The eutectic characteristic of MC-type carbide precipitation in a DS nickel-base superalloy

The precipitation of MC-type carbide in a DS nickel-base superalloy has been studied. The nucleating temperature of the MC carbide was determined by analysis. The MC carbide and γ phase were complementary in composition, and the eutectic reaction of (γ+MC) took place in different forms at varied solidification rate. The (γ+MC) eutectic reaction was an important factor of determining the carbide morphology. At an extremely high solidification rate, such as quenching, the compositional ranges for the (γ+MC) eutectic reaction was broadened and the carbide precipitated through the quasi-eutectic reaction of (γ+MC). As a result, the carbide appearing frequency was increased noticeably. The dendrites of the quasi-eutectic carbide formed in quenching paralleled to each other closely for facilitating diffusion between the two eutectic components of carbide and γ phase. The script carbide precipitated at a relatively high drawing rate around 50 μm/s. At this rate, diffusion was insufficient and the eutectic reaction of (γ+MC) was suppressed at most. The carbide grew quickly along the interdendritic liquid passages due to the high degree of segregation of carbide forming elements there, and hence formed a very complicated dendritic structure. The faceted carbides precipitated when the drawing rate was lower than 5 μm/s, and some of them contained a γ phase core. The incorporation of γ phase into the carbide body was caused by the eutectic reaction of (γ+MC) in the midst of the carbide precipitation. Two forms of the (γ+MC) eutectic reaction were favored by sufficient diffusion at 0.5 μm/s drawing rate. The (γ+MC) eutectic reaction took place at the final stage of the carbide precipitation in an isolated liquid pool and formed small ridges distributed uniformly on the carbide surface. Diffusion operated sufficiently throughout the whole residual liquid passage guaranteed the continual eutectic reaction by a sandwich like mode of γ/MC/γ, and the extremely long carbide was formed along the grain boundaries as a result.