Fault architecture-controlled γ↔ε transformation and reversion under strain-controlled fatigue in a metastable high-entropy alloy

Fatigue failure remains the dominant mode of structural degradation; yet the mechanistic coupling between cyclic loading, microstructural fault topology, and deformation reversibility in metastable high-entropy alloys (MHEAs) remains poorly understood. Owing to their low stacking-fault energy (SFE) and marginal γ–ε free-energy difference, MHEAs exhibit deformation pathways, including partial-dislocation glide, stacking fault evolution, and γ↔ε martensitic transformation, that differ fundamentally from conventional alloys. Yet, the interplay between endurance strength, amplitude-dependent cyclic plasticity, and reversible martensitic transformation has not been established. Here, we present a novel integrated mechanical–microstructural framework by combining stress-controlled endurance-limit quantification with strain-controlled low-cycle fatigue (±0.1–0.6%), and multi-scale characterization (EBSD, synchrotron XRD, TEM, APT). A maximum-likelihood random endurance-limit model defines the cyclic stability threshold, while strain-controlled tests reveal pronounced amplitude-dependent cyclic plasticity, non-Masing behavior, and a bilinear Coffin–Manson response marking a transition from transformation-assisted to reversion-assisted fatigue. Microstructurally, γ→ε transformation initiates at ±0.1% under nominally elastic conditions, enabled by pre-existing stacking-fault tetrahedra. At ±0.4%, ε formation reaches a maximum, where dense Shockley-partial networks, interacting fault assemblies, and mixed planar–wavy slip produce peak back-stress hardening and maximum plastic strain accumulation. Upon increasing the strain amplitude to ±0.6%, partial ε→γ reversion occurs via reverse partial glide and fault annihilation, facilitated by three-dimensional stacking-fault parallelepipeds that impede glide while promoting structural reordering, providing the first integrated experimental evidence of room-temperature reversible γ↔ε transformation under strain-controlled cyclic loading in an HEA. This work establishes a unified, physics-based framework linking cyclic mechanics to fault topology and identifies metastability and fault architecture as key design variables for fatigue-resistant alloys.