Studies of nanomechanical properties and fatigue strength of annealed Ni–Ti shape memory alloy

The nanomechanical properties and fatigue strength of annealed Ni–Ti alloys are studied using cyclic nanoindentation. The alloys are annealed for 2 h at 400, 500, 600, and 700 °C, respectively. The experimental results show that the hardness and the elastic recovery ratio increase with increasing number of cyclic indentations. The Young's modulus decreases with increasing annealing temperature. The grain size increases with annealing temperature until 500 °C. At 600 °C, the grain size gradually decreases and hardness increases with increasing annealing temperature due to the partial dissolution of Ti3Ni4 precipitates, which prevent dislocation. The elastic recovery ratio of Ni–Ti alloys increases with increasing hardness. The hardness increases with increasing annealing temperature after recrystallization.

► The hardness of Ni-Ti alloys decreases with number of indentation cycles.
► The elastic recovery ratio of Ni-Ti alloys increases with hardness.
► The hardness increases with annealing temperature after crystallization.
► The elastic recovery ratio of Ni-Ti alloys increases with indentation cycles.