Formation of solidification and homogenisation micropores in two single crystal superalloys produced by HRS and LMC processes

Abstract

Effects of processing techniques and alloying chemistry on the formation of solidification and homogenization micropores were investigated in this study for two single crystal nickel-based superalloys with different compositions produced by Bridgman (HRS) and LMC processes. The results show that the volume fraction of micropores in as-cast HRS alloy is lower than that in as-cast LMC alloy, and they are mainly influenced by alloy chemistry. After solution heat treatment, external and internal homogenization micropores are observed to form at the Al-depletion zone near the surface region and the internal zone away from the surface, respectively. The number of external homogenization micropores reduces with increasing the distance away from the surface. Responding to high temperature oxidizing environment (in air), the formation of such homogenization micropores is ascribed to fast outward diffusion of Al, which lead to the formation of Kirkendall voids within the Al-depletion zone. Additionally, the formation of internal homogenization micropores at high temperature is due to Kirkendall effect during the diffusion process of alloying elements between dendrite and interdendrite regions. Smaller primary dendrite arm spacing and lower level of solidification segregation in the LMC alloy results in much less internal homogenization micropores in comparison with the HRS alloy.