Chemically disordered uniform promotes negative temperature-dependent superelasticity in shape memory alloys

An increasing number of studies on shape memory alloys (SMAs) have shown anomalous temperature dependence of superelasticity, i.e., the superelastic stress with temperature (dσ/dT) deviates from the behavior expected from the Clausius–Clapeyron relationship, especially showing a negative dσ/dT 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 dσ/dT and fluctuation in chemical concentration. By comparing the superelastic behavior of Ni₅₀Ti_47.2Nb_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 dσ/dT 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.