Mechanism underlying Mg regulating inclusion formation and carbide morphology evolution in Ni-based cast superalloys

In the investment casting of nickel-based superalloys, interaction with magnesium-containing crucibles inevitably introduces trace Mg into the alloy melt. Therefore, it is essential to elucidate the occurrence of magnesium in cast superalloys and its subsequent influence on the microstructure. This study investigates the precipitation behavior of inclusions in cast superalloys with varying Mg contents. The results indicate that Mg readily reacts with oxygen and sulfur, promoting inclusion precipitation and thereby refining the average inclusion size. Thermodynamic simulations reveal that with increasing Mg content, the oxide composition evolves from Al 2 O 3 to MgAl 2 O 4 and MgO, while the sulfide transitions from TiS to MgS. Furthermore, Mg-bearing inclusions act as potent nucleation sites that promote the precipitation of nitrides and MC carbides. The resulting oxide–nitride–MC carbide composite structure significantly reduces the interfacial energy barrier during nucleation. This increased density of nucleation sites, coupled with the altered solute field during solidification, leads to the refinement and spheroidization of MC carbides. This work provides critical theoretical insights for the clean smelting and purification of cast superalloys.