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 bee n studied. The nucleating temperature of the MC carbide was determined by a nalysis. The MC carbide and gamma phase were complementary in composition, and the eutectic reaction of (gamma + MC) took place in different forms at varied solidification rate. The (gamma + MC) eutectic reaction was an impor tant factor of determining the carbide morphology. At an extremely high sol idification rate, such as quenching, the compositional ranges for the (gamm a + MC) eutectic reaction was broadened and the carbide precipitated throug h the quasi-eutectic reaction of (gamma + MG). As a result, the carbide app earing frequency was increased noticeably The dendrites of the quasi-eutect ic carbide formed in quenching paralleled to each other closely for facilit ating diffusion between the two eutectic components of carbide and gamma ph ase. The script carbide precipitated at a relatively high drawing rate arou nd 50 mu m/s. At this rate, diffusion was insufficient and the eutectic rea ction of (gamma + MC) was suppressed at most. The carbide grew quickly alon g the interdendritic liquid passages due to the high degree of segregation of carbide forming elements there, and hence formed a very complicated dend ritic structure. The faceted carbides precipitated when the drawing rate wa s lower than 5 mu m/s, and some of them contained a gamma phase core. The i ncorporation of gamma phase into the carbide body was caused by the eutecti c reaction of (gamma + MC) in the midst of the carbide precipitation. Two f orms of the (gamma + MC) eutectic reaction were favored by sufficient diffu sion at 0.5 mu m/s drawing rate. The (gamma + MC) eutectic reaction took pl ace at the final stage of the carbide precipitation in an isolated liquid p ool and formed small ridges distributed uniformly on the carbide surface. D iffusion operated sufficiently throughout the whole residual liquid passage guaranteed the continual eutectic reaction by a sandwich like mode of gamm a/MC/gamma, and the extremely long carbide was formed along the grain bound aries as a result. (C) 1999 Elsevier Science S.A. All rights reserved.