Optimizing the hydrogen embrittlement resistance by tuning the structures of Cu-rich nanoprecipitates in high strength martensite stainless steels

Hydrogen is notoriously known to embrittle high-strength steels, critically endangering the materials service safety. Cu-rich nanoprecipitates can mitigate the hydrogen embrittlement (HE) presumably by imposing hydrogen traps; in the meantime, they may also contribute to strengthening and hence effectively tackle the trade-off between strength and HE resistance. However, as the Cu-rich precipitates grow, they go through sequential transformations, raising a critical question as to exactly which phase of the Cu-rich precipitates prompts the best counter-HE effects. Here, by tuning the structures of the Cu-rich precipitates in a tempered martensitic steel, we find that the HE susceptibility of the steel can be significantly reduced by up to 55.3%, while maintaining roughly the same ultimate tensile strength of the steels. Further, by using high-resolution transmission electron microscopy and hydrogen permeation tests, it is found that, while all Cu-rich precipitates may trap hydrogen and impede hydrogen diffusion, the 9R-structured Cu-rich precipitates (9R-Cu) have the strongest binding with hydrogen and highest trapping capacity. This is further corroborated by first-principles calculations which reveal the precipitates/matrix interface as favorable trapping sites. The findings provide deep insights for remedying the HE susceptibility of high-strength steels through Cu-rich precipitates.