Effects of Homogenization Treatment on the Microsegregation of a Ni–Co Based Superalloy Produced by Directional Solidification

To reduce microsegregation,a series of homogenization treatments were carried out on a Ni-Co based superalloy prepared through directional solidification(DS).The element segregation characteristics and microstructural evolution were investigated by optical microscopy(OM),scanning electron microscopy(SEM),and electron probe microanalysis(EPMA).The results show that the elements are non-uniformly distributed in the solidified superalloy,in which W and Ti have the greatest tendency of microsegregation.Furthermore,severe microsegregation leads to complicated precipitations,including η-Ni_3Ti and eutectic(γ+γ').EPMA results show that Al and Mo are uniformly distributed between the eutectic(γ+γ')and γ matrix,whereas Ti is segregated in the eutectic(γ+γ') and η phases.The positive segregation element Ti,which is continuously rejected into the remaining liquid during γ matrix solidification,promotes the formation of eutectic(γ+γ') and the transformation of the η phase.According to the homogenization effect,the optimal single-stage homogenization process of this alloy is 1180 ℃ for 2 h because of the sufficient diffusion segregation of the elements.In the present study,a kinetic diffusion model was built to reflect the degree of element segregation during homogenization,and the diffusion coefficients of W and Ti were estimated.