Excellent tensile properties and deformation mechanisms in a FeCoNi-based medium entropy alloy with dual-heterogeneous structures

A dual-heterogeneous structure with both heterogeneous grain structure and coherent L1(2) nano-precipitate was obtained in a (FeCoNi)(86)Al7Ti7 medium-entropy alloy (MEA) after cold rolling, critical annealing, and aging treatments. The volume fraction of the L1(2) phase is found to increase from 0% to 34.5% after aging treatment, resulting in severer heterogeneity. The unaged samples of (FeCoNi)(86)Al7Ti7 MEA are found to have a much better synergy of strength and ductility as compared to the FeCoNi MEA. Moreover, the tensile properties for the aged samples of (FeCoNi)(86)Al7Ti7 MEA with dual-heterogeneous structures are even better than those for the corresponding unaged samples. The hetero-deformation-induced hardening plays a more important role in the aged samples than in the unaged samples, producing a higher density of geometrically necessary dislocations. High density dislocations are discovered in grain interiors for the aged samples and the deformation is dominated by the planar slip in the unaged samples. Meanwhile, a high density of dislocations are observed inside L1(2) nano-precipitates; thus, L1(2) nano-precipitates can be sheared by dislocations, indicating that L1(2) nano-precipitates are strong but deformable. The strong interactions between dislocations and L1(2) nano-precipitates should provide additional strain hardening for better tensile properties in the aged samples.