Effects of Nb/Ti Ratio on the Microstructures of Two Experimental Ni-Based Cast Superalloys

Abstract

Effects of Nb/Ti ratio on the microstructures of two experimental Ni-based cast superalloys of the Nb/Ti ratio 0.46 and 0.03 were investigated. The Nb/Ti ratio had a significant effect on the dendritic microstructures, especially the γ precipitate, of the two alloys under the as-cast condition. The amount and composition of primary MC carbides were closely related to the Nb/Ti ratio for that the Nb atoms have a stronger tendency to form the MC carbides than the Ti atoms. During thermal exposure, the primary MC carbides gradually degraded through two reactions: firstly MC + γ M6C + γ and then MC + γ M23C6+ γ. The Nb/Ti ratio had little influence on the reaction formation but obvious influence on the decomposition degree of the primary MC carbides decomposition. In addition, the second M6C and M23C6carbides on the boundaries (GBs) gradually formed a continuous chain during thermal exposure. The γ-free zones appeared along the GBs as the growth of the second carbides and γ precipitates on the GBs when thermal exposure time last up to 3000 h.