Topological passivation makes high strength alloys insensitive to hydrogen embrittlement
Infrastructure parts for a hydrogen (H) economy need alloys that are mechanically strong and at the same time resistant to the most dangerous and abrupt type of failure mode, namely, H embrittlement. These two properties are in fundamental conflict, as increasing strength typically amplifies susceptibility to H-related failure. Here, we introduce a new approach to make alloys resistant to H embrittlement, by creating a topological passivation layer (up to a few hundred micrometers thick) near the material surface, the region that is most vulnerable to H ingress and attack. It features instead a layer of ultrafine laminated grains with tens of times higher dislocation density than conventional materials, altering H diffusion, trapping and crack evolution. We tested the concept on a face-centered cubic (FCC) CoCrNi medium entropy model alloy which undergoes severe H-induced intergranular cracking. Two key mechanisms create the topological passivation: First, the high density (up to ~1.3e15 m-2) of H-trapping dislocations within the passivating grain layer decelerates H migration by up to about an order of magnitude, delaying H-induced crack initiation at grain boundaries. More importantly, once unavoidable micro-sized H-induced intergranular cracks emerge in the topmost surface region, they become completely arrested by the laminated grains, due to a transition in the embrittlement mechanism from H-enhanced grain boundary decohesion to highly energy-dissipative dislocation-associated cracking. These effects almost completely eliminate H embrittlement, at even doubled yield strength, when exposing the so architected material to harsh H attack. Our approach leverages surface mechanical treatments to tailor metallic microstructures in surface regions most susceptible to H attack, providing a scalable solution to protect alloys from H-induced damage.