A novel methodology to estimate hydrogen diffusivity and its applications in revealing hydrogen effects in CoCrNi medium-entropy alloy versus 316L stainless steel

Hydrogen diffusivity is one of the important factors affecting the susceptibility to hydrogen embrittlement of metals and alloys. Here, we proposed a novel approach to estimate hydrogen diffusivity in FCC alloys by performing nanoindentation experiments with continuous stiffness measurement. In consideration of the plastic zone size around the indentation, the relationship between hydrogen-induced hardness change and the depth from the sample surface was established for two typical FCC alloys—316L stainless steel (SS) and CoCrNi medium-entropy alloy (MEA)—subjected to hydrogen-charging and subsequent room-temperature aging. By combining with the estimation of the through-thickness distribution of hydrogen concentration, hydrogen diffusivity values were successfully determined, which agreed well with the literature data. It was revealed that CoCrNi MEA exhibited a hydrogen diffusivity ∼0.68 times higher than that in SS316L, and that hydrogen-induced softening upon long-term aging (at room temperature) was observed in CoCrNi but not in SS316L. The underlying mechanisms for the distinct hydrogen-related phenomena in CoCrNi MEA were elucidated based on the density functional theory calculations of hydrogen solution energy and vacancy formation energy.