A high-performance β-solidifying TiAl alloy sheet: Multi-type lamellar microstructure and phase transformation

TiAl alloys are widely used in the aerospace industry and, hence, a thorough understanding of the corresponding microstructural evolution is crucial for providing insight into the fabrication of high-performance TiAl alloy sheets. In this study, a high-performance β-solidifying Ti-43Al-9V-0.2Y alloy sheet with multi-type lamellar structures was obtained via hot-rolling at an initial rolling temperature of 1260 °C. Owing to the temperature drop and reheat associated with multi-pass rolling, relatively fine irregular β/γ and regular α₂/γ lamellar microstructures (average size: ~25 μm) with nano-scaled antiphase domains (APDs) were generated by the β → γ and α + γ → α₂ + γ/γ_T phase transformations. These two lamellar structures have different formation mechanisms. The results of electron backscatter diffraction and transmission electron microscopy revealed that the nucleation and growth of (i) β/γ and (ii) α₂/γ lamellar structures are induced mainly by (i) stress and (ii) a combination of the stress and temperature drop during hot-rolling, respectively. Thin γ_T precipitates from the α/α₂ or γ phases. This precipitation is induced by the higher stress and lower temperature associated with the later stage (compared with those occurring in the initial stages) of hot-rolling. Moreover, compared with the corresponding as-forged alloy, the as-rolled TiAl alloy sheet exhibits better performance at both room and elevated temperatures. The improvement in the tensile properties is attributed to the duplex microstructure consisting of fine multi-type lamellar structures with nano-scaled APDs. The interface boundaries of β/γ, α₂/γ and APDs, which are effective in retarding dislocation motion, contribute mainly to strengthening of the alloy sheet.