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 degrees 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 alpha grain boundaries and inside alpha grains by the eutectoid transformation of alpha -> gamma + tau(1) at 10 K/min. While the rod-like tau(1) phase replaced net-like morphology and dispersed in gamma matrix at 100 K/min condition. For Ti-45Al-1.0Ru-0.2B alloy, the mixture of blocky gamma phase and dispersive fine tau(1) particles emanated from alpha 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 gamma, gamma/tau(1) mixture and B2 phase. The near-equilibrium phase transformation of the present two Ru-containing TiAl alloys follows the sequence: L -> L + beta -> alpha + beta -> alpha -> alpha + gamma -> alpha + gamma + tau(1), which clarifies that the tau(1) containing mixtures precipitate after the formation of alpha grain, rather than during solidification process. The related crystallographic orientation and microstructure evolution mechanism were investigated. In addition, ifs found that the Ru element promotes the formation of duplex microstructure. The duplex microstructure containing dispersive tau(1 )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.