Active Eutectoid Decomposition of α → γ + τ₁ and the Morphological Evolution in a Ru-Containing TiAl Alloy

The active eutectoid decomposition and its morphological evolution during slow cooling and isothermal holding were investigated in a Ru-containing TiAl alloy. The fine τ₁/γ active eutectoid nodules precipitated at α grain boundary and interior during water quenching. The active eutectoid microstructure evolved from sheaf-like, granular to bulky net-like sluggish eutectoid morphology gradually with the decrease in quenching/holding temperatures. The active eutectoid reaction occurs from 1220 to 1290 °C, while the beginning temperature of sluggish eutectoid locates at 1305 ± 5 °C. The combination of the intact τ₁ phase and immature nano-sized γ laths suggests a short incubation period of τ₁ phase in the active type. Furtherly, the semiquantitative estimation shows that the growth velocity of active eutectoid is about ninety times higher than sluggish type. In addition, a representative feature of γ phase abruptly ripening along {111} crystallographic plane was also observed in the active eutectoid.