Effects of Rhenium on Microstructure and Phase Stability of MAR-M247 Ni-Base Fine-Grain Superalloy

The effects of Re additions on the microstructure and phase stability of the cast fine-grain Mar-M247 superalloy were investigated in this study. The results showed that the grain sizes of the alloys with 0, 3, and 4.5 mass% Re content were 90, 60, and 50 mu m, respectively. The primary gamma' phase became finer and more cuboidal as Re content increased. The addition of 3 mass% Re caused strip-like MC carbides within the grain to degenerate into discontinuous M23C6 carbides and initiated the formation of a deleterious topological closed-packed (TCP) phase within the grain interior. The addition of 4.5 mass% Re promoted phase instabilities that led to the precipitation of large amounts of needle-like P phase in the grain interior, attributable to Re and W segregation. This study found that 3 mass% Re was a critical addition to maintain optimal microstructure and phase stability, and improved the ultimate tensile strength and the yield strength at room temperature and 760 degrees C. However, the addition of 4.5 mass% Re obviously resulted in a decrease in tensile properties at room temperature and 760 degrees C. [doi:10.2320/matertrans.M2009331]