Characterization of a New Microstructure in a β-Solidifying TiAl Alloy after Air-Cooling from a β Phase Field and Subsequent Tempering

Abstract

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.

Keywords

• titanium aluminides
• martensitic transformation
• variant selection
• microstructure
• electron back-scattered diffraction