The evolution of carbides during long-term creep/aging of nickel based superalloy K444

Abstract

In K444 alloy, the evolution of carbides during stress aging (creep) and stress-free aging at 900 °C for 5000 h, and interrupted at 3000 h were investigated, combining microstructure characterization and quantitative statistics method. After stress-free aging for 5000 h, the volume fraction of intragranular MC and M6C carbides decrease from 0.48 % to 0.66 %–0.26 % and 0.48 %, respectively, while that of M23C6 carbides increase to 0.28 %. The two degradation behaviors can be concluded as MC + γ → M6C + M23C6 + γ′ and M6C + γ → M23C6 + γ′ respectively. The former controlled by the diffusion of Ti occurred earlier, while the latter determined by the diffusion of W and Mo mainly happened later. The applied stress did not change degradation reactions while accelerate the degradation rate slightly. During stress-free aging, the morphology of grain boundaries (GBs) changed from discrete distribution to semi-continuous and then to continuous chain, and the content of MC and M6C carbides decreases while that of M23C6 carbides increases. The applied stress markedly promotes above behaviors. Especially, the completed continuous chain formed before 3000 h. Moreover, the applied stress induced the formation of cavitation on the GBs, which is attributed to the following reasons. First, it can accelerate the degradation of GBs and decline the GBs strength. What's more, dislocations pile up on the GBs and cause a high level of stress concentration. Finally, the new generated phase interfaces have a greater deviation with its adjacent carbides, weakening the binding force of GBs.