Effect of Mo addition on the microstructural evolution and mechanical properties of Fe-Ni-Cr-Mn-Al-Ti high entropy alloys

High-strength alloys are in urgent demand for engineering applications. Inducing multiple strengthening phases in a ductile matrix by microalloying is an effective alloy-strengthening strategy, especially the simultaneous formation of geometrically close-packed (GCP) phases and topologically close-packed (TCP) precipitation in high entropy alloys (HEAs). In this study, the microstructural evolution and mechanical properties of a series of (FeNi)67Cr15Mn10-xAl4Ti4Mox HEAs with x values of 1, 2, 3, 4, and 5 were analyzed in detail, with the substi-tution of Mn by Mo. This substitution does not change the L12 phase forming elements (Ni, Al and Ti) in the alloy, and enhance the a phase precipitation, which meantime does not modify the oxidation resistance in thermal mechanical treatment. After aging at a temperature of 1023 K for 5 h, the matrix of the HEAs still maintains a face-cubic-centered structure. However, as Mn is replaced by Mo, hard a precipitates are introduced into the matrix and the grains are significantly refined when x < 5. Therefore, the yield strength monotonically increases with increasing x. However, the formation of the hard a precipitates also hinders the sustainability of the work hardening process, and thereby decreases the plasticity of the HEAs. Hence, the elongation at break mono-tonically decreases with increasing x. However, the high density of a phase precipitates obtained at x = 5 prematurely terminates the work hardening process. Therefore, the highest ultimate tensile stress is obtained at x = 4, rather than x = 5. This also changes the fracture morphology from a coarse grain morphology for x < 5 to a typical brittle fracture at x = 5. Accordingly, (FeNi)67Cr15Mn6Al4Ti4Mo4 obtains the optimal mechanical prop-erties overall.