Phase equilibrium of Ti-Pt-Nb high-temperature shape memory alloys studied using cluster expansion techniques

The effect of adding Nb on TiPt high-temperature shape memory alloys (HTSMAs) was investigated using the universal cluster expansion and first-principle approach. In this study, cluster expansion was utilized to predict ground state structures containing three (3) elements: Ti, Pt, and Nb. The cluster expansion method generated 45 new Ti-Pt-Nb structures, and these were ranked as stable and meta-stable structures based on their formation energies. Among the six (6) predicted structures, the Ti₄Nb₂Pt₂ system was selected around 50:50 on Platinum (Pt)-rich sites since it is the most stable structure on the ground state line. The supercell approach in MedeA (VASP) was used to create large supercells of about 64 atoms. In addition, the Ti₄Nb₂Pt₂ system was studied further by determining the structural, thermodynamic and mechanical properties using the first-principle density functional theory. The Ti₄Nb₂Pt₂ system was found to be the most thermodynamically stable structure due to its negative heat of formation (-0.361 eV/atom). The materials have similar properties as tetragonal Nb-doped TiPt. The mechanical properties of these compounds revealed that they are ductile in nature and mechanically stable. Furthermore, the phonon dispersion curves showed the vibrational stability of the Ti₄Nb₂Pt₂ alloy due to the absence of soft modes. This work suggests that introducing Nb stabilizes the TiPt SMAs, making them potential candidates for high-temperature applications.