Chemical short-range order in Fe₅₀Mn₃₀Co₁₀Cr₁₀ high-entropy alloy

Chemical short-range order (CSRO) is generally possible in concentrated solid solutions and currently of considerable interest for multi-principal element alloys. However, a convincing demonstration of CSRO has been challenging and achieved thus far only for ternary medium-entropy alloys such as VCoNi. Here, we report definitive proof of CSRO in a quaternary face-centered-cubic Fe₅₀Mn₃₀Co₁₀Cr₁₀ high-entropy alloy, acquired from systematic electron microscopy experiments. The evidence includes extra diffuse disks in nano-beam electron diffraction patterns, images in state-of-the-art aberration-corrected scanning transmission electron microscope, as well as compositional profiles across neighboring atomic planes/columns in atomic-resolution chemical maps. The CSRO regions are found to occupy an areal fraction of 20% and have dimensions on a sub-nanometer scale. This length scale, as well as the diffraction features of the CSRO, are different from those of intermetallic compound precipitates; as such, the CSRO is not a growing stage of a nucleated second phase, the precipitation of which has been dealt with previously in classical alloys. We further conducted a spatial correlation analysis of the concentrations in atomic columns in the chemical map, enabling us to uncover a general tendency toward nearest-neighbor chemical ordering, specifically, preference for unlike species (such as Fe–Mn) and avoidance for like-species (such as Fe–Fe). The persistence of this trend, the same as that found in VCoNi MEA, recently, is somewhat intriguing for a high-entropy alloy in which all the constituent elements are similar in atomic size and have rather a small enthalpy of mixing.