On the Strong Dependence of Transformation Temperatures on Alloy Composition in NiTi-Based Shape Memory Alloys

It is well known that the martensite start temperature (M_S) in NiTi-based shape memory alloys (SMAs) strongly depends on alloy composition. Materials scientists struggle to explain this effect since 1965, when Wang, Buehler and Pickard first described the strong dependence of M_S on Ni-concentration in near stoichiometric binary NiTi. In the present study we take a new look on compositional effects in binary NiTi, ternary NiTiX (X=Cr, Cu, Hf, Pd, Pt, Zr) and quaternary NiTiYZ (Y: Cu, Z: Co, Pd) SMAs. Using differential scanning calorimetry and density functional theory simulations we show that the heat of transformation ΔH decreases as the Ni-concentration increases from 50.0 to 51.2 at.%. This causes a shift in the Gibbs free enthalpy difference of austenite G_A(T) and martensite G_M(T), which in turn results in a lower martensite start temperature. We discuss the roles of various mechanisms including the formation of point defects, changes in lattice constants, and changes in valence electron concentrations, which affect the enthalpy difference between austenite and martensite.