Effect Of Long-term Aging On Microstructure Evolvement And Fatigue Crack Growth Behavior Of GH4169Alloy

The precipitation strengthening type superalloy GH4169is one of the most significant materials in modern aircraft engines in China, mostly applied in critical components like turbine disks. When working at650℃, it has satisfactory performance such as strength, plasticity and fatigue crack growth resistance, etc. A turbine disk is serviced for a long term under high temperature and cyclic loading, during serving, microstructure evolvement will certainly change the fatigue crack resistance. The current research published about fatigue crack growth behavious mostly focused on the effect of external environment and loading conditions but little on that of microstructure evolvement, Concerning this issue, the microstructure evolvement and fatigue crack growth rate (FCGR) of GH4169alloy after long-term aging was researched in this paper and the micromechanism of microstructure reflecting on FCGR was also analyzed.It is found that the GH4169alloy has good microstructure stability and has not exhibited apparent microstructure changing during long-term aging at650℃. The size of both y’and y" phase increased, while the total volume fraction of them kept almost constant with the increasing of aging time.δ phase precipitated in the shape of granules or short bars, mostly on the grain and twin boundaries and some inside the grains, which increased in amount but kept in the same size.The FCGR test was carried out for the specimens after different aging treatments, and the fractograph was examined. The results shows that the FCGR increased slightly with the increasing of aging time at650℃, inconspicuous in Paris region and more obvious in near-threshold region and fast propagation region. Besides, when the alloy was long-term aged at750℃, the FCGR increased evidently with the increasing of aging time, while the fatigue crack growth threshold decreased. It is considered that the coarsening of γ’ and γ" phase long-term aged at650℃and the transition from y" phase to needle-like δ phase at750℃which degraded the strength and weakens the crack closure effect and crack tip stress shielding effect should be blamed for the decreasing of fatigue crack growth resistance and consequently, the increasing of FCGR. The crack closure effect is mainly induced by roughness, while the crack tip stress shielding effect is caused by crack branching in this research.