Polycrystalline Cast Superalloys

The initial cast superalloy developments in the United States centered on cobalt-base materials. Nickel-base and nickel-iron-base superalloys owe their high-temperature strength potential to their gamma prime content. For polycrystalline superalloy components, high-temperature strength is affected by the condition of the grain boundaries and, in particular, the grain-boundary carbide morphology and distribution. Vacuum induction melting offers more control over alloy composition and homogeneity than all other vacuum melting processes. The primary purification reaction occurring in the process is the removal of melt contained oxygen by means of a reaction with carbon to form carbon monoxide. A number of casting processes can provide near-net shape superalloy cast parts, but essentially all components are produced by investment casting. The solidification of investment cast superalloy components is precisely controlled so that the microstructure, which ultimately determines mechanical properties, remains consistent. Heat treating cast superalloys involves homogenization and solution heat treatments or aging heat treatments.