Microstructural Instability and Precipitation Behaviors of Intermetallic Phases in a Nb-Containing CoNi-Based Superalloy

Exploring the alloying effect of various elements plays a crucial role in optimizing the alloy design for superalloys. In this study, the influence of Nb additions on a γ′-strengthening CoNi-based superalloy has been investigated. Undesirable intermetallic phases including the topologically close-packed D85 phase and the ordered D019 phase were observed to preferentially form in the dendritic and interdendritic regions of the investigated alloy, respectively. Precipitation behaviors of the D019 phase and the D85 phase including growth mechanisms and crystallographic orientation relationships have been characterized and analyzed in detail using advanced electron microscopy techniques. First-principle calculations using density function theory have been carried out to evaluate the effect of Nb additions on the thermodynamic stability of the L12-γ′ phase. It turns out that the supercell with L12 structure would be energetically destabilized when the Nb-substituted atom sits on the Co or Al sites, which could correspond to the microstructural instability of the Nb-containing CoNi-based superalloy.