Low-Temperature Mechanical Instabilities Govern High-Temperature Thermodynamics in the Austenite Phase of Shape Memory Alloy Constituents: Ab Initio Simulations of NiTi, NiZr, NiHf, PdTi, and PtTi

Recent studies on the constituent binaries of common NiTi-based shape memory alloys (NiTi, NiZr, NiHf, PdTi, PtTi) have led to questions regarding accuracy current phase diagrams, including existence theoretical phases, and atomistic nature stability in observed martensitic austenitic phases. These issues are investigated here with ab initio molecular dynamics. Theoretically identified ground state structures found become mechanically unstable well below room temperature, while experimentally determined martensite be stabilized by anharmonic effects at elevated temperatures. The phases all transition a austenite structure (cubic B2), temperatures agreement available experiments. Interestingly, B2 low temperature but large that also govern transition. An signature this anharmonicity is transitory symmetry breaking distorted, glassy known as B2’. energy B2’ quantified and, across binaries, linearly correlated entropy from high-temperature B2. implication finding properties low-temperature ultimately transitions Means utilizing these results simplify characterization thermodynamics associated important effect discussed. A reduced order model presented captures competition between crystal lattice differences (B2 vs martensite) degree characterized B2-B2’ for sytems.