Roles of lattice and grain boundary on hydrogen diffusion and trap behaviors in single-and poly-crystalline CrCoNi medium-entropy alloy

In this study, single-crystalline and poly-crystalline CrCoNi alloys are utilized as model systems to analyze the distinct roles of each GB and interstitial lattice sites. To effectively reveal hydrogen behavior, both electrochemical and gaseous hydrogen pre-charging methods are applied. Hydrogen content, diffusivity, and trap behaviors are quantified using thermal desorption analysis and hydrogen permeation tests, which determines (1) changes in hydrogen behavior depending on the presence of GB and (2) alterations in hydrogen behavior depending on lattice crystallographic orientation. The results indicate that GB and interstitial lattice sites exhibit comparable binding energies for hydrogen trapping. However, the introduction of GB alters the primary trapping sites from interstitial lattice sites to GB. In this case, the hydrogen content in the poly-crystalline alloy is determined by the trap site density of the primary trapping site. On the other hand, in the single-crystalline alloy, where only interstitial lattice sites exist, the crystallographic orientation of the hydrogen-charged plane is an important variable that determines hydrogen content and hydrogen diffusivity. Such insights contribute to a deeper understanding of hydrogen behavior within a more intricate microstructure, suggesting the alloy design approach to enhance resistance to HE.