2018 Volume 59 Issue 10 Pages 1560-1566
Ti–15V–7Al alloy, which has a composition exhibiting peculiar phenomena upon tempering, was prepared and its martensitic transformation behavior during cooling and heating was investigated. The structure of the quenched specimen mostly consisted of the β-phase and a small quantity of α′′ martensite near the grain boundaries. When heating an elastically bent strip of the specimen, spontaneous bending occurred, as reported for some other alloys. Subzero treatment using LN2 newly induced some martensites around the prior martensites formed by quenching, but no martensites were formed in the single β region. The formation of the martensites by the subzero treatment exhibited time dependence. Even tempering at 550°C for 3 s, induced the formation of coarse martensites throughout the specimen. All martensites formed by the quenching, the subzero treatment or the tempering disappeared completely upon heat treatment at 200°C for 300 s, resulting in a single β-phase. However, the coarse martensites were regenerated from the single β-phase by tempering at 550°C for a short time, which means that the martensite behavior in the range of 200–550°C is reversible. Continuous isothermal aging at 550°C led to marked hardening through the process β → coarse α′′ → fine α′′ → β + fine α. Both an MS curve and a free-energy model, which can explain the martensite formation at low and high temperatures and the annihilation at 200°C, are proposed.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 82 (2018) 307–313.
