Polycrystalline γ(Ni)γ(Ni₃Al)-δ(Ni ₃Nb) eutectic Ni-base superalloys: Chemistry, solidification and microstructure

A novel type of γ/γ′-δ eutectic polycrystalline Ni-base superalloy has been recently demonstrated to possess promising mechanical properties at 700-800°C, yet much of our knowledge regarding this eutectic alloy system is still qualitative in nature. A matrix of alloys spanning a broad range of alloy compositions were examined to improve our understanding of various alloying effects on the solidification behavior and as-cast microstructure of multi-component γ/γ′-δ eutectic alloys. While the volume fraction of constituent phases was observed to be closely related with the Nb concentration, it could also be effectively modified by additions of other elements. Solidus and liquidus temperatures of these γ/γ′-δ alloys were also found to vary with different solute concentrations, with a general decrease in solidus temperature as solute content increases. Interestingly, Cr, Mo and W were found to induce similar changes on the solidification and microstructure of this eutectic system. Extensive comparison between these experimental results and Computherm Pandat predictions was carried out, which confirmed the reliability of Pandat in predicting the qualitative trends for this eutectic system. Although satisfactory agreement was found between the measured and calculated solidus temperatures, additional phase equilibria information of multi-component γ/γ′-δ alloys is required to improve the predictive capability on other properties.