Growth behavior of columnar grains in TiAl alloy during directional induction heat treatment
Columnar grains are often obtained by directional solidification. In this study, columnar grains were achieved by directional induction heat treatment (DHT). TiAl ingot with equiaxed grains was induction heated at 1750 K for 20 minutes and pulled down with velocity of 4.17 μm/s. Liquid Ga-In alloy was used for cooling the ingot at the bottom. Results show that grains grow from equiaxed to columnar after competitive growth. The length of directional growth zone is about 25 mm and the maximum length of columnar grain is 15.5 mm. Lamellae direction with small angle (less than 60°) is about 90% in all columnar grains, which is higher than that of the ingot prepared by traditional directioal solidification. The driving force for columnar grain growth is bending interfacial tension that caused by heat transfer from the induction heating zone to the bottom. Atoms in front of the interface will diffuse downward under the action of interfacial tension during DHT, grains will grow up along axial direction. The interface migration rate (that is growth rate) of columnar grains is calculated based on the model of interface continuous moving. Calculated results indicate that the interface migration rate is ranged from 0.350 - 0.505 μm/s at 1750 K. Columnar grains will be obtained through atoms diffusion during DHT.