Formation mechanism of carbide-coated oxide core-shell structure in FGH96 superalloys

The carbide-encapsulated oxide is a crucial component of prior particle boundaries (PPB), yet there still lacks direct evidence regarding its microscopic formation mechanism. In this work, Atom Probe Tomography and Transmission Electron Microscopy techniques were employed to investigate PPB in intermediate sintering states. It was found that the core-shell structure is related to the carbides and oxides already present on the powder surfaces, which slightly differs from the previously held conclusion that carbide nucleation and growth occur around isolated oxides as cores. When oxides and carbides (primarily M(Ti, Nb)C) from different powder surfaces come into contact, carbon elements from the interior of the powders migrate to the contact surfaces during sintering, triggering the growth of existing carbides. Ultimately, these growing carbides encapsulate adjacent oxide particles, forming a carbide-encapsulated oxide core-shell structure. Furthermore, inferring from the poor orientational relationship between MC carbides and the matrix, it is more likely that the carbides represent the growth of existing carbides rather than new nucleation. These findings provide insights into the formation mechanism of core-shell structure appeared in powder metallurgy superalloys, and suggests O may not influence the MC particles during sintering.