Study On Plastic Deformation Behavior Of Directionally Solidified U720Li Superalloy

U720Li, a new kind of hard-worked Ni-based superalloy, shows an excellent high temperature performance. It is contained with many alloy elements, which is contented by more than 42% ?' phase. High temperature strength is greatly improved by high alloying, but the melting point is also lowered by that. At the same time, both of the solution and dynamic recrystallization temperature increase with the increasing of ?'phase. Those two factors not only narrow the hot working temperature range, but also increase the thermal deformation resistance rapidly, therefore it is difficult to deform. By far, this is the key restriction for U720Li alloy to get further.For the directional solidification alloy, the properties are different in different directions: the lowest elastic modulus on the dendrite growth direction; and the highest on the direction of a 45-degree angle with the dendrite direction; shear modulus also has the same change law. Based on the anisotropy of directional solidification, this paper launches the research on the study of deformation behavior in different directions in order to find the optical deformation direction. And then do some research on the law of hot deformation behavior and law of dynamic recrystallizaiton on the optical direction. Therefore new methods would be found to expand hot working temperature range of U720Li alloy, and to lay a theoretical foundation for the further application of the alloy.The experiments were performed on the multi-function thermal simulation experiment machine. First of all, this paper studies the deformation properties of different direction. The results show that the optical direction is 0= 90° direction. Samples under this direction performance lower deformation resistance, good surface quality and good comprehensive properties with no inner crack. Then this paper studies the deformation property and recrystallization rules of this optical direction. Deformation temperature was divided into 1025?,1050?,1075?,1100?,1125?,1150?, strain rate were 0.005 s-1,0.01 s-1,0.1 s-1,1 s-1, and a total deformation of 30 percent. According to the revised true stress-true strain curves and deformed microstructures, the hot compression deformation behavior and dynamic recrystallization rules of as-cast U720Li alloy were analyzed. The results are shown as follows:(1) The critical temperature for as-cast U720Li is 1075?; (2) With the decrease of strain rates (higher than 0.005 s-1) and increase of temperatures the deformation properties of the directional solidification alloy will be improved. After deformation, the microstructure was uniform and no crack was found in directional solidification U720Li superalloy, which is much better than that of normal solidification U720Li superalloy; (3) The thermal activation energy of directional solidification U720Li superalloy is 766 kJ/mol, which is 482 kJ/mol lower than that of normal solidification U720Li superalloy. This indicates that dynamic recrystallization is easier to happen in directional solidification U720Li superalloy.There are two kinds of dynamic recrystallization nucleation in directional solidification U720Li superalloy:dislocation pile-up induced nucleation, which is the main way of nucleation in the test alloy, and grain boundaries arched nucleation. In directional solidification U720Li superalloy, dislocation pile up around the second phase, then dynamic recrystallization grains nucleus here before it reaches the critical temperature; all the (?+?') phase, large size ?' phase and carbide distribute regularly in the interdendritic between the orientated dendrite, which makes the regularly distribution of dynamic recrystallization and homogeneous deformed microstructure without any crack.