In-situ synchrotron high energy X-ray diffraction study of spontaneous reorientation of R phase upon cooling in nanocrystalline Ti₅₀Ni_45.5Fe_4.5 alloy

This study investigated a peculiar phenomenon of self-reorientation of thermally formed R phase in nanocrystalline Ti₅₀Ni_45.5Fe_4.5 by means of in-situ synchrotron high energy X-ray diffraction (HE-XRD). Two samples with different average grain sizes of 40 and 90 nm were investigated. R phase in the 40-nm grain size sample was found to self-reorient gradually upon cooling, whereas the same phenomenon did not occur in the 90-nm grain size sample. This self-reorientation process is attributed to the development and evolution of an internal stress anisotropy caused by the second order continuous lattice distortion of R phase upon further cooling in the small nanograined matrix, which lacks the self-accommodation mechanism for internal stress cancellation.