Mechanism of hydrogen-induced vacancy cluster decomposition in CrCoNi medium-entropy alloy

Hydrogen-vacancy interactions in CrCoNi medium entropy alloy (MEA) were investigated using special quasirandom structures (SQS) and first-principles density functional theory (DFT). Hydrogen lowers the vacancy formation energy and prefers off-center interstitial sites near vacancies rather than the vacancy core. Across multivacancy configurations, hydrogen preferentially destabilizes larger clusters and promotes their breakup into smaller ones. This size-selective destabilization correlates with a reduction of the unstable stacking-fault energy, lowering the barrier for partial slip and facilitating cluster breakup. The local elastic response is size dependent. In hydrogen, large clusters are stiffer and more brittle, but smaller decomposition-derived clusters are more compliant. Thus, hydrogen does not act as a uniform embrittling agent. Its effect depends on defect size and local chemistry. Overall, the study establishes an atomistic link between hydrogen exposure, vacancy-cluster evolution, stacking-fault energetics, and twinning in CrCoNi MEA. • First-principles identify hydrogen–vacancy interactions shaping deformation in CrCoNi MEA. • Hydrogen lowers stacking-fault energy in defected CrCoNi, promoting twinning and ductility. • A prescribed amount of hydrogen limits vacancy clustering, guiding hydrogen-service alloy design.