Cooling rate effects on the microstructure evolution in the βo zones of cast Ti–45Al–8.5Nb–(W, B, Y) alloy

• The βo → ωo phase transformation process in high Nb–TiAl alloy is imaged by HRTEM.
• The ordered ω formation is very sensitive to the cooling rate and both the βo → ω″ and ω″ → ωo phase transformations are easily finished.
• The ordered ω formation is bound to happen in the βo matrix as long as the βo phase is formed as a result of Nb segregation during the ingot producing process of high Nb–TiAl alloys.

The cooling rate effects on the βo → ωo phase transformation in Ti–45Al–8.5Nb–(W, B, Y) (at.%) alloy were investigated by annealing the alloy at 950 °C followed by different cooling methods. The morphology and the distribution of the ω-related phases were analyzed by TEM. The amount and morphology of the ω-related phases are very sensitive to the cooling rate. The ω-related phases could not be resolved in the water-quenched sample whereas it grew into nano-particles in the air-cooled sample. In the furnace-cooled sample, the ωo phase with B82 structure grew into micron-sized particles and occupied the whole βo area. The nucleation of the ordered ω embryos can be explained by the well accepted displacive mechanism because of the instability of the βo(B2) structure, accompanied by a short-range diffusion process between neighboring {111}βo planes. However, the growth of the ω-related phases is controlled by a long-range diffusion process. Due to ready nucleation and growth, the ordered ω formation is bound to occur in as-cast and heat-treated high Nb–TiAl alloys.