Co–Fe–Ni-Based High-Entropy Superalloys with γ/γ′ Microstructure

Abstract

Six multi-principal-element (“high-entropy”) Co–Fe–Ni-based superalloys are produced with 1) various ratios of Co, Fe, and Ni; 2) a constant concentration (13 at%) of γ′ formers (sum of Al, Ti, V, Nb, and/or Ta) without W; and 3) up to 8% Cr. The role of different ratios of γ′ to γ formers on stability of the γ + γ′ microstructure is investigated via calorimetry and metallographic imaging after aging for 1000 h at 850 °C, culminating in a W-free superalloy with equiatomic concentrations of γ- and γ′-forming elements: (Co_0.33Fe_0.33Ni_0.33)₈₇(Al_0.33Ti_0.33V_0.33)₁₃. This new alloy displays a γ/γ′ microstructure with γ′ volume fraction of ≈40% and no additional phases. Herein, a stable, continuous γ + γ′ phase field is found when transitioning from W-free Co-based superalloys to this new equiatomic (CoFeNi)₈₇(AlTiV)₁₃ composition. An intermediate Co–Ni–Fe superalloy with high Cr content (31Co–30Ni–18Fe–8Cr–5Al–3V–2Ti–1.5Nb–1.5Ta, at%) shows a higher solvus temperature and larger γ′ volume fraction, consistent with the addition of two refractory elements (Nb and Ta).