Effect of hafnium and tantalum on the microstructure of PM Ni-based superalloys

The effect of Hf and Ta on the microstructure of powder metallurgy Ni-based superalloys after heat treatment was investigated. Hf and Ta change the distribution and main components of MC carbides, inhibiting TiC carbides precipitate along prior particle boundaries by forming stable Hf- and Ta-rich carbides inside powders during the process of hot isostatic pressing. The partition of Hf and Ta in γ′ phase causes the repartition of other elements in γ and γ′ phases, which leads to the change of the lattice mismatch of γ and γ′ phases. The size of secondary γ′ precipitates increases and their shapes transform from spheres to cuboids when Hf or Ta content is raised, and some secondary γ′ precipitates begin to split in high Hf and Ta content. This transformation of morphology is related to the increase of the lattice mismatch of γ and γ′ phases, and the increase in size and volume fraction of secondary γ′ precipitates.