Microstructure transition of γ-TiAl alloys with abrupt cross-sections in yttria ceramic moulds during directional solidification by electromagnetic cold crucible

Directional solidification of γ-TiAl alloys with abruptly varying cross-sections was performed in yttria molds using an electromagnetic cold crucible. The solid–liquid interface at different growth positions was revealed by quenching using a liquid metal coolant, and the temperature field was mapped with the assistance of a finite element simulation. On the basis of these findings, the process of grain growth was summarized. The columnar–equiaxed transition was found to occur during directional solidification, and a criterion for continuous growth was established, which demonstrates that increasing the withdrawing rate or decreasing the nucleation rate can promote continuous growth. The continuity of columnar grains is confirmed by analysis in orientation evolution which also reconstructs the orientation of α and β grains at elevated temperatures. The degree of directionality of microstructures at elevated temperatures is determined from the angle between the preferential growth direction and the actual growth direction of the β phase. It is found that the degree of directionality increases with increasing withdrawing rate and nucleation rate.