Effects of nanotwins and stacking faults on the mechanical properties of CrCoNi medium-entropy alloys

Experiments have shown that planar defects play an important role in strengthening and toughening mediumand high-entropy alloys (M/HEAs). Understanding the underlying defect dependent deformation mechanisms is helpful in designing strong and ductile M/HEAs. In this work, the compressive mechanical responses of columnar-grained CrCoNi MEAs are investigated via molecular dynamics simulations, considering the interspacing effect and deformation difference of pre-existing nanotwins (NTs) and stacking faults (SFs). It is found that the strength of MEAs is enhanced by the introduction of planar defects through impeding dislocation motion. "Smaller and stronger" is found between the NT/SF interspacing and strength, which is attributed to the stronger obstruction of dislocation motion for the higher fraction of planar defects at smaller interspacing. During compression, as compared with NTs, dislocations are easier to penetrate the SF planes, and partial SFs are annihilated due to the reaction between partial dislocations and SFs. Thus, at an identical NT/SF interspacing, the stacking faulted specimen manifests lower strength than the nanotwinned counterpart. The strain localization and shear deformation are observed in the specimen with a small NT/ST interspacing but dislocation motion dominates plastic deformation. Only at sufficiently small NT interspacing, less than 1 nm, the dominant deformation of the nanotwinned specimen is transformed into strain localization deformation, leading to softening. Our finding provides further atomic insights for understanding the strengthening mechanisms of M/HEAs with pre-existing planar defects.