Effects of annealing on softening and hardening behaviors of 60NiTi alloy

Abstract

60NiTi alloy is an important biomedical, structural and functional material. In this paper, the effects of heat treatment (temperature, time and cooling rate) on the microstructure and properties of 60NiTi alloy were studied, and the softening and hardening mechanisms were revealed. The results showed that the number of Ni4Ti3 phase enhanced with an increase of the cooling rate and the precipitation of Ni3Ti phase was inhibited after annealing at 950∼1075 °C followed by water-cooling. The orientation relationship between NiTi matrix and Ni4Ti3 phase obey the rule: [111]NiTi//[0001]Ni4Ti3 and (123¯)NiTi//(112¯0)Ni4Ti3. The faster the cooling rate was, the higher the hardness of the alloy was. The hardness of alloy under different cooling rate ranked from high to low was water-cooling, oil-cooling, liquid nitrogen-cooling, air-cooling and furnace-cooling. The hardness of the 60NiTi alloy was 316 HV after annealing at 1075 °C for 4 h followed by furnace-cooling, and the hardness sharply increased to 669 HV after annealing at 1075 °C for 4 h followed by water-cooling. The phase transformation of Ni4Ti3→Ni3Ti during cooling mainly contributed to the hardness decrease of the 60NiTi alloy.

Keywords

• 60NiTi alloy
• Heat treatment
• Hardening behavior
• Softening behavior