Effect of minor additions on the formation of TCP phases in modified RR2086 SX superalloys

The effect of minor additions (C and Hf) on the formation of topologically closed-packed (TCP) phases in single crystal (SX) superalloy RR2086 has been investigated. The base RR2086 and the modified RR2086 were exposed at 950 °C for 200–2000 h. It was found that both alloys were very stable in terms of TCP phase precipitation. TCP phases initiated in the dendrite regions in both the base and modified RR2086 and near MC carbides in the modified material as well. However, TCP phases formed faster in the modified RR2086 than in the base RR2086. This indicated that the minor additions in the modified RR2086 expedited the formation of TCP phases. EDX measurements showed that Re, W, Cr and Co were predominant in TCP phases, in agreement with the Scheil simulation; that they also segregated in the dendrite cores in both alloys and near MC carbides in the modified RR2086; and that the segregation was intensified by the minor additions in the modified material. These EDX results can explain the initiation of TCP phases in the dendrite cores in both alloys, the formation of TCP phases near MC precipitates, and the faster TCP phase precipitation in the modified RR2086. In contrast with the RR2086, TCP phase formation was retarded by the same minor additions in the modified RR2072. It was thought that this difference was to due to the formation of M 23C 6 phase which was influenced by the concentration of chromium. The higher level of chromium in RR2072 promoted M 23C 6 phase prior to the precipitation of TCP phases in the modified RR2072 and subsequently retarded the occurrence of σ phase to a later stage than in the base RR2072. The lower level of chromium in RR2086 suppressed M 23C 6 precipitation in the modified alloy, and as a result TCP phase precipitation was only influenced by the microsegregation, which led to the faster formation of TCP phases in the modified RR2086 than in the base material.