Effect Of Long-term Aging On Tensile Deformation And Fatigue Crack Growth Behavior Of GH4742 Superalloy

GH4742 superalloy is a Ni-base wrought superalloy that can be used as the turbine disc above 700?.The turbine disc is often subjected to the long-term cyclic loading at high temperature,high pressure,high speed,high load conditions,and it is necessary to have sufficient thermal strength property and ensure high fatigue crack resistance.If the crack growth rate can be effectively controlled,the safety service of the aircraft engine will bring huge economic and social benefits.The aim of the present research is to study the effects of the microstructure of the long-time aged GH4742 superalloy on the tensile deformation and fatigue crack growth behavior,and as well as the influence mechanism.The results will provide the fundamental data and the theoretical guidance for the microstructure stability and the fatigue life assessment in the long-term service process of the turbine disc.After standard heat treatment,the microstructure of GH4742 superalloy mainly consists of the primary ?',the secondary ?',the thrice ?' phase and MC type carbide.After the aging at 650?,the morphology of the primary ?' and the secondary ?' phases have no obvious change,and the thrice ?' phase precipitated out complementally.After the long-term aging at 750?,the primary block ?' phase grows up further and has the smooth boundary,and the thrice ?' phase redissolves into the matrix or aggregates or coarsen into corner square ?' phase.The results showed that the strength of the long-time aged superalloy increases at first and then decreases at both room temperature and 800?.The ductility does not change at room temperature.The evolution of the thrice ?'phase in the long-term aging at 650? and the coarsening of the primary ?' or the secondary ?' phases in the long-term aging at 750? are considered as the main factors affecting the tensile properties at room temperature and 800?..For the tensile testing at 800?,the elongation of the superalloy aged at 750?for 500h and 1000h increases nearly 100 percent.The MC carbides become the sites of the microcrackings in the tensile process.That is,there are two weak parts during the tensile testing at high temperatures,one is the boundary between dense region of the secondary ?' phase and dense region of the primary or thrice ?' phases,the other is MC carbides.With the increasing of the aging time at 650? and 750?,the fatigue crack growth rate increases.After long-term aging,the fatigue crack propagates of the alloy is in a zigzag feature.The fatigue crack easily propagates across the region without the primary or the secondary ?' phase.When the crack meets an area with dense primary ?' phase or the secondary ?'phase,the crack by pass the large size ?' phase.In the high stress intensity range region,fatigue crack propagates in a sawtooth feature..The fatigue crack growth rate is mainly affected by the crack tip stress shielding effect and the crack closure effect,and the crack propagation resistance will be improved by the two factors.In the near threshold region,the crack growth rate mainly depended on the crack closure effect induced by the surface roughness.