In Situ Structural Characterization of Functionally Graded Ni–Ti Shape Memory Alloy During Tensile Loading
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A functionally graded NiTi shape memory alloy wire was investigated by in situ synchrotron radiation-based X-ray diffraction (SR-XRD) during cyclic tensile deformation. The transformation temperatures were determined by DSC and the thermomechanical behaviour was analysed by three-point bending test. The present study focussed on the localized heat treatment (Joule heat effect, reaching 300 °C, 350 and 400 °C pulses for 10 min) of NiTi wires, using an equipment that allows a large variety of graded conditions. Structural, mechanical and thermomechanical characterization is presented to get a perspective of the different types of graded functionality. A combination of two strategies has been used for the in situ analysis by SR-XRD of the tensile tests: (i) continuously following the structural evolution at one single point (at the center of the heat-treated segment) all long the load/unload cycle and (ii) scanning the full heat-treated segmentat previously defined discrete steps of the stress–strain curve. The combined information from both types of tests provided detailed information about the phase transformations taking place in different regions of the functionally graded segment, at different steps of the tensile load/unload cycle, giving a better understanding of the overall mechanical, namely the evidence of the sequence B2 ↔ R ↔ B19′ for the direct and reverse transformations.
KeywordsShape memory alloy X-ray diffraction Synchrotron Functionally graded material
Authors gratefully acknowledge the funding of Project POCI-01-0145-FEDER-016414 (FIBR3D), cofinanced by Programa Operacional Competitividade e Internacionalização and Programa Operacional Regional de Lisboa, through Fundo Europeu de Desenvolvimento Regional (FEDER) and by National Funds through FCT - Fundação para a Ciência e a Tecnologia. P.F.R., EC and F.M.B.F acknowledge the funding of CENIMAT/I3N by National Funds through FCT - Portuguese Foundation for Science and Technology, Reference UID/CTM/50025/2019 and FCT/MCTES. P.F.R. acknowledges the funding of CAPES (CsF/ Brazil—BEX 11943-13-0). TS and PI acknowledge Fundação para a Ciência e a Tecnologia (FCT - MCTES) for its financial support of UNIDEMI via the project UID/EMS/00667/2019. Parts of this research were carried out at beamline P-07-HEMS at DESY, a member of the Helmholtz Association. The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020.
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