Microstructure of TiNi shape-memory alloy synthesized by explosive shock-wave compression of Ti-Ni powder mixture

Abstract

Cylinders of TiNi shape-memory alloy were synthesized from mixtures of equiatomic fine irregular titanium and nickel powders by explosive-wave compression with a detonation velocity of about 6500 m s-1. B2 type parent phase, R phase, B19′ type martensite, Ti2Ni, Ti3Ni4 and Ti2Ni3 phases were observed in this as-synthesized material. In the B2 matrix high density dislocations existed. The Burgers vectors of many dislocations were determined to be parallel to \(\left\langle {111} \right\rangle \) directions. The R phase variants formed (0 0 1) B2 twinning structure. The substructure of the B19′ martensite was (0 0 1) B19′ type I twin and stacking faults on the (0 0 1) B19′ plane. When increasing the temperature of the as-synthesized material in a differential scanning calorimeter, no B19 ′ → R → B2 transitions were observed on the temperature range −50 to 100 °C. However, B2 → B19′(R) transitions occurred during the cooling cycle. After heat treating the specimen at 800 °C for 1 h and then ageing at 400 °C for 10 min, both B2 → R → B19′ and B19′(R) → B2 phase transitions were observed.