In-situ synchrotron high energy X-ray diffraction study of micro-mechanical behaviour of R phase reorientation in nanocrystalline NiTi alloy

This study investigated the mechanisms and temperature dependence of the R phase variant reorientation deformation in a nanocrystalline NiTi wire by means of in-situ synchrotron high-energy X-ray diffraction technique during tensile deformation. The NiTi wire exhibited a single stage B2 → R transformation upon cooling with the R → B19′ martensitic transformation completely suppressed down to the liquid nitrogen temperature. The R phase variant reorientation deformation in tension was revealed to proceed in a Lüders-manner, as evidenced by sudden changes of diffraction peak intensity and increases of lattice elastic strains of the R phase during the deformation. The variant reorientation obeys the principles to yield maximum crystallographic strain in the direction of the applied load. Thus, the variants with larger d-spacing values aligned to the axis of tension grew at the expense of those of lower d-spacing values. The increases of lattice elastic strains are attributed to the internal load transfer among the different crystallographic variants during the R phase reorientation. The crystallographic strain of R phase reorientation was found to increase with decreasing the deformation temperature. This corresponds directly to the continued evolution of the structure of the R phase of increased structural distortion relative the B2 phase with decreasing the temperature. This reflects the second order nature of the R phase.