Hydrogen resistance of a 1 GPa strong equiatomic CoCrNi medium entropy alloy

•The influence of hydrogen on the deformation behavior in an equiatomic CoCrNi medium entropy alloy (MEA) is investigated.•Hydrogen influences the deformation microstructure of the MEA.•The primary hydrogen-induced failure mode of the alloy is intergranular cracking.•Two main competing mechanisms are found in the CoCrNi MEA when exposed to hydrogen. In this work, we study the influence of hydrogen on the deformation behavior and microstructure evolution in an equiatomic CoCrNi medium entropy alloy (MEA) with an ultimate tensile strength of ∼1 GPa. Upon deformation, hydrogen-charged samples exhibit enhanced dislocation activity and nanotwinning. Hydrogen shows both positive and negative effects on the deformation behavior of the CoCrNi MEA. More specifically, it weakens grain boundaries during loading, leading to intergranular cracking. Also, it promotes the formation of twins which enhance the material’s resistance to crack propagation. The underlying mechanisms responsible for the hydrogen resistance of the CoCrNi MEA are discussed in detail.