Microstructural changes of a creep-damaged nickel-based K002 superalloy containing hf element under different HIP temperatures

Effects of hot isostatic pressing (HIP) temperature on the microstructural evolution of a nickel-based K002 superalloy containing Hf element after long-term service were investigated using three different soaking temperatures during HIP. The degraded γ′ precipitates represented coarse and irregular morphology after long-term service. These γ′ precipitates still were of coarse and irregular shape, but the size and volume fraction of γ′ precipitates were markedly reduced under HIP condition of 1,190°C/200 MPa/4 h, indicating that the γ′ precipitates were experiencing a dissolution process. Meanwhile, the concentrically oriented N-type γ′ rafting structure around the cavities was formed. With HIP temperature increase to 1,220°C and 1,250°C, the small-sized, cubic and regular γ′ precipitates were re-precipitated, and the concentrically oriented γ′ structure vanished. The unstable morphology induced by the nucleation and growth of γ matrix was found near the creep cavities, indicating that the solute atoms diffused inward the creep-induced cavities during HIP. However, at HIP temperature of 1,220°C and 1,250°C, a large number of blocky MC₍₂₎-type carbides containing amounts of Hf elements were precipitated, demonstrating that HIP treatment at higher temperatures can result in the formation of a large number of blocky MC₍₂₎-type carbides.