Origins of the transformability of nickel-titanium shape memory alloys

Abstract

The near equiatomic NiTi alloy is the most successful shape memory alloy by a large margin. It is widely and increasingly used in biomedical devices. Yet, despite having a repeatable superelastic effect and excellent shape-memory, NiTi is very far from satisfying the conditions that characterize the most reversible phase-transforming materials. Thus, the scientific reasons underlying its vast success present an enigma. In this paper, we perform rigorous mathematical derivations and accurate density-functional theory calculations of transformation mechanisms to seek previously unrecognized twinlike defects that we term involution domains, and we observe them in real space in NiTi by aberration-corrected scanning transmission electron microscopy. Involution domains lead to an additional 216 compatible interfaces between phases in NiTi, and we theorize that this feature contributes importantly to its reliability. They are expected to arise in other transformations and to alter the conventional interpretation of the mechanism of the martensitic transformation.