High-strain-amplitude cyclic response in CoCrFeMnNi high-entropy alloy: Experiments, microstructure analysis and physics-based constitutive model

The high-strain-amplitude cyclic deformation behavior of the CoCrFeMnNi high-entropy alloys (HEAs), a promising candidate for extreme-load engineering applications, was systematically investigated using mechanical testing, multi-scale microstructure characterization, and a physics-based constitutive model. Strain-controlled cyclic tests uncovered significant cyclic softening, a hallmark feature of high-strain-amplitude loading that is successfully reproduced by the proposed model. In-situ and post-mortem electron backscatter diffraction and transmission electron microscopy revealed the underlying deformation mechanism: cyclic loading uniquely activates secondary twinning systems oriented at 60° to primary twins, forming a hierarchical twin network that fundamentally mitigates stress concentration and enhances fatigue resistance. This twinning mechanism also governs the observed tension-compression asymmetry: the alloy maintains symmetric hardening below an approximate plastic strain of 0.25, while the significantly higher twin nucleation rate under tension drives pronounced asymmetric hardening at larger strains. These mechanistic insights are synthesized into a dislocation-density-based constitutive framework, with flow stress explicitly decomposed into three physically meaningful components: matrix strengthening from dislocation evolution, twin-induced hardening from twin volume fraction evolution, and back stress based on dislocation pile-up theory. The model accurately reproduces the macroscopic mechanical response under both monotonic and high-strain-amplitude cyclic loading, providing a physically grounded predictive framework for analyzing mechanical behavior and assessing fatigue life of HEA components subjected to severe cyclic overloads in aerospace, nuclear power, and other extreme-service applications.