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 transformation temperatures were determined by DSC and the thermomechanical behaviour was analysed by three-point bending. The present study focussed on the localized heat treatment (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 characterization is presented in order to get a perspective of the optimization parameters for the adequate graded functionality. Experimental results put in evidence the structural gradient resulting from the localized heat treatments. The analysis of the in-situ X-ray diffraction (XRD) provide detailed information about the phase transformations in different regions of the wire, at different steps of the cyclic load/unload, giving a better understanding of the overall mechanical behaviour of a functionally graded material.
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