Segregation and diffusion behavior of niobium in a highly alloyed nickel-base superalloy

The segregation during solidification and high temperature diffusion during homogenization of niobium in a highly alloyed nickel-base superalloy were investigated. Niobium is seriously segregated into the interdendritic regions with segregation coefficient as high as 4.30. Various niobium-enriched phases including Laves phase, δ phase, (γ+γ′) eutectic, MC and M6C types of carbides precipitated in the interdendritic regions. The soluble temperature of niobium-enriched phases and the degree of dendritic segregation were determined by differential thermal analysis and homogenization treatments at varied temperatures and times. The calculation of elemental diffusion indicates that the diffusion rate of niobium increases remarkably with the homogenization temperature increasing, so that the annealing time can be effectively reduced. A three-step homogenization treatment technology without incipient melting was established to eliminate the segregation of niobium and to obtain a uniform microstructure.