Phase transformation behavior and kinetics of high Nb-TiAl alloy during continuous cooling

Phase transformation behavior and kinetics of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y (at. %) alloy during continuous cooling under the cooling rates of 3 °C/min, 5 °C/min, 10 °C/min, 20 °C/min, 40 °C/min and water quenching (WQ) were investigated in this study. The phase composition and microstructure of the alloy were studied using X-ray diffraction (XRD) and optical microscopy (OM), respectively. The expansion and the volume fraction evolution of phases during continuous cooling were investigated by dilatometry (DIL), and the microhardness of the alloy suffered from cooling was also tested through micro-sclerometer. The results indicate that the phase transformation from parent α phase to γ and α₂ take place during continuous cooling, the α→γ phase transformation occurs in the temperature range of 1150 °C-1310 °C while the α→α₂ phase transformation takes place between 1100 °C and 1220 °C. The transformation temperature decreases with the cooling rate increasing, and the higher the cooling rate is, the higher the phase transformation rate is. The relative fraction of γ phase and α₂ phase much depend on the cooling rate, and the lamellar colony sizes and interlamellar spacing of crystal structure decrease with the cooling rate increasing. The microhardness of high Nb-TiAl alloy increases with the cooling rate increasing due to the variations of phase composition and crystal structure.