In Situ Structural Characterization of Functionally Graded Ni–Ti Shape Memory Alloy During Tensile Loading

A functionally graded NiTi shape memory alloywire was investigated by in situ synchrotron radiationbasedX-ray diffraction (SR-XRD) during cyclic tensiledeformation. The transformation temperatures were determinedby DSC and the thermomechanical behaviour wasanalysed by three-point bending test. The present studyfocussed on the localized heat treatment (Joule heat effect,reaching 300 C, 350 and 400 C pulses for 10 min) ofNiTi wires, using an equipment that allows a large varietyof graded conditions. Structural, mechanical and thermomechanicalcharacterization is presented to get a perspectiveof the different types of graded functionality. Acombination of two strategies has been used for the in situanalysis by SR-XRD of the tensile tests: (i) continuouslyfollowing the structural evolution at one single point (at thecenter of the heat-treated segment) all long the load/unloadcycle and (ii) scanning the full heat-treated segmentatpreviously defined discrete steps of the stress–strain curve.The combined information from both types of testsprovided detailed information about the phase transformationstaking place in different regions of the functionallygraded segment, at different steps of the tensile load/unloadcycle, giving a better understanding of the overallmechanical, namely the evidence of the sequence B2 $R $ B190 for the direct and reverse transformations