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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]

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]

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]

[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]