Three-dimensional in situ characterization of phase transformation induced austenite grain refinement in nickel-titanium

TL;DR

This study uses advanced 3D in situ imaging techniques to analyze phase transformation-induced austenite grain refinement in nickel-titanium shape memory alloys during thermal cycling under load.

Contribution

It provides the first 3D in situ visualization of microstructural evolution and grain boundary formation in NiTi alloys during thermal cycling.

Findings

Significant subgrain refinement after one cycle

Emergence of heterogeneous {} and {} grain boundaries

Heterogeneous distribution explains previous microscopy inconsistencies

Abstract

Near-field and far-field high-energy diffraction microscopy and microcomputed tomography X-ray techniques were used to study a bulk single crystal nickel-titanium shape memory alloy sample subjected to thermal cycling under a constant applied load. Three-dimensional in situ reconstructions of the austenite microstructure are presented, including the structure and distribution of emergent grain boundaries. After one cycle, the subgrain structure is significantly refined, and heterogeneous {\Sigma}3 and {\Sigma}9 grain boundaries emerge. The low volume and uneven dispersion of the emergent {\Sigma} boundaries across the volume show why previous transmission electron microscopy investigations of {\Sigma} grain boundary formation were inconsistent.