Effect of elemental combination on friction stress and Hall-Petch relationship in face-centered cubic high / medium entropy alloys

In this study, we report the effect of elemental combinations on the friction stress and Hall-Petch relationship in medium entropy alloys (MEAs) and high entropy alloys (HEAs) which are defined as the alloys composed of four or less and five or more principal elements, respectively, with (near-) equiatomic concentrations. The MEAs (CoCrFeNi, CoCrNi, etc.), which are subsystems of equi-atomic CoCr-FeMnNi HEA, were highly deformed by high-pressure torsion (HPT) and subsequently annealed at different temperatures. The specimens with fully-recrystallized microstructures of FCC single-phase with various mean grain sizes down to sub-micrometer scale were obtained. Subsequently, tensile tests were performed at room temperature to obtain precise Hall-Petch relationships and friction stresses of the materials. Co-20(CrNi)(80) was successfully predicted as the alloy showing the highest strength among the MEAs by the modified Labusch model (so-called mean field Labusch model) for solution hardening. Experimental values of the friction stresses were found to fit with the model very well, indicating that the strength of the alloys was closely related to entirely distorted crystal lattice acting as high-density obstacles for dislocation motion in the alloys. At the same time, values of the average lattice distortion in the alloys were found comparable to those in some dilute alloys, although severe lattice distortion had been believed as a reason for the higher strength than dilute systems. Finally, a strengthening mechanism by element-element interaction was proposed as an additional mechanism to enhance the strength in FCC HEAs and MEAs. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.