A Study of the Microstructure and Chemical Inhomogeneities in CMSX-10 Polycrystalline Ingots Aimed at Optimizing the Solidification Process

Abstract

This study investigated the effects of different melting processes—VIM, VIM+VIM, and VIM + VIM + VAR—on the microstructure, chemical inhomogeneities, elemental segregation, and inclusion distribution in the polycrystalline nickel-based superalloy CMSX-10. Microstructural analysis via OM and SEM revealed that the VIM + VIM + VAR sample exhibited a more developed dendritic structure than the VIM + VIM sample, with both showing greater dendritic advancement than the single VIM-melted sample. This progression correlated with increased dendrite volume, secondary dendrite formation, and reduced eutectic structure size. Elemental segregation analysis identified Ta and Al as positively segregating elements. Their segregation intensified with additional melting steps: segregation coefficients for Ta and Al increased from 0.35 and 0.44 (single VIM) to 0.71 and 0.58 (double VIM), and further to 0.77 and 0.87 (triple VIM + VAR). Conversely, segregation of Co, W, Re, Mo, and Cr decreased in the multi-step processes. The most significant reductions were observed for W, Mo, and Re. For instance, Re’s segregation coefficient dropped sharply from 4.17 (single VIM) to 2.56 (double VIM) and 1.69 (VIM + VIM + VAR). In conclusion, the VIM and VAR remelting steps prior to Bridgman solidification are crucial for refining and homogenizing the dendritic structure. Producing an initial ingot with fine, uniform dendrites allows for better control of the solidification rate in the Bridgman furnace, reduces the likelihood of interdendritic cracking, and facilitates the successful formation of a single-crystal structure.