Influence of Microsegregation on the TiN Inclusions Formation Behavior in a K418 Superalloy during the Continuous Unidirectional Solidification Process

Abstract

Controlling the formation behavior of inclusions during solidification is an effective approach to improve the purity of Ni-based superalloys. In this study, the effects of the microsegregation on the formation behavior of inclusions in the continuous unidirectional solidification (CUS) process were investigated. Three types of inclusions were found in revert and the CUS solidified K418 alloy: nitrides, oxides and SiC, with the nitrides as the dominant inclusions. With the application of the CUS process, the precipitation and growth behaviors of inclusions were significantly suppressed by the larger cooling rate, refinement of microstructure, and a reduction of microsegregation. Compared to the revert K418 alloy, the removal rates of the inclusion index and contents of N and O were 93, 84.1, and 57.7%, respectively in the CUS solidified K418 alloy at the withdrawal velocity of 27 mm/min. Numerical simulations based on a coupled microstructure-microsegregation-nucleation-growth model were carried out in order to verify the experimental results of the formation behaviors of inclusions during solidification. This study provided a theoretical and practical approach to the recovery of the revert K418 alloy.