Influence of solution heat treatment on the microstructural evolution and mechanical behavior of 60NiTi

The microstructures and mechanical properties of 60NiTi alloys after solution treated at 980 °C, 1005 °C, 1030 °C and 1055 °C for 2 h, 4 h, 8 h were studied. The results showed that there was undissolved Ni₃Ti phase in the specimens after heat treated at 980 °C and 1005 °C for 8 h, and the specimens consisted of Ni₃Ti, B2 NiTi and Ni₄Ti₃ phase. The volume fraction of the Ni₄Ti₃ phase was about 36% and 46% respectively. After heated at 1030 °C and 1055 °C for 8 h, specimens consisted of B2 NiTi and Ni₄Ti₃ phase, and the Ni₃Ti phase was totally dissolved into the matrix. As a result, the volume fraction of Ni₄Ti₃ phase increased to 69% and 64%. Higher density of the Ni₄Ti₃ phase contributed to the higher hardness and higher ultimate compressive strength. The fracture modes of samples after solution treated were mainly quasi-cleavage fracture. The undissolved Ni₃Ti phase improved the fracture toughness of the binary alloy. Crack propagation path in Ni₃Ti rich area was featured by a zigzag mode, which indicated that Ni₃Ti phase displayed positive effect on fracture toughness. The toughening mechanism of 60NiTi with Ni₃Ti phase is emphasized.