Microstructure and Properties of NiTi Alloy Annealed by Pulsed Current

The cold-rolled NiTi alloy was annealed at different temperatures using pulsed current and muffle furnace,respectively, and the effect of pulsed current annealing on the microstructure and phase transformation behavior of the alloy was studied. The results show that when the pulse current annealing is used, granular and elliptical flake-like Ti₃ Ni₄ precipitates are precipitated in the alloy after annealing at 450 ℃ and 550 ℃, and the microstructure changes from martensite twins in cold rolled state to equiaxed austenite. Recrystallization can be completed after annealing at 650 ℃. As the pulse current annealing temperature increases, the phase transformation behavior of the alloy after annealing changes. During the cooling process of the DSC phase transformation behavior test, the phase transformation type changes from B2 →B19’ to B2→R→B19’ type transformation（where B2:austenite phase A; B19’:martensite phase M）, the peak transformation temperatures of martensite transformation M_p and austenite transformation A_p gradually decrease, and when annealing at 650 ℃, the final peak transformation temperature M_p =-53.81 ℃, A_p =-45.15 ℃. With the increase of annealing temperature, the hardness of NiTi alloy generally shows a decreasing trend. Ti₃ Ni₄ phase precipitated during annealing at 550 ℃ acts as dispersed inclusion particles, which increases the matrix strength and resulting in an abnormal phenomenon of hardness increase. After annealing at 650 ℃, recrystallization is completed and the hardness drops to 252 HV0.2. Compared with the muffle furnace annealed sample, pulse current annealing reduces the recrystallization temperature of the NiTi alloy by 100 ℃ and reduces the precipitation temperature of the Ti₃ Ni₄ precipitated phase. Compared with that of the muffle furnace annealed sample, the hardness decreases faster with the increase of the pulse current annealing temperature.