Nonbasal Slip Systems Enable a Strong and Ductile Hexagonal-Close-Packed High-Entropy Phase

Linear defects, referred to as dislocations, determine the strength, formability, and toughness of crystalline metallic alloys. The associated deformation mechanisms are well understood for traditional metallic materials consisting of one or two prevalent matrix elements such as steels or aluminum alloys. In the recently developed high-entropy alloys (HEAs) containing multiple principal elements, the relationship between dislocations and the mechanical behavior is less understood. Particularly HEAs with a hexagonal close-packed (hcp) structure can suffer from intrinsic brittleness due to their insufficient number of slip systems. Here we report on the surprisingly high formability of a novel high-entropy phase with hcp structure. Through in situ tensile testing and postmortem characterization by transmission electron microscopy we reveal that the hcp phase in a dual-phase HEA (${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{30}{\mathrm{Co}}_{10}{\mathrm{Cr}}_{10}$, at. %) activates three types of dislocations, i.e., $⟨a⟩$, $⟨c⟩$, and $⟨c+a⟩$. Specifically, nonbasal $⟨c+a⟩$ dislocations occupy a high line fraction of $\ensuremath{\sim}31%$ allowing for frequent double cross slip which explains the high deformability of this high-entropy phase. The hcp structure has a $c/a$ ratio of 1.616, i.e., below the ideal value of 1.633. This modest change in the structure parameters promotes nonbasal $⟨c+a⟩$ slip, suggesting that ductile HEAs with hcp structure can be designed by shifting the $c/a$ ratio into regimes where nonbasal slip systems are activated. This simple alloy design principle is particularly suited for HEAs due to their characteristic massive solid solution content which readily allows tuning the $c/a$ ratio of hcp phases into regimes promoting nonbasal slip activation.