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

摘要

The cooling rate effects on the β_o -> ω_o 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 co-related phases were analyzed by TEM. The amount and morphology of the co-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 ω_o phase with B8_2 structure grew into micron-sized particles and occupied the whole β_o area. The nucleation of the ordered co embryos can be explained by the well accepted displacive mechanism because of the instability of the β_o(B2) structure, accompanied by a short-range diffusion process between neighboring {111}β_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 co formation is bound to occur in as-cast and heat-treated high Nb-TiAl alloys.