Ti15V7Al 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 ¢-phase and a small quantity of ¡AA 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 LN 2 newly induced some martensites around the prior martensites formed by quenching, but no martensites were formed in the single ¢ 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 ¢phase. However, the coarse martensites were regenerated from the single ¢-phase by tempering at 550°C for a short time, which means that the martensite behavior in the range of 200550°C is reversible. Continuous isothermal aging at 550°C led to marked hardening through the process ¢ ¼ coarse ¡AA ¼ fine ¡AA ¼ ¢ + fine ¡. 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°C, are proposed.
Martensitic transformation behavior during cooling and heating of Ti-15V-7Al was investigated. The alloy was heated at 1050℃ 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 LN 2 , some martensites were newly formed at around prior martensites formed by quench, but no martensite was formed at the single β region. The martensite formation by the sub zero treatment exhibited a time dependence. Even tempering at 550℃ 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℃ 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℃ for a short time, which means the martensite behavior in the range of 200~550℃ is reversible. Continuous isothermal aging at 550℃ led to a remarkable hardening through the process of β → coarse α" → fine α" → β+fine α. 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.
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