THERMAL STABILIZATION TREATMENT AND THE EFFECT ON THE MICROSTRUCTURE IN DIRECTIONALLY SOLIDIFIED Ti-52%Al ALLOY

Thermal stabilization treatment with different treatment time and the corresponding directional solidification experiments are conducted for Ti-52%Al alloy in Bridgman-type directional solidified furnace. The formation of the mushy zone as well as the microstructure, interfacial morphologies and solute distributions of the mushy zone are investigated. Simultaneously the effect of the thermal stabilization treatment on the directionally solidified microstructure is studied. The mushy zone is composed of α phase grains, Al- rich droplets and liquid channels, and a completely liquid region is observed ahead of the mushy zone. With the increase of thermal stable time, α phase grains become well-aligned along the growth direction, and Al-rich droplets and liquid channels disappear gradually. Finally the mushy zone is occupied by single orientated a phase, and solute Al distribution along the growth direction in mushy zone evolves with a phase solidus line. Solute Al content in fully melted zone is calculated to be 52.21% according to the solute balance in the whole system, which is higher than the original content and consisted with the measuring value. Additionally the effect of thermal stabilization treatment on the subsequent directionally solidified microstructure is investigated. It can be seen that an appropriate thermal stable time(≥30 min) promotes a stable temperature and concentration field at the mushy zone before the directional solidification, in which conditions the column grains can be aligned parallel to the growth direction and the favorable directionally solidified microstructure can be obtained finally.