Characteristic deformation microstructure evolution and deformation mechanisms in face-centered cubic high/medium entropy alloys

Face-centered cubic (FCC) high/medium entropy alloys (HEAs/MEAs), novel multi-principal element alloys, are known to exhibit exceptional mechanical properties at room temperature; however, the origin is still elusive. Here, we report the deformation microstructure evolutions in a tensile-deformed Co₂₀Cr₄₀Ni₄₀ representative MEA and Co₆₀Ni₄₀ alloy, a conventional binary alloy for comparison. These FCC alloys have high/low friction stresses, fundamental resistance to dislocation glide in solid solutions, respectively, and share similar other material properties, including stacking fault energy. The Co₂₀Cr₄₀Ni₄₀ MEA exhibited higher yield strength and work-hardening ability than in the Co₆₀Ni₄₀ alloy. Deformation microstructures in the Co₆₀Ni₄₀ alloy were marked by the presence of coarse dislocation cells (DCs) regardless of grain orientation and a few deformation twins (DTs) in grains with the tensile axis (TA) near 〈1 1 1〉. In contrast, the MEA developed three distinct deformation microstructures depending on grain orientations: fine DCs in grains with the TA near 〈1 0 0〉, planar dislocation structure (PDS) in grains with other orientations, and a high density of DTs along with PDS in grains oriented 〈1 1 1〉. Three-dimensional electron tomography revealed that PDS in the MEA confined dislocations within specific {1 1 1} planes, indicating suppression of cross-slip of screw dislocations and dynamic recovery. In-situ X-ray diffraction during tensile deformation showed a higher dislocation density in the MEA than in the Co₆₀Ni₄₀ alloy. These findings demonstrate that FCC HEAs/MEAs with high friction stresses naturally develop unique deformation microstructures which is beneficial for realizing superior mechanical properties compared to conventional materials.