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

The cooling rate effects on the beta(O) -> omega(O) phase transformation in Ti-45Al-8.5Nb-(W, B, Y) (at.%) alloy were investigated by annealing the alloy at 950 degrees C followed by different cooling methods. The morphology and the distribution of the co-related phases were analyzed by TEM. The amount and morphology of the omega(O)-related phases are very sensitive to the cooling rate. The co-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 omega(O) phase with B8(2) structure grew into micron-sized particles and occupied the whole beta(O) area. The nucleation of the ordered omega embryos can be explained by the well accepted displacive mechanism because of the instability of the beta(O)(B2) structure, accompanied by a short-range diffusion process between neighboring {111}(beta O) planes. However, the growth of the co-related phases is controlled by a long-range diffusion process. Due to ready nucleation and growth, the ordered omega formation is bound to occur in as-cast and heat-treated high Nb-TiAl alloys. (C) 2014 Elsevier Inc. All rights reserved.