Martensitic transformation induced at low and high temperatures on Ti-15V-7Al alloy

Martensitic transformation behavior during cooling and heating of Ti-15V-7Al was investigated. The alloy was heated at 1050°C under a vacuum and quenched into an iced water. The structure of the quenched specimen consist of most β-phase and a small quantity of α" martensite at near grain boundaries. The elastic bent strip using a jig exhibited spontaneous shape change into the bending direction with heating. By sub zero treatment using LN₂, some martensites were newly formed at around prior martensites formed by quench, but no martensite was formed at the single region. The martensite formation by the sub zero treatment exhibited a time dependence. Even tempering at 550°C for a few second dramatically induced coarse martensites all over the specimen. All martensites, formed by the quenching, the sub zero treatment, or the tempering, disappeared completely by the heat treatment at 200°C for 300 s, and turned into a single β phase. However, the coarse martensites were regenerated again from the single β phase by the tempering at 550°C for a short time, which means the martensite behavior in the range of 200∼550°C is reversible. Continuous isothermal aging at 550°C led to a remarkable hardening through the process of β → coarse α" fine α → β+ fineα. Both the Ms curve and the free energy model with a spinodal decomposition of α which can explain the martensite formation at low and high temperatures, were proposed.