Martensitic Transformation Induced in Ti-15V-7Al Alloy at Low and High Temperatures

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 beta-phase and a small quantity of alpha '' 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 beta region. The formation of the martensites by the subzero treatment exhibited time dependence. Even tempering at 550 degrees 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 degrees C for 300 s, resulting in a single beta-phase. However, the coarse martensites were regenerated from the single beta-phase by tempering at 550 degrees C for a short time, which means that the martensite behavior in the range of 200-550 degrees C is reversible. Continuous isothermal aging at 550 degrees C led to marked hardening through the process beta -> coarse alpha '' -> fine alpha '' -> beta + fine alpha. Both an M-S curve and a free-energy model, which can explain the martensite formation at low and high temperatures and the annihilation at 200 degrees C, are proposed.