Influence of carbon on the mechanical behavior and microstructure evolution of CoCrFeMnNi processed by high pressure torsion

Abstract In this study, a Cantor type high entropy alloys with the addition of C interstitials (, 0, 0.5 and 2 at. %) were processed via high pressure torsion (HPT) under 6.5 GPa by 0.5, 1 and 3 turns at room temperature. The microstructures and mechanical properties of samples before and following HPT were investigated. In all compositions studied, HPT deformation led to a dramatic grain size refinement down to a nanoscale range and also resulted in a considerable increase in dislocation density. Atom Probe Tomography studies reveal that in the alloy with 2 at. % C, C atoms segregated at the boundaries of the nano-grains. The hardness of specimens with 0, 0.5 and 2 % at. C approached a maximum value at a plateau at 490 HV, 550 HV and 640 HV, respectively. This plateau was reached at an HPT shear strain corresponding to more than ∼20. The yield and ultimate tensile strength values of the as-cast alloys increased with increasing C content. The uniform elongation of all three alloys studied exceeded 30%. The yield strength values of the HPT processed samples increased significantly and reached 1.7 GPa, 1.9 GPa and 2.4 GPa, respectively, with 0, 0.5 and 2 at. % C; however, a dramatic decrease of ductility was observed. Analysis of the factors contributing to the strengthening of HPT processed alloys revealed that the conventional approach based on dislocation motion resulted in significantly overestimated yield stress values in comparison with the experimentally obtained ones. It was proposed that the discrepancy between the theoretical estimate and experimental results is related to the emergence of grain boundary sliding. The present results provide new insights on the leverage of strength versus ductility by combining HPT processing with alloying with an interstitial-type element.