Interstitial equiatomic CoCrFeMnNi high-entropy alloys: carbon content, microstructure, and compositional homogeneity effects on deformation behavior

While interstitial alloying has been utilized to improve mechanical properties of multi-component high-entropy alloys (HEAs), its effectiveness depends on the interstitial content, microstructure and compositional homogeneity states. Here we present and discuss the influences of these factors on the mechanical behavior of interstitial equiatomic CoCrFeMnNi HEAs at room temperature. Interstitial HEAs containing carbon of 0.2, 0.5 and 0.8 at. % were processed to different compositional homogeneity states and grain sizes. We found that deformation of the various interstitial HEAs at early deformation stages is accommodated by dislocation slip whereas twinning occurs at the later stages. Upon an identical local strain at the later stages of deformation, nano-twin density decreases as the increase of carbon content due to the increased stacking fault energy. Also, the increase of C content leads to significantly higher energy barrier to recrystallization during annealing. Partially recrystallized (∼20 vol %) interstitial HEA with C content of 0.8 at. % shows more than five times higher yield strength compared to the as-homogenized coarse-grained (∼200 μm) reference material, suggesting the significant beneficial effect of interstitials enabled microstructural adjustment on performance of the interstitial HEAs. Further, the compositionally inhomogeneous coarse-grained (∼200 μm) interstitial HEAs exhibit lower work-hardening ability and ultimate strength compared to the homogenized reference material due to that the compositional inhomogeneity promotes the localized plasticity. Some more insights for the design and processing of interstitial HEAs are generalized and discussed.