Dual-interstitials promoted multiple mechanisms enhance strength-ductility synergy of an equiatomic high-entropy alloy

Abstract

Although minor additions of interstitials have been employed to enhance the yield strength of high-entropy alloys (HEAs) with multi-principal elements, the significant loss of ductility occurs as the cost. In this work, to further improve strength-ductility synergy of the model face-centered cubic (FCC) CoCrFeMnNi HEA, dual-interstitial alloying of C and N is manipulated with the emphasis on tuning the microstructures and multiple strengthening mechanisms. C (0.6 at%) and N (0.8 at%) are completely dissolved in the FCC matrix after annealing at or above 1150 °C. The dissolution of the dual-interstitials in the matrix refines grains by impeding grain boundary migration via solute atmospheres. Nanoprecipitates including M23C6, Cr3C2 and Cr2(N, C) are dispersed in the recrystallized FCC matrix of the dual-interstitial HEAs upon annealing at 950–1000 °C. The Zener pinning pressure induced by these nanoprecipitates retards grain boundary migration and hence reduces grain sizes. The primary deformation mechanism is dislocation slip in both dual-interstitial HEAs with and without precipitates at the early deformation stages (e.g., 40% local strain), the mechanical twinning occurs at the medium and late deformation stages (e.g., 60% and 90% local strains, respectively) contributing to the high ductility of the HEAs. The synergetic strengthening effects of nanoprecipitates, grain boundaries, interstitial and substitutional solid-solution brings about high yield strength of 772 MPa and ultimate tensile strength of 1178 MPa at a large elongation of 34% in the dual-interstitial HEA containing carbides/carbonitrides nanoprecipitates. The contributions of the multiple mechanisms are quantified and discussed.