In-situ high energy X-ray diffraction study of microscopic deformation behavior of martensite variant reorientation in NiTi wire

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This study investigated the collective evolution of martensite variant structures, the development of crystallographic textures and the load partitioning among the variants during tensile deformation in a commercial polycrystalline NiTi wire by means of in-situ high-energy X-ray diffraction technique. The cold-worked and then crystallized NiTi wire was found to have three fiber textures along the wire axial direction in its thermally formed self-accommodating B19′ martensite, including (1¯20)/(120), (1¯02) and (102). These textural orientations are inheritance of the texture of the B2 phase and are associated with the <011> type II twins and (001) compound twins. Tensile deformation via martensite variant reorientation converted the (102) and (120) fiber textures into (1¯02). The texture evolutions and variant selections are driven by the difference in d-spacing values between the pairing variants in each twin structure in order to achieve maximum elongation deformation in the tensile loading direction. At the meantime, the lattice strains of the favored variants increased whereas those of the unfavored variants decreased (even to negative) abruptly at the occurrence of the Lüders deformation event when at the global stress remained constant over the stress plateau. These changes of the lattice strains are attributed to load transfer and repartitioning among the variants. Based on the lattice strain measurements, the load partitioning among variants is also estimated.