Characterization of a new microstructure in a β-solidifying tial alloy after air-cooling from a β phase field and subsequent tempering

In this study, we found that well-developed α₂^′ martensite was formed in a Ti-40Al-10V (atomic percent or at.%) alloy after air-cooling from a β phase field, rather than the traditional α₂ /γ lamellar colonies. The martensitic laths were produced according to the Burgers orientation relationship (OR), the same as those during quenching. Local variant selection detected that three (or six) α₂^′ variants sharing one (or two) common [11.0]_α2′ axes were predominant, while no global variant selection was observed. Subsequent to the martensitic transformation, the retained β phase was decomposed mainly via a β→γ transformation. The γ laths always nucleated at the α₂^′/β interface according to a Blackburn orientation relationship. In order to stabilize the microstructure, the air-cooled samples were tempered at 800–1000° C. During tempering, the microstructure decomposed mainly via an α₂^′ →γ transformation. The martensite was almost completely transformed after tempering at 1000° C for 4 h, and hence a fine β-γ microstructure was obtained. Such a treatment resembling the quenching–tempering in steels may be a new strategy for the microstructural design of TiAl alloys, while an unexpected quenching process can be avoided.