Effect of Nb Additions on the Microsegregation and Homogenization of Ni–Co Based Superalloys

In this work, the segregation and homogenization organization of as-cast Ni–Co based superalloys with 4, 5 and 6% Nb contents are investigated, respectively. It’s found that the Nb element is the main cause of segregation in Ni–Co alloys, which is mainly attributed to the high melting point and slow solidification rate of Nb, resulting in the alloy being very susceptible to macroscopic segregation, and the content of Nb should be controlled within 5%. By means of high-temperature homogenization, which is heating at 1150 °C for 48 h, the dendritic structure can be completely eliminated and macroscopic segregation can be eliminated, but the high temperature leads to the formation of refractory compounds, such as NbC or TiN, and the amount of refractory compounds increases with the increase of Nb content. In addition, when the Nb content exceeds 5%, it leads to the formation of the Laves phase, which is detrimental to the thermoplasticity of the alloy.