Tensile deformation of NiTi shape memory alloy thermally loaded under applied stress

Abstract

Constitutive behavior of engineering materials is typically characterized by stress–strain curves from isothermal tensile and/or compression tests until fracture. Strain reversible behavior of martensitically transforming shape memory alloys (SMA) is additionally characterized by cyclic stress–strain and strain-temperature curves limited to temperatures and stresses, at which the recorded strain responses are reversible in closed loop cycles. In this work focussing coupled martensitic transformation and plastic deformation of NiTi, we deformed nanocrystalline NiTi SMA wire in isothermal and isostress tensile tests beyond the temperature and stress limits stemming from the requirement on the strain reversibility in closed loop cyclic tests. Stress–strain-temperature responses of NiTi wire in such tests were recorded and analysed. To detect and characterize deformation mechanisms activated in performed thermomechanical loads, electric resistance and dynamic elastic modulus of the wire were evaluated in-situ during tensile tests. Martensite variant microstructures and lattice defects in austenite evolving upon heating deformed NiTi wire under 750 MPa stress were analyzed by post mortem transmission electron microscopy. Stress-temperature diagram showing critical stress–temperature conditions for activation of 5 different deformation/transformation processes in thermomechanically loaded NiTi was constructed from the results of isothermal and isostress tests and the recorded stress–strain-temperature responses were discussed based on this diagram