IN-SITU DEFORMATION STUDIES OF NiTi SHAPE MEMORY ALLOY USING SYNCHROTRON DIFFRACTION

Abstract

The in-situ deformation behavior of a superelasticNiTi Shape Memory alloy with Ni-49.4 at% Ti was examined using synchrotron x-ray diffraction. Two tensile specimens, one with uniform cross-section the other with varying cross-section along the gauge length were tested to understand the evolution of fraction of martensitic phases, lattice strain and crystallographic texture with deformation. Further, the sample with uniform cross-section was provided loading and unloading up to three cycles and subsequent changes in the crystallographic texture are reported. The changes in lattice strain calculated from the changes in d spacings of different crystallographic planes suggested that NiTi is elastically anisotropic. Quantitative phase analysis at certain loads was reported for both tensile specimens. Inhomogeneous (localized) phase transformation along the gauge length has been observed. Recalculated pole figures of the austenite and martensite phases exhibited a non-uniform change in the intensities of different variants of the same texture component. This suggests a possibility for the heterogeneous nature of deformation not only at the macroscopic scale, but also at the microscopic scale. All these results would contribute to a better understanding of phase transformation during uni-axial loading of superelasticNiTi alloys.