Study On Hot Deformation Behavior Of Hard-to-Deform Superalloy U720Li

Ni-based U720Li is aγ′precipitation-hardened wrought superalloy that is widely used for advanced turbine disk application owing to its superior high-temperature strength,creep resistance and fatigue resistance.However,its hot workability is poor and the microstructural instability is prone to occur during its hot working due to the extremely high alloying degree andγ′content.Reasonable selection of thermal processing parameters plays an important role in obtaining uniform microstructure and avoiding microstructural instability.At present,there have been many reports on the effect of deformation degree,strain rate and deformation temperature on the hot workability of U720Li alloy,but few studies have been conducted on the influence mechanism of holding time before deformation.In this study,the following research work has been conducted on the forged U720Li alloy:The evolutions ofγ′characteristic and grain structure with heating temperature and holding time have been studied by conducting isothermal heating experiments.It has been found that the area fraction and size of primaryγ′decrease with the increase of heating temperature and holding time.The dissolution temperature of fine secondaryγ′is lower and the dissolution rate is faster compared with those of coares primaryγ′.The secondaryγ′was completely dissolved after holding at 1100℃for 30min,while the primaryγ′was completely dissolved after holding at 1180℃for 30min.The continuous dissolution ofγ′weakened its pinning effect on grain boundaries,which results in the increase of average grain size with the increasing heating temperature and holding time.Moreover,the grains grow rapidly above 1160℃（higher than the solvus temperature ofγ′）.According to the grain growth behaviors at different heating conditions,a grain growth kinetic model has been established:d_t^{0.6 07}-d₀^{0.6 07}=1.25?10¹⁹t^0.782exp（-528989/RT）。Based on the above results,hot compression experiments were carried out to study the effect of deformation condition on high temperature deformation behavior of U720Li alloy.The results show that the flow stress increases with the increase of strain rate,but decreases with the increasing of deformation temperature and holding time before deformation.The Arrhenius constitutive model and hot processing map under two holding time（5 min and 10 min,respectively）have been established.The optimum processing interval for the holding time of 5 min is 1050～1110℃/0.01～0.1s^-1 and 1110～1180℃/0.01～1 s^-1,while that for the holding time of 10 min is1080～1120℃/0.01～0.5 s^-1 and 1120～1160℃/0.01～0.3 s^-1.Accoring to the microstructure observation and hot processing map analysis,the holding time of 5min possesses better hot working performance.Finally,the dynamic recrystallization（DRX）behavior of U720Li alloy under different deformation conditions was studied.It was found that,in most cases,the DRX grain size increases with the increase of deformation temperature and deformation rate.After deformation at 1050℃,the original grain boundaries disappearred and fine subgrains formed within the orginal grains.Besides,the subgrain morphology became more obvious with the increase of strain rate.During the deformation at 1100℃,for the strain rate of slower than 0.1 s^-1,fine subgrains formed within the grains.When the strain rate reached above 1 s^-1,sufficient DRX occurred.The main dynamic recrystallization mechanism at 1100℃should be the continuous dynamic recrystallization（CDRX）.However,as the deformation temperature reached 1130℃and above,DRX occurrd at all strain rates,and the dynamic recrystallization mechanism transformed to the discontinuous dynamic recrystallization（DDRX）.