Hydrogen-enhanced decohesion driven by grain boundary γ′/σ precipitates in CoCrFeNi-based medium-entropy alloys

The influence of titanium addition and thermal treatments on the microstructure, mechanical behavior, and hydrogen embrittlement (HE) susceptibility of the CoCrFeNi-based medium-entropy alloys (MEAs) was investigated using the Small Punch Test (SPT). The single-phase CoCrFeNi alloy exhibited negligible HE, owing to gaseous hydrogen exposure (150 atm/200 o C/200 hrs). The Ti-containing CoCrFeNiTi 0.2 alloy showed varying degrees of susceptibility, depending on the precipitation morphology. A performance trade-off was observed: while aging at 700 o C maximized strength in the uncharged state due to intergranular precipitation, it led to severe intergranular embrittlement after hydrogen charging. The recrystallized condition, containing only coherent intragranular γ′ nano-precipitates, retained higher ductility and resistance to hydrogen embrittlement. Fractographic analysis confirmed that grain boundary precipitates (γ′ and σ) promote hydrogen enhanced decohesion (HEDE).