采用热模拟实验研究了定向凝固与等轴晶U720Li合金的高温塑性变形行为, 利用OM, SEM和EBSD分析了不同变形条件下合金的组织特征及动态再结晶机制. 结果表明, 定向凝固与等轴晶合金的变形抗力均随变形温度升高而降低. 定向凝固U720Li合金沿垂直柱状晶方向变形时变形抗力较低, 枝晶间协调变形能力良好, 未出现裂纹. 与等轴晶合金相比, 相同变形条件下定向凝固合金的动态再结晶组织均匀; 高温塑性变形过程中, 定向凝固合金的动态再结晶主要以晶界弓弯和位错塞积方式形核. 定向凝固U720Li合金的变形激活能为766 kJ/mol, 比等轴晶合金降低了38.6%, 定向凝固合金呈现出更好的热加工特性.

U720Li, a kind of precipitation type nickel-based superalloy, shows excellent mechanical properties at elevated temperature, which is also known as the difficult-to-deform alloy because of the high-alloying. To solve its deformation problem, new methods would be developed to enlarge the temperature deforming window and improve its plasticity. The hot compression deformation behaviors of directionally solidified and equiaxed grain U720Li alloys were studied by the MMS-300 testing system, as well as the dynamic recrystallization nucleation and growth mechanisms during the hot deformation were discussed. The microstructural characteristics of the alloy under different deformation conditions were examined using OM, SEM and EBSD. The results show that the deforming resistances of both directionally solidified and equiaxed grain U720Li alloys decrease with the increasing of deforming temperature. When the angle θ between the compression deforming direction and dendrite growth direction is 90°, the deforming resistance of directionally solidified U720Li alloy would be lower. With this direction, the coordination deformation between the dendrites becomes better and no crack can be found after deformation, which indicates that the deforming ability is best along θ=90° and it can be considered as the optimal deforming direction for directionally solidified U720Li alloy. Compared with equiaxed grain alloy, directionally solidified U720Li alloy performs higher deformation ability and more homogenous microstructures. During the deformation of directionally solidified U720Li alloy, bulging nucleation of grain boundary migration and dislocation pile-up induced nucleation are found as the main mechanism for the nucleation of dynamic recrystallization. In addition, the deformation activation energy of directionally solidified U720Li alloy is 766 kJ/mol, which is 482 kJ/mol lower than that of equiaxed grain alloy, indicating the directionally solidified U720Li alloy exhibits better hot-working plasticity.