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 alpha(2)' martensite was formed in a Ti-40Al-10V (atomic percent or at.%) alloy after air-cooling from a beta phase field, rather than the traditional alpha(2)/gamma 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) alpha(2)' variants sharing one (or two) common [11.0](alpha 2') axes were predominant, while no global variant selection was observed. Subsequent to the martensitic transformation, the retained beta phase was decomposed mainly via a beta ->gamma transformation. The gamma laths always nucleated at the alpha(2)'/beta interface according to a Blackburn orientation relationship. In order to stabilize the microstructure, the air-cooled samples were tempered at 800-1000 degrees C. During tempering, the microstructure decomposed mainly via an alpha(2)'->gamma transformation. The martensite was almost completely transformed after tempering at 1000 degrees C for 4 h, and hence a fine beta-gamma 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.