Effects of Long-Time Aging on Fatigue Crack Propagation Behaviors of a Hardworking Ni-Based Superalloy

Abstract

Fatigue crack propagation behaviors coupled with the microstructure evolution of a hardworking Ni-based superalloy after long-time aging at 1023K were studied, to provide fundamental data for the fatigue life prediction of the superalloy. The results show that the microstructures remarkably change and the fatigue crack propagation resistance decreases with the aging time prolongs. It is found that the precipitation and the growth of topologically close packed (TCP) phases as well as the coarsening of γ' phase and carbides on grain boundaries can significantly affect the fatigue crack growth rate. On one hand, coarsened γ' phase and carbides at grain boundaries block dislocation movements near the crack tip, thus the fatigue crack propagation is hindered in near-threshold region and Paris region. On the other hand, the stress concentration accumulates continually with carbides precipitation increases, so that the grain boundaries become the main fatigue crack propagation rout. As well as, the effect of the TCP phases on the fatigue crack propagation behavior ascribes to the size and the distribution of TCP phases. Very small quantity of TCP phases contribute to pinning dislocation and enlarging fatigue crack propagation absorption energy, but high quantity of TCP phases with short rod shape changed to the needle, which gradually precipitate uniformly within the grain after 1000h besides on grain boundaries in the earlier aging, leads to much higher stress concentration degree. Those discussed above are the most important reasons why the fatigue crack growth rate increases after long-time aging.