Origins of the transformability of Nickel-Titanium shape memory alloys

TL;DR

This paper investigates the fundamental transformation mechanisms of NiTi shape memory alloys, revealing new twin-like defects called involution domains that enhance phase compatibility and may explain NiTi's exceptional reliability.

Contribution

It introduces the concept of involution domains in NiTi, supported by mathematical derivation, DFT calculations, and experimental observation, offering new insights into its transformation behavior.

Findings

Involution domains increase compatible phase interfaces in NiTi.

DFT calculations support the existence of twin-like defects.

Involution domains may influence transformation mechanisms in other 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 work, we perform rigorous mathematical derivation and accurate DFT calculation of transformation mechanisms to seek previously unrecognized twin-like defects that we term involution domains, and we observe them in real space in NiTi by the 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.