Development and Microstructural Characterization of a New Wrought High Entropy Superalloy

In this study, a newly developed γ′ precipitation hardened high entropy superalloy (designated as HES-A1 hereafter) was produced and its microstructural features were characterized. The new alloy composition is based on the major elements of conventional IN718 alloy, however other elements were added or removed to minimize possible presence of δ (or γʺ) phase and increase the volume fraction of very fine γ′ precipitates instead. Broadening the hot working window, lower density, and avoiding the use of expensive elements were other considerations taken into account in the design of the new alloy. CALPHAD (CALculation of PHAse Diagrams) and PhaComp methods were used for the prediction of phases and their evolutions. The microstructure of the HES-A1 alloy in as-cast, homogenized, hot rolled, annealed and aged conditions were characterized using optical, scanning, and transmission electron microscopes as well as X-ray diffraction technique. The as-cast microstructure of HES-A1 contained 5.5% of Laves phase, which was reduced to less than 0.3% through the development of a homogenization treatment. Hot rolling with reductions up to 36% at 1015 °C did not produce any cracking, indicating a good forming potential for the new alloy. The application of double aging treatment, similar to the one for IN718 alloy, showed no presence of γʺ or formation of δ phase in the microstructure. HES-A1 has been designed with a higher Al/(Ti + Nb) ratio and higher proportion of Al + Ti + Nb so that it could be mainly strengthened by γ′ precipitation. The average size of monomodal γ′ precipitates was smaller than that observed in conventional alloys after similar treatments.