In situ Structural Characterization of Functionally Graded Ni-Ti Shape Memory Alloy During Tensile Loading

A functionally graded NiTi shape memory alloy wire was investigated by in-situ synchrotron radiation-based x-ray diffraction during cyclic tensile deformation. The transfor-mation temperatures were determined by DSC and the ther-momechanical behaviour was analysed by three-point bend-ing. The present study focussed on the localized heat treat-ment (Joule heat effect, reaching 300ºC, 350 and 400ºC pulses for 10 minutes) of NiTi wires, using an equipment that allows a large variety of localized heat treatments. Structural, mechanical and thermomechanical characteriza-tion is presented in order to get a perspective of the optimi-zation parameters for the adequate graded functionality. Ex-perimental results put in evidence the structural gradient re-sulting from the localized heat treatments. The analysis of the in-situ X-ray diffraction (XRD)provide detailed infor-mation about the phase transformations in different regions of the wire, at differentsteps of the cyclic load/unload, giv-ing a better understanding of the overall mechanical behav-iour of a functionally graded material.