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 eta-Ni3Ti and eutectic (gamma + gamma '). EPMA results show that Al and Mo are uniformly distributed between the eutectic (gamma + gamma ') and gamma matrix, whereas Ti is segregated in the eutectic (gamma + gamma ') and eta phases. The positive segregation element Ti, which is continuously rejected into the remaining liquid during gamma matrix solidification, promotes the formation of eutectic (gamma + gamma ') and the transformation of the eta phase. According to the homogenization effect, the optimal single-stage homogenization process of this alloy is 1180 degrees C 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.