Unveiling the role of microstructure in hydrogen-induced nanomechanical degradation of CoCrFeMnNi

Hydrogen embrittlement (HE) remains a key obstacle for structural alloys in hydrogen-rich environments. High-entropy alloys (HEAs) have shown superior resistance to HE compared with conventional alloys, yet the underlying microstructure-dependent mechanisms remain insufficiently understood. Here, the nanomechanical response of CoCrFeMnNi was examined in as-cast, rolled, and annealed states to clarify hydrogen–defect interactions. Various processing routes produced differences in grain morphology, boundary character, and defect density, which governed deformation pathways under hydrogen charging. The as-cast HEA exhibited minimal softening due to its coarse grains and limited density of trap sites. The rolled alloy showed significant hardness degradation, driven by hydrogen-assisted dislocation activity and twinning. The annealed alloy underwent severe softening, with partial recovery during aging, which is consistent with hydrogen-facilitated dislocation glide and stacking fault activity within its recrystallized, twin-rich microstructure. These findings underscore the critical role of microstructural state in dictating HE susceptibility, offering guidance for the microstructure-informed design of hydrogen-tolerant HEAs.