Compositional variation effects on the microstructure and properties of a refractory high-entropy superalloy AlMo0.5NbTa0.5TiZr

An AlMo0.5NbTa0.5TiZr baseline alloy was shown earlier to have good high temperature strength but poor ductility below 600 °C due to coarse intermetallic grain boundary particles and a continuous ordered B2 matrix phase. Systematic composition changes intended to remove the deleterious microstructural features and to improve mechanical properties were explored in the present work. The baseline alloy and the new alloys studied here, AlMo0.5NbTa0.5TiZr0.5, AlNbTa0.5TiZr0.5, Al0.5Mo0.5NbTa0.5TiZr and Al0.25NbTaTiZr, all had an ordered B2 matrix crystal structure. Additionally, coherent BCC nanoscale precipitates were present at a high volume fraction inside the B2 matrix grains in AlMo0.5NbTa0.5TiZr, Al0.5Mo0.5NbTa0.5TiZr and Al0.25NbTaTiZr, and/or coarse, grain-boundary particles existed in AlMo0.5NbTa0.5TiZr and AlMo0.5NbTa0.5TiZr0.5. The mechanical properties were assessed with microhardness and compression testing at 25 °C and 1000 °C. Al0.5Mo0.5NbTa0.5TiZr showed the highest hardness (Hv = 6.4 GPa) and strength (σ0.2 = 2350 MPa) at 25 °C and modest strength (σ0.2 = 579 MPa) at 1000 °C. AlMo0.5NbTa0.5TiZr0.5 had the highest strength (σ0.2 = 935 MPa) at 1000 °C, but was brittle at 25 °C. High-temperature deformation produced a desirable microstructure in Al0.5Mo0.5NbTa0.5TiZr and Al0.25NbTaTiZr alloys consisting of a continuous BCC phase and discontinuous B2 nano-precipitates. The relationships between the composition, microstructure, and properties were identified and discussed. Keywords: High entropy alloy, Superalloy, Microstructure, Mechanical properties