Origin of the asymmetry in martensitic phase transitions in off-stoichiometric NiTi near equiatomic compositions

Nitinol (NiTi) is the most commonly used and most extensively studied shape memory alloy (SMA). However, it remains a mystery that a small increase of Ni concentration $({x}_{\mathrm{Ni}})$ from 50% to 51% in NiTi results in a dramatic drop of about 110 K in the martensitic transition temperature (MTT), while a similar increase of Ti barely changes MTT. This weird behavior drastically affects both the manufacturing and applications of the NiTi-based SMAs, which represent the majority of ongoing scientific research efforts in this field. Using the most accurate first-principles methods, we find that this trend in MTT can be well reproduced theoretically. Furthermore, the physical origin of this striking asymmetry in MTT is revealed to be the fact that the atomic size of Ni is much larger than that of Ti in NiTi; though for free atoms, Ti is larger than Ni. Replacing smaller Ti atoms with bigger Ni atoms in NiTi leads to a much larger deviation from the ideal cubic B2 structure in the austenite phase than does the other way around.