Effect of High Temperature Oxidation on Surface Pit and Recrystallization of Directionally Solidified CM247LC Superalloy in Creep Gauge

Abstract

This study analyzed a recrystallization phenomenon that occurred simultaneously with hightemperature oxidation on the surface of a directionally solidified CM247LC creep specimen, using optical and scanning electron microscopy. After heat treatment, the surface of the specimen subjected to the creep test at 982oC was oxidized by exposure to high temperature and underwent microstructural changes due to high temperature stress. The outermost layer of the oxidized surface pits was found to consist of an oxide of the Cr/Co component, and the lower layer contained an oxide of the Al component. The area adjacent to the surface oxide layer is a precipitated free zone (PFZ) depleted of the y’ precipitated phase. The PFZ is caused by the diffusion of the Al component from this area to the surface oxide layer, resulting in the depletion of y'-Ni3Al as the main Al component. The area adjacent to the PFZ is a y' coarsening layer, which is the result of increasing y' phase fraction and coarsening as the Cr/Co component of this region diffuses into the PFZ of the y phase composition. This y' coarsening and y' rafting occurs in the direction perpendicular to the creep stress. In the EBSD analysis, the PFZ and y' coarsened layer were observed to be recrystallized regions, with the recrystallization composed of a single grain including the PFZ and y' coarsened layer. It is presumed that this recrystallization is caused by the residual stress of one-way solidification or the residual stress of the specimen surface processing. Accordingly, high-temperature oxidation in DS CM247LC creep gauge caused surface pits associated with recrystallization.