Microstructures and Mechanical Properties of Equiatomic NiTi Shape Memory Alloy Undergoing Local Canning Compression and Subsequent Annealing

Abstract

Local canning compression is imposed on equiatomic NiTi shape memory alloy (SMA) with complete B19′ martensite. A fraction of retained nanocrytalline grains are embedded in the dominant amorphous phase. The compressed NiTi samples are annealed for 2 h at various temperatures, including 300, 450 and 600 °C. Grain size increases with increasing crystallization temperature during heat treatment. Under annealing at 300 °C, nanocrystalline grains are dominant in NiTi SMA, where there exists local amorphous zone. Under annealing at 450 °C, almost complete nanocrystalline can be obtained. Under annealing at 600 °C, grain size increases substantially, but there still exists a small amount of nanocrystalline grains. Under annealing at 300 °C, NiTi SMA exhibits extremely high elastic limit, but its plasticity is poor. Under annealing at 450 °C, NiTi SMA exhibits very high yield stress and it simultaneously keeps high plasticity. Under annealing at 600 °C, NiTi SMA exhibits relatively low yield stress, but its yield stress is still higher than that of as-received NiTi SMA and its plasticity is also high.