Interfacial segregation of magnesium in nickel base superalloy: Carbide morphology and properties

It is shown that microadditions of Mg to cast M17 superalloy significantly improve the impact toughness, the high cycle bending fatigue endurance, and the stress rupture properties at high temperature. There is a relationship between these improvements and Mg refinement of the MC primary carbide. Analysis using Auger electron spectroscopy shows Mg segregating to various interfaces and in particular to the carbide/matrix interface. A thermodynamic analysis of the diffusion process is presented to show that the Mg segregation results from uphill diffusion of Mg to the Ni enriched interfacial liquid surrounding the growing carbide particle, which derives from the negative excess Gibbs free energy change for the Ni(rich)–Mg liquid solution. MST/991.