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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]

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]

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]

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]

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]

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: 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]