Understanding the asymmetric orientations and stress states in polycrystalline NiTi SMA by in-situ synchrotron-based high-energy X-ray diffraction

The stress-induced martensite transformations (SIMTs) dramatically affect the recoverable strain and mechanical response of polycrystalline NiTi. An in-depth understanding of the propagation manner and orientation preference of SIMTs is therefore crucial. In this work, we present a unique asymmetric anisotropy of SIMTs and lattice strains induced by Luders-type deformation in polycrystalline NiTi, achieved through a combination of in-situ synchrotron X-ray diffraction and uniaxial tensile loading and unloading experiments. Our experimental findings reveal that in polycrystalline NiTi under uniaxial deformation, the asymmetry of SIMTs is attributed to the inhomogeneous strain field caused by the Luders-type mechanism. The asymmetrical SIMT starts with the forward Luder band and disappears along with the backward Luder band. The austenite with the favored orientation of < 110 & rang;A//loading direction (LD) transformed and recovered back at a higher rate compared to other orientations during both loading and unloading.