Solidification characteristics and segregation behavior of Ni-based superalloy K418 for auto turbocharger turbine

摘要

The solidification characteristics and segregation behavior of K418 alloy were investigated by the DSC measurement and the isothermal solidification quenching process. Phase transformation temperatures, together with the solid fraction with temperature were measured by experiments and comparatively calculated by Thermo-Calc. It is demonstrated that the solidification begins with the formation of primary y dendrite and terminates with the y/y' eutectic reaction, being accompanied by the formation of MC carbide and M(C, B) carboboride. During the solidification, interdendritic region is significantly enriched with Nb, Ti, Zr, B, C and Mo, which results in the formation of carbide, carboboride and y/y' eutectic. The slow increase of solid fraction at the final stage of solidification is also caused by the strong segregation of the positive segregation elements in interdendritic area. In the case of K418, equilibrium model is shown to be a valuable tool to predict the solidification sequence and phase transition temperatures. Scheil calculation can be used to investigate the segregation behavior during solidification.