In-situ observation of microstructure evolution and phase transformation under continuous cooling in Ru-containing TiAl alloys

Ruthenium is a promising element to heighten the mechanical properties of TiAl alloys. The microstructure evolution and phase transformation of two Ru-containing TiAl alloys have been in-situ observed by the high-temperature laser-scanning confocal microscopy from melting to 1000 °C under the cooling rates of 10 K/min and 100 K/min. For Ti-45Al-2.0Ru-0.2B alloy, some net-like and spherical microstructures precipitated at α grain boundaries and inside α grains by the eutectoid transformation of α → γ + τ₁ at 10 K/min. While the rod-like τ₁ phase replaced net-like morphology and dispersed in γ matrix at 100 K/min condition. For Ti-45Al-1.0Ru-0.2B alloy, the mixture of blocky γ phase and dispersive fine τ₁ particles emanated from α grain boundaries via a divorced eutectoid transformation at 10 K/min. Upon 100 K/min, a complex microstructure appeared which consists of lamellar colony, blocky γ, γ/τ₁ mixture and B2 phase. The near-equilibrium phase transformation of the present two Ru-containing TiAl alloys follows the sequence: L → L + β → α + β → α → α + γ → α + γ + τ₁, which clarifies that the τ₁-containing mixtures precipitate after the formation of α grain, rather than during solidification process. The related crystallographic orientation and microstructure evolution mechanism were investigated. In addition, it's found that the Ru element promotes the formation of duplex microstructure. The duplex microstructure containing dispersive τ₁ particles of Ti-45Al-1.0Ru-0.2B alloy at 10 K/min obtained the maximum microhardness and can be considered as a potential optimal microstructure.