3D in-situ characterization of stress-induced martensitic phase transformation in CuAlNi shape memory alloys using dark-field x-ray microscopy

During the forward and reverse martensitic phase transformation in polycrystalline shape memory alloys (SMAs), local stress fields develop throughout the microstructure, acting as driving forces for transformation. In this research, we employ non-destructive, high-resolution, in situ dark-field X-ray microscopy (DFXM) to obtain three-dimensional (3D) subgrain-scale maps of relative misorientation, elastic strain, and phase domains with a spatial resolution of 100 nm. These measurements reveal the microstructural evolution of two CuAlNi SMA grains during tensile loading, unveiling a consistent martensite growth mechanism: Martensite nucleates at triple junctions where stress is highly localized, then propagates along the habit plane toward a grain boundary or free surface. This propagation is driven by stress concentrations generated in the parent phase at the martensite domain tips. Finally, the martensite domain thickens in the habit-plane-normal direction, and new martensite domains (of the same variant) form adjacent to it. This experiment represents one of the first characterizations of martensitic phase transformation throughout the thickness of a bulk sample, providing a deeper fundamental understanding of the origins of the heterogeneous martensitic phase transformation in polycrystalline SMAs, shaped by the interplay between granular interactions, local stress concentrations, and the nucleation and growth of martensite domains.