Defeating hydrogen-induced grain-boundary embrittlement via triggering unusual interfacial segregation in FeCrCoNi-type high-entropy alloys

Metallic materials are mostly susceptible to hydrogen embrittlement (HE), which severely deteriorates their mechanical properties and causes catastrophic failures with poor ductility. In this study, we found that such a long-standing HE problem can be effectively eliminated in the Fex(CrCoNi)1-x face-centered-cubic (fcc) high-entropy alloys (HEAs) by triggering the localized segregation of Cr at grain boundaries (GBs). It was revealed that increasing the Fe concentration from 2.5 to 25 at. % leads to substantially improved HE resistance, i.e., the ductility loss decreases from 70% to 6%. Meanwhile, the fracture mode transformed from the intergranular to the transgranular mode. Multiscale microstructural analyses demonstrated that the Fe2.5Cr32.5Co32.5Ni32.5 and Fe25Cr25Co25Ni25 alloys show negligible differences in the phase structure, grain size, and grain-boundary (GB) character. However, interestingly, the near atomic-resolution elemental mapping revealed that an increased Fe concentration promotes the nanoscale Cr segregation at the GBs, which is primarily motivated by the strong repulsive force between Cr and Fe and the low self-binding energy of Cr. Such unusual interfacial segregation of Cr, which has not been reported before in the Fe25Cr25Co25Ni25 alloy, helps enhance the GBs’ cohesive strength and suppresses the local hydrogen segregation at GBs due to the deceased GB energy, leading to the outstanding HE resistance. These findings decipher the origins of the vastly-improved HE resistance in current FeCrCoNi-type HEAs, and meanwhile, provide new insight into the future development of novel high-performance structural alloys with extraordinary immunity to hydrogen-induced damages.