Effect of Cold Work and Annealed on Tensile Properties and Microstructure in Ti50Ni47Fe3 Shape Memory Alloy

Up to now, Ti₅₀Ni₄₇Fe₃ shape memory alloy has been the most important and successful tube coupling material. The microstructure, tensile and recovery properties of Ti₅₀Ni₄₇Fe₃ alloy are very sensitive to working procedure and heat treatment process. The present work systematically investigated the effects of working procedure and heat treatment on tensile properties and microstructure in Ti₅₀Ni₄₇Fe₃ alloy. The results are as follows. For the hot-worked Ti₅₀Ni₄₇Fe₃ material, the yield and ultimate strength are about 300 MPa and 600MPa, respectively, even if the working temperature decreases to 773K. The yield and ultimate strength of hot-worked material do not increase with the hot-worked deformation rising. When the heat treatment temperature increases, the yield and ultimate strength and the martensite transformation temperatures decrease, whereas the R phase and reverse martensite transformation temperatures change slightly. The cold-worked procedure increases the strength, and the yield and ultimate strength increase with the cold-worked deformation rising. For cold-worked Ti₅₀Ni₄₇Fe₃ material, the afterward heat treatment temperature affects the tensile strength significantly. With the temperature increasing, the yield and ultimate strength decrease and the elongation increases. The microstructure observation indicates that the mechanical twin occurs in substructure. With the annealed temperature increasing the mechanical twin gradually lessens, when annealed temperature exceeds 823K, the mechanical twin nearly disappears. For cold-worked and middle temperature annealed sample, it recovers completely after 8% pre-strain at 103K.