GRAIN-GROWTH AND CARBIDE PRECIPITATION IN SUPERALLOY, UDIMET-520

The results of an experimental study on the grain coarsening behavior, M23C6 carbide precipitation, and secondary MC carbide precipitation k inetics in UDIMET 520 are presented. Primary MC carbides and M(C, N) c arbonitrides strongly influence the grain growth, with their dissoluti on near 1190 degrees C and 1250 degrees C, respectively, resulting in two distinct grain coarsening temperatures (GCTs). M23C6 carbides prec ipitate in the alloy over a wide range of temperatures varying between 600 degrees C and 1050 degrees C, A discrete M23C6 grain boundary car bide morphology is observed at aging temperatures below 850 degrees C. Secondary MC carbides formed at temperatures ranging between 1100 deg rees C and 1177 degrees C, in specimens in which primary MC dissolutio n had been obtained at solution treatment temperatures of 1190 degrees C to 1250 degrees C. A schematic time-temperature-transformation (TTT ) diagram for understanding the microstructure and precipitation inter -relationships in UDIMET 520 alloy is also presented.