定向凝固Ti-52％Al合金热稳定处理及其对微观组织的影响

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

以Ti-52%Al合金为研究对象,在Bridgman定向凝固炉中进行不同时间的热稳定处理实验及相关的定向凝固实验,研究了定向凝固前糊状区的形成及其微观组织演化、界面形态和溶质分布,并考察了热稳定处理对定向凝固组织的影响.结果表明,试样熔化后形成了由α相晶粒、晶粒间富Al液滴和液相通道组成的糊状区,糊状区前沿为完全液相区.随着热稳定处理时间的增加,糊状区内α晶粒逐渐融合且沿温度梯度方向定向排布,富Al液滴和液相通道逐渐消失,固/液界面变得光滑平整.热稳定处理时间足够长时,糊状区内为单一的α相,溶质Al的成分在生长方向近似沿着α固相线变化.根据溶质守恒计算得出完全液相区内溶质Al含量为52.21%,高于试样原始成分,与实验测量结果相一致.进一步研究热稳定处理对后续定向凝固组织的影响,发现合理的热稳定处理时间(≥30 min)确保了定向凝固启动界面处存在稳定的温度场和浓度场,柱状晶沿生长方向能够平行并列生长,试样具有良好的定向效果.

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 a 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, a 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.