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