperature range 430-545K. Our results show that the effective activation energy strongly depends on the peak pressures of shock compression. In addition, we confirm that the orientation relationship in Zr for this reverse transformation is consistent with the original suggestion by Silcock in Ti for a direct α→ω martensitic transition. Combined with large-scale molecular dynamics simulations, we find that the difference in the effective activation energy, which is related to the nature of the isothermal kinetics, is controlled by heterogeneous nucleation from defects in the microstructure. [ABSTRACT FROM AUTHOR]

Abstract: Diffuse scattering is an important precursor phenomenon prior to the martensitic transformation (MT). It is related to the correlated atomic position fluctuations prior to the MT and can provide important hints of the transformation mechanism. However, the role of this precursor phenomenon in the MT is not clear so far. Here we study the evolution of diffraction patterns prior to temperature- and stress-induced MTs and consider the evolution of atomic configurations during the whole MT process, using molecular dynamics simulations on a generic body-centered cubic–hexagonal close-packed transformation as an example. Our results show that, although the diffuse scattering changes with external fields, there exists a general relationship between the transformation pathways, the diffuse scattering streaks and the martensitic products. Two preferred transformation pathways with opposite shuffle directions lead to a single specific diffuse scattering streak prior to the MT and form one pair of anti-variants after the MT. Thus the distribution of diffuse scattering acts as an indicator of the selection of martensitic variants. In addition, we find that the applied stress can change the shear order parameter of the phase transformation, and subsequently determines the preferred transformation pathways and the distribution of diffuse scattering streaks. This work establishes a relationship between the transformation mechanism, the precursor phenomenon and the products after the MT under the influence of external fields. [Copyright &y& Elsevier]

Abstract: We investigate shock-induced phase transformations in titanium (α-Ti) single crystals induced by shock loading along the [0001], and directions using molecular dynamics simulations. We find a significant dependence of the microstructure evolution on the crystallographic shock direction, providing insight into the nature of the coupling between deformation and phase transformation. For shock along the c-axis, the orientation relationships (ORs) and between parent and product phases are observed, which differs from that previously reported for Ti. For shock compression along the and directions, there is a reorientation of the hexagonally close-packed α phase before the α→ω martensitic transformation, and the OR is consistent with the previously proposed Silcock relationship. We associate the reorientation with a shuffle and shear mechanism and suggest that shear stress is an underlying factor for the anisotropic phase transformation sensitivity. [Copyright &y& Elsevier]

Abstract: An analytical solution is presented that interprets the effects of grain size, surface and grain boundary diffusivities, surface and grain boundary free energies, as well as grain boundary grooving on the creep rate in free-standing polycrystalline thin metal films. The Coble creep in the film plane is also taken into account; this has a significant effect on the creep rate of the film. The effects of diffusion ratio and free energy ratio between surface and grain boundary on film agglomeration are illustrated as well. A closed-form expression for stress relaxation in films under constrained strain conditions is derived from this solution. An exponential decay in stress is found as a function of the film microstructure. Results predicted by the solution are shown to be in agreement with the experiments. [Copyright &y& Elsevier]

Abstract: During repeatedly imposed thermally induced martensitic transformations in Ti–Ni shape memory alloys, the martensite start temperature Ms decreases. This has been rationalized on the basis of a scenario where an increasing dislocation density makes it more and more difficult for martensite to form. However, it is not clear why dislocations which form because they accommodate the growth of martensite during the first cooling cycle should act as obstacles during subsequent transformation cycles. In the present work we use diffraction contrast transmission electron microscopy to monitor the formation of unique leaf-like dislocation substructures which form as the martensite start temperature decreases during thermal cycling. We interpret our microstructural results on the basis of a microstructural scenario where dislocations play different roles with respect to the propagation of a big martensite needle in one transformation cycle and the nucleation and growth of new martensite needles in the following cycles. As a consequence, chestnut-leaf-like dislocation arrays spread out in different crystallographic directions. [Copyright &y& Elsevier]

Abstract: Pristine single crystalline gold particles with sizes ranging from 300 to 700nm have been fabricated through high-temperature (1150°C) liquid de-wetting of gold thin films atop a specially designed SiO2/Si substrate for in situ transmission electron microscopy testing. Quantitative compression tests showed that these particles display cataclysmic structural collapse immediately following elastic loading to very high stresses (over 1GPa), resulting in a nearly pristine postmortem microstructure despite the large plastic deformation experienced by the particle. This distinct class of dislocation plasticity behavior is attributed to the very high degree of structural perfection of the initial sample, resulting from high-temperature formation or annealing around the melting point. Temporally correlated dislocation nucleation from the contact interface together with the inability to form stable junctions inside is proposed to explain the pristine-to-pristine structural collapse. Upon further compression, once the contact diameter d increases to above a critical value (∼250nm), continuous plastic deformation begins to set in under relatively low flow stress with the postmortem microstructure containing a high density of tangled dislocations, suggesting that a critical dislocation tangling volume under multiple slip is needed for the onset of dislocation storage (robust dislocation jamming) and more conventional plasticity. [Copyright &y& Elsevier]

Abstract: We revisit the martensite aging effect in Au–Cd alloys, by studying the evolution of dynamic properties (i.e. storage modulus and internal friction) with aging time. We found that the storage modulus shows a gradual increase while the internal friction shows a gradual decrease with increasing aging time. Moreover, the evolution of dynamic properties with aging time obeys a simple relaxation function with activation energy of 0.46eV in AuCd49.5 alloy. The dependence of such evolution on temperature, defect concentration, frequency and displacement amplitude was also well characterized. All the observed characteristics are understood by the gradual stabilization of martensite phase during aging, driven by a symmetry-conforming short-range ordering tendency of point defects. Furthermore, we proposed a homogenous Landau-type model to describe the response of storage modulus during aging by introducing a time-dependent variable coupled with the order parameter. Such systematic investigation on the dynamic properties during martensite aging is crucial for applications under mechanical vibrations. [Copyright &y& Elsevier]

Abstract: Strain glass is a new glassy state discovered recently in Ni-rich Ti–Ni ferroelastic alloys. However, it remains unclear whether or not strain glass can be found in a wide range of ferroelastic systems. Here, we investigated the transition behavior of three different defect-doped systems Ti50(Ni50− x D x ) (D=Co, Cr, Mn) and found a striking similarity in their transition behavior as a function of defect concentration x. In all these systems, there exists a critical doping level x c at which the transition behavior shows an interesting crossover. At x < x c, these materials undergo a martensitic transition and the transition temperature decreases with increasing x. However, at x > x c, the martensitic transition is suppressed and a strain glass transition occurs. These results imply that strain glass may be quite general in defect-containing ferroelastic systems. An analysis with a modified Landau-type free energy landscape suggests that although point defects always favor the formation of strain glass, to actually form a strain glass they need to destabilize the martensite of the system. [ABSTRACT FROM AUTHOR]

In the present study, the reversible adiabatic temperature change (Δ T a d ) of shape memory alloys was shown to be proportional to the mechanical work released by the reverse martensitic transformation during unloading (Δ W u). As there exists a considerable amount of data (Δ W u) on superelastic stress-strain measurements in shape memory alloys, the proposed relationship allows us to predict Δ T a d without caloric measurements. The estimated Δ T a d from the Δ W u of different Ti-Ni alloys shows good linear relationship with the directly measured values. Moreover, following such a design criterion and by tuning of composition and thermo-mechanical treatment, a group of Ti-Ni binary shape memory alloys with directly measured Δ T a d larger than 35 K under tension were achieved. The large Δ T a d and Δ W u can be ascribed to the grain refinement and the heterogeneous internal stress fields after the thermo-mechanical treatment, which have enhanced the critical stress of superelasticity and recoverability of martensitic transformation during unloading. [ABSTRACT FROM AUTHOR]

Abstract: Previous work by Honma et al. suggested that there is an order–disorder transition around 1090°C in Ti–Ni alloys, but its existence has not been confirmed by independent investigation. In the present work, various techniques including TG-DTA, resistance measurement and TEM were employed to detect the possible signs of such a transition. We found interestingly that when strong measures were taken to prevent sample oxidation, no sign for order–disorder transition was detected; but when the sample was allowed to be oxidized during measurement, we did find a clear DTA peak at 1118°C. With SEM-EPMA analysis this peak was identified as originating from a eutectic reaction in the oxidation-affected layer, where selective oxidation of Ti causes significant enrichment of Ni and hence drives the oxidation-affected layer into the eutectic regime. Therefore, our results disprove the existence of “order–disorder transition” in the B2 portion of the Ti–Ni phase diagram. [Copyright &y& Elsevier]

Abstract: After aging at intermediate temperatures (400–500°C), Ni-rich Ti–Ni alloys undergo an abnormal 3-stage martensitic transformation behavior (1-stage R and 2-stage B19′), which stems from a preferential Ti3Ni4 precipitation around grain boundary. On the other hand, if aged at low-temperatures (250–300°C), they undergo 2-stage R-phase transformation, but the origin of this strange phenomenon is unclear. In the present study, we made a systematic study of this phenomenon by considering the grain boundary effect and composition effect. We found that all single crystals undergo 1-stage R-phase transformation; in contrast, the transformation behavior of polycrystals is dependent on Ni content: low-Ni (50.6Ni, 51Ni) polycrystals undergo 2-stage R-phase transformation while high-Ni (52Ni) polycrystals undergo 1-stage R-phase transformation. The abnormal 2-stage R-phase transformation is attributed to a large-scale compositional heterogeneity in B2 matrix between grain boundary region and grain interior, due to the heterogeneity in precipitate density between the grain boundary and grain interior. But for high-Ni polycrystals, precipitates are essentially homogeneously distributed across the whole grain and this leads to normal 1-stage R-phase transformation. The different transformation behavior of low-Ni and high-Ni polycrystals stems from a competition between two opposing tendencies: (1) for preferential precipitation in the grain boundary; (2) for homogeneous precipitation across the whole grain with high-Ni content. The difference between the effect of intermediate-temperature and low-temperature aging lies in the difference in the ability for long-range diffusion of Ni (from the grain interior to the grain boundary), which results in whether or not Ti3Ni4 precipitates can form in the grain interior. Our results lead to a unified explanation for different transformation behaviors of both low-temperature and intermediate-temperature aged alloys in terms of the kinetics of precipitation in supersaturated polycrystals. [Copyright &y& Elsevier]

The natural aging (NA) response of a commercial Al–Zn–Mg alloy has been tracked to investigate the effects of solute clusters on its mechanical properties. It has been observed that the increase of yield strength during NA is not accompanied by the degradation of uniform elongation due to the simultaneously enhanced strain hardening ability. As a consequence, the Al–Zn–Mg alloy with dense solute clusters shows a comparable yield strength, better strain hardening ability and uniform tensile strain relative to its artificially aged counterparts containing precipitates. This positive effect of solute clusters on strain hardening has been systematically studied by tracing the microstructure evolution during deformation through synchrotron X-ray diffraction and atom probe tomography. We found that the dislocation multiplication dominates over the entire deformation process until failure in NA alloys; however, no effect of solute clusters on the dislocation density evolution can be identified. On the other hand, solute clusters themselves dramatically evolve, showing a dissolution-to-coarsening transition during deformation, which can be understood on the basis of a kinetic model. The experimental evidence strongly suggest that the dislocation storage and strain-induced evolution of solute clusters are insufficient to account for the observed high strain hardening rate, and the contribution from other possible mechanisms are estimated in a semi-quantitative manner. [ABSTRACT FROM AUTHOR]