NiTi shape-memory transformations: minimum-energy pathways between austenite, martensites, and kinetically-limited intermediate states

TL;DR

This study uses advanced computational methods to elucidate atomic-scale transformation pathways and energy barriers in NiTi shape-memory alloy, revealing new stable phases and intermediate states crucial for its shape-memory behavior.

Contribution

It introduces a generalized solid-state nudged elastic band method applied via density-functional theory to detail NiTi's transformation pathways and energy barriers, including a newly identified stable austenite structure.

Findings

Identification of a stable austenite structure with B2-like features

Detailed energy barriers for phase transformations

Observation of kinetically limited intermediate R-phase

Abstract

NiTi is the most used shape-memory alloy, nonetheless, a lack of understanding remains regarding the associated structures and transitions, including their barriers. Using a generalized solid-state nudge elastic band (GSSNEB) method implemented via density-functional theory, we detail the structural transformations in NiTi relevant to shape memory: those between body-centered orthorhombic (BCO) groundstate and a newly identified stable austenite ("glassy" B2-like) structure, including energy barriers (hysteresis) and intermediate structures (observed as a kinetically limited R-phase), and between martensite variants (BCO orientations). All results are in good agreement with available experiment. We contrast the austenite results to those from the often-assumed, but unstable B2. These high- and low-temperature structures and structural transformations provide much needed atomic-scale detail for transitions responsible for NiTi shape-memory effects.