Reveling the orientation preference along with localized Lüders-type deformation in polycrystalline NiTi SMA by in-situ synchrotron-based high energy X-ray diffraction

The stress-induced martensite transformations (SIMTs) in near-equiatomic NiTi shape memory alloys (SMAs) predominantly occur through localized, inhomogeneous, and intense Lüders-type mechanisms, which significantly influence the recoverable strain and mechanical response of the material. An in-depth understanding of the propagation manner and orientation preference of SIMTs is therefore crucial. In this study, we present a unique asymmetric anisotropy of SIMTs and lattice strains induced by Lüders-type deformation in polycrystalline NiTi, achieved through a combination of in-situ synchrotron X-ray diffraction and uniaxial tensile loading experiments. Our experimental findings reveal that in polycrystalline NiTi under uniaxial tensile loading, the austenite with the favored orientation of ⟨110⟩A//loading direction (LD) is consumed faster compared to other orientations, resulting in residual austenite with an orientation of ⟨431⟩//LD within the Lüders banding area. In contrast, the high-strain residual austenite with few favored orientations transforms fairly slowly and remains well beyond the transformational plateau. Our work provides valuable new insights into the microstructural nature of the Lüders-type deformation mechanism of polycrystalline NiTi, and the enhanced understanding of these complex interactions holds promise for optimizing the performance and design of SMAs in practical applications.