Stress-induced phase transformation and detwinning in NiTi polycrystalline shape memory alloy tubes

Deformation behavior associated with initial austenite (A), rhombohedral (R) and martensite (M) phase structures was studied in polycrystalline NiTi shape memory alloy tubes by tensile testing at different temperatures. The nominal stress–strain curves of the tubes from room temperature (23 °C) to 70 °C were recorded. The deformation of NiTi tubes with initial structure of R-phase proceeded via R → M martensitic transformation, while the deformation of NiTi tubes with initial structure of M-phase proceeded via martensitic detwinning. It was found that the R → M martensitic type transformation was realized, at the macroscopic level, by nucleation and growth of an inclined cylindrical band, while the detwinning process of the tube was macroscopically homogeneous. Further, two-stage yielding, which is associated with austenite to rhombohedral (A → R) and R → M phase transformations, was observed in the stress–strain curves of NiTi tubes in a certain testing temperature range. With a further increase in temperature, the shape of the nucleated band remained cylindrical until 60 °C (>Af) when the shape of the initial band suddenly became helical which was well observed in the superelastic microtubing.