A single-phase Al0.1CoCrFeNi high-entropy alloy (HEA) with face-centered cubic structure was subjected to cryo-rolling at the liquid N2 temperature and subsequent annealing. Microstructural characterization of post-annealed samples by electron backscattered diffraction and transmission electron microscopy revealed that, the dominant grain boundaries (GBs) are of Σ3 type low energy twin boundaries owing to the low stacking fault energy of the HEA. The resulting ultrafine-grained single-phase HEA with abundant Σ3 twin boundaries shows a high strength above 1.0 GPa and a tensile strain larger than 20%. The quantitative analysis on the grain size dependence of strength suggests that a high lattice friction stress and a high GB strengthening via low energy Σ3 twin boundaries are two major contributions for the excellent strength-ductility balance of the ultra-fine grained HEA.
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