Understanding the Metal-Mold interfacial reactions during directional solidification of superalloys using Al2O3-rich ceramic mold

This study investigates the reactions occurring at the metal-mold interface during directional solidification of Ni-based superalloys, exerting a significant influence on both the surface quality and the desired composition of the alloy. The origin and nature of these reactions are systematically investigated by integrating thermodynamics and microstructural evolution of the alloy. It was observed that Hf from the melt reacts with the Al2O3-based ceramic mold, leading to the formation of HfO2 at the interface. This compound adheres to the surface of the ingot and also infiltrates into the melt near the interface. In order to validate these findings, an exploration of four alternative Ni-based superalloys, both with and without Hf content, was conducted, revealing an absence of comparable reactions at the interface in Hf-free alloys. Additionally, it was explored that the interface formed due to the metal-mold reaction may serve as a preferential nucleation site for MC-type carbides in C-containing Ni-based superalloys. This study concludes that the Hf content is the primary determinant influencing reactions at the metal-mold interface.