The morphological change of the coarsening γ'-precipitates has been investigated in oxide dispersion plus gamma prime strengthened Ni-base superalloys. The chemical compositions of the alloys investigated were MA6000($15Cr-2Mo-4W-2Ta-4.5Al-2.5Ti-0.15Zr-0.01Br-0.05C-69Ni-1.1Y_2O_3$) vacuum arc melted(Alloy AMMA6000) and vacuum arc melted MA6000 alloy composition without $Y_2O_3$(Alloy PA). When the both alloys were aged at 1140℃ after furnace cooling directly from the solution treatment temperature of 1250℃, the γ'-precipitates underwent Ostwald ripening from the early stage of the aging.
When the PA alloy was aged at 1140℃, the shape of γ'-precipitates was initially cuboids and splitted into doublets or triplets, and then octets as they coarsened. The observed critical sizes for these transformations were comparable to those predicted by Khachaturyan et al.. As the octets coarsened they rearranged into aligned cubes until the coherency between γ-matrix and γ'-precipitates broke down. Since all the cuboids that constitute aligned cubes could not maintain their faceted shapes, each of them coarsened into spheres
For further study to examine the role of the dispersoid $Y_2O_3$, the MA ODS alloy of MA6000 was vacuum arc melted(AMMA6000). Then, the behavior of γ' coarsening in the AMMA6000 was compared with that of PA alloy(MA6000 without $Y_2O_3$). At the same temperature of 1140℃, the cuboids appeared to rearrange into aligned cubes and no splitted cubes were found in AMMA6000, while in the PA alloy the shape of γ'-precipitates was initially cuboids and splitted into doublets or triplets, and then octets as they coarsened. It appeared in this work that the dispersoid $Y_2O_3$ lowered volume fraction of γ'-precipitates and the γ' solvus temperature. Futher conformation by DTA is underway.
