Dissolution and Precipitation Behavior of the γ′ Phase During Heat Treatment of the Nickel-Based High-Temperature Alloy K444

The morphology, volume fraction, and size distribution of the γ′ phase significantly influence the fabrication of high-temperature alloys and their performance under severe service conditions. This study systematically investigates the dissolution, precipitation, and growth behavior of various γ′ phases during heat treatment. The results demonstrate that the γ/γ′ eutectic structure between dendrites decomposes first during the solution treatment, forming coarsened γ′ agglomerates, which subsequently dissolve into the γ matrix. The splitting and dissolution mechanism of the coarsened primary γ′ phase within the γ′ agglomerate regions is clarified. Increasing the solution temperature and prolonging the solution time enhance the growth of secondary γ′ phases by accelerating the splitting and dissolution of the γ/γ″ eutectic structure. During heat treatment, carbide segregation at grain boundaries induces γ′ coarsening at these boundaries. The combined effects of carbides with varying sizes, the coarsened γ′ precipitates at grain boundaries, and the pinning effect of precipitates on local grain boundaries lead to the formation of serrated grain boundary morphologies. These findings enhance the understanding of the evolution of γ′ phases in nickel-based superalloys during heat treatment and provide a theoretical basis for optimizing the heat treatment processes of the K444 alloy.