Microstructure and Defect Effects on Strength and Hydrogen Embrittlement of High-Entropy Alloy CrMnFeCoNi Processed by High-Pressure Torsion

High-entropy alloys (HEAs) are considered as new hydrogen compatible materials, but enhancing their yield strength without deteriorating their hydrogen embrittlement resistance is challenging. In this study, various kinds of defects are introduced into a CrMnFeCoNi Cantor alloy by plastic straining via the high-pressure torsion method, and the correlations of applied strain, microstructural features, strength, and hydrogen embrittlement are studied. The unstrained coarse-grained alloy shows elongations over 80% under hydrogen, but its yield strength is only 220 MPa. Twinning is a major deformation mechanism at the early stages of straining, resulting in over 1 GPa yield strength and 9% elongation in the presence of hydrogen. With a further increase in strain, dislocation-based defects including Lomer-Cottrell locks and D-Frank partial dislocations with low mobility are formed, enhancing the strength further. At large strains, nanograins with high-angle boundaries are generated, resulting in over 1900 MPa strength with poor hydrogen embrittlement resistance due to large hydrogen diffusion and hydrogen-enhanced decohesion. These results suggest that twins and defects with low mobility such as Lomer-Cottrell locks and D-Frank partial dislocations are effective to achieve a combination of high yield strength and good hydrogen embrittlement resistance by suppressing the hydrogen-enhanced localized plasticity in HEAs.