Ab initio study of martensitic transformation in nitipt high temperature shape memory alloys

The crystal structures and martensitic transformation of Ti₅₀Ni_50−xPt_x alloys (x = 0, 6.25, 8.33, 10.42, 12.5, 18.75, 25) were studied by means of density functional theory (DFT). The computational results indicate that the lattice parameters of Ti-Ni-Pt alloys continuously increase with increasing the Pt content. It is found that at ≤ 12.5 at.% Pt, the martensite structure is monoclinic B19^′ phase, and the energy differences between parent and martensite phases (∆E) decrease slightly with a minimum observed at 6.25 at.% Pt. However, when the Pt content is increased to around 15 at.%, the most stable martensite phase is the orthorhombic B19 structure, and the ∆E increases sharply with Pt concentration. It was found that the phase transition temperatures are closely related to the energy differences ∆E between parent and martensite phases. The electronic structures of martensite B19 and B19^′ phases are also discussed.