Dissolution and diffusion of oxide films and formation mechanism of prior particle boundary carbides for FGH96 alloys

To make certain the formation mechanism of carbides at prior particle boundary (PPB) during the densification of powder metallurgy (PM) superalloys, the FGH96 alloys with different relative density were employed by interrupted sintering method in this study. The dissolution behavior of oxide films and the growth behavior of carbides during densification were systematically investigated through scanning electron microscope and transmission electron microscope, elucidating the formation mechanism of a core-shell structure where the carbide phases encapsulated the oxide particles at PPB. The results indicate that the surface layers of FGH96 alloy powders consist of the amorphous oxide layers and M(Ti,Nb)C-type carbides with TiO₂. Prior to the sinter neck formation, the amorphous oxide layers transform into the crystalline oxide particles, while the MC-type carbides at the powder surface grow by consuming the amorphous TiO₂ and the carbon elements, diffusing from the matrix. Following the sinter neck formation, the partial oxide particles dissolve, leaving only the residual Al₂O₃ and ZrO₂ particles at sintering neck interface. Meanwhile, the carbides progressively encapsulate the adjacent oxide particles during the subsequent growth stage, ultimately forming a core-shell structure with oxide cores enveloped by carbide shells.