| GH4586superalloy was designed as a new material for gas turbine engine discs for aerospace applications. It is a kind of wrought superalloy with independent intellectual property. GH4586superalloy exhibits excellent performances at temperatures up to650℃, so it has been successfully used as turbine rotator material, satisfying the need of aerospace industry. In order to spread the application of GH4586superalloy to aviation industry and produce aeroengine discs meeting requirements, present research was carried out to study the dynamic recrystallization behavior during hot deformation. It is expected to provide fundamental knowledge for the microstructure control during plastic deformation. Meanwhile, when the superalloy is used for aviation applications, properties related to long duration should be taken into account, because the microstructure characteristics will be changed due to the long-term heating at high temperatures. Therefore, present research has systematically studied the effect of long-term aging at relevant service temperature on the fatigue crack propagation characteristics of the superalloy.It is found that when GH4586superalloy was deformed at temperature from1000℃to1100℃, dynamic recrystallization initiated at original grain boundaries first, forming necklace structure. During deformation at high temperatures, original grain boundaries can not bulge because of the inhibition of grain boundary carbides and the formation of deformation twins. Deformation twins absorb the work done by external load and therefore reduce the driving force for grain boundary migration. Results manifest that the dynamic recrystallization nuclei evolve from the subgrains formed during deformation and dynamic recovery. Subgrains can grow by coalescence of neighbours, and finally become recrystallization nuclei when they reach critical size and gain enough misorientation. Meanwhile, since the formation of deformation twins provides strain energy and strain gradient, dynamic recrystallization is prone to take place at deformation twins near grain boundaries.The secondary phases, γ’ phase and carbides at grain boundaries and inside grains, have substantial influences on dynamic recrystallization behavior. During deformation at1000℃, the existence of undissolved γ1phase raised the flow stress. γ1phase also imposes pinning force on subgrain boundaries and the boundaries of recrystallized nuclei, preventing them from migrating. Large carbides inside grains promote dynamic recrystallization more efficiently than small carbides. However, the amount of γ’ phase is much larger than the amount of carbides inside grains, so the carbide stimulated nucleation mechanism is weakened by the existence of γ’ phase. Grain boundary carbides effectively prevent grain boundaries from gliding at lower temperature such as1000℃,resulting in the pile up of dislocations. It increases the stored energy of deformation and lays the foundation for recrystallization. As deformation temperature increases to1050℃and1100℃, γ’ phase nearly dissolves completely. Some of the carbides dissolve too. Therefore, the pinning force for dislocations reduces, and dynamic recrystallization easily occurs.Annealing twins form during dynamic recrystallization, and the formation mechanism is different, depending on the stage of dynamic recrystallization of GH4586superalloy. At the beginning of dynamic recrystallization, annealing twins form by the stacking faults during the growth of recrystallization nuclei. The driving force for the formation of annealing twins arises from the lowering of dislocation density and release of stored energy of deformation. The growth of annealing twins in the direction parallel to the coherent twin boundary is realized by the migration of high-angle grain boundaries or by the slip of dislocations at the noncoherent twin boundaries. Coalescence of neighbouring twins assists the growth of annealing twins in the direction perpendicular to the coherent twin boundary. At the beginning of dynamic recrystallization without the occurrence of grain growth, the frequency of twin boundary in all the boundaries including low-angle boundaries increases with the extent of dynamic recrystallization. It demonstrates that the formation of annealing twins is beneficial to the reduction of dislocation density, and the driving force for twin formation is the stored energy of deformation. Therefore, it is suggested that repetitious dynamic recrstallization at high temperatures will increase the twin boundary frequency, as long as recrystallized grain growth does not take place. During the growth of recrystallized grains, new twin boundaries are produced to increase the grain growth rate by the boundary reaction between previously formed twin boundaries and other boundaries.GH4586superalloy was long-term aged at relevant service temperature700℃and750℃for500h,1000h and1500h respectively. Fatigue crack growth rate results indicate that aging at700℃for500h can enhance the crack growth resistance in the near-threshold regime, comparing to the case of standard heat treatment. The threshold value increases from14.6MPa·m1/2for standard heat treatment to17.33MPa-m1/2for aging at700℃for500h. It demonstrates that precipitation of a small quantity of TCP phases and proper coarsening of y’ phase can increase the crack growth resistance in the near-threshold regime. Either aging at700℃for1000h and1500h or aging at750℃for different time lowers the crack growth resistance, and results in the decrease in fatigue crack growth threshold. It is attributed to the precipitation and growth of a large quantity of TCP phases and coarsening of γ’ phase associating with long-term aging, which weakens the crack closure effect and crack tip stress shielding effect. The crack closure effect is mainly induced by roughness, while the crack tip stress shielding effect is caused by crack branching. |
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