Martensite evolution in a NiTi shape memory alloy when thermal cycling under an applied load

The microstructural evolution of a near equiatomic NiTi shape memory alloy has been studied by in situ synchrotron diffraction, whilst being thermally cycled under an applied load. The martensite texture was found to be cyclically dynamic and an elongation of the sample was observed with each thermal cycle. The martensite formed when cooling under load exhibited progressive variant selection, in an attempt to accommodate the applied stress, accompanied by strain relaxation of the material. The characteristic transformation temperatures were found to decrease with cycling, possibly as a result of the generation and accumulation of dislocations. The magnitude of elongation, variant selection, lattice relaxation and damage accumulation were found to increase as a function of applied stress and their rate decrease with cycle number.