Cooperative deformation mechanisms in a fatigued CoCrNi multi-principal element alloy: A case of low stacking fault energy

Multi-principal element alloys (MPEAs) of low stacking fault energies (SFEs) often exhibit outstanding damage tolerance. In contrast to the well-understood monotonic deformation behaviors, fatigue deformation mechanisms of the low-SFE MPEAs remain fundamental questions. Using TEM, this work demonstrated two main dislocation configurations cooperating in an exemplary low-SFE CoCrNi MPEA, namely slip bands and veins/walls/cells, which are usually formed in materials with low- and high-SFEs, respectively. Under low strain amplitude of 0.3%, the deformation features are found to include primary and double slip bands (dominating in ≈71% and 29% grains, respectively). Upon increasing strain amplitude, apart from the slip bands, dislocation substructures formed (i.e., veins/walls/cells present in ≈ 33% and 60% grains under strain amplitude of 0.5% and 0.7%, respectively) due to cross slip, despite the alloy's low-SFE. Moreover, the slip bands and walls/cells are found to serve more likely as geometrically necessary dislocations and statistically stored dislocations, respectively. Besides, the constraints from neighboring grains and Copley-Kear effect (rather than grain orientation) were suggested to play more important role in determining these dislocation patterns formation. Lastly, this work evidenced unexpected partial dislocations and stacking faults (in addition to usually observed screw dislocations) shuttling between the walls/cells by planar slip. This predominant planar slip behavior explains well the exceptional damage tolerance of low-SFE MPEAs. These insights can also advance the understanding of fatigue deformation of conventional alloys with low-SFEs.