Hydrogen-grain boundary interaction in Fe, Fe-C, and Fe-N systems

Highly accurate predictions of hydrogen's influence on material strength and development of materials with minimal hydrogen effects are essential to prevent failure under various hydrogen environments. Here, we investigated the influence of misorientation angle and solute elements (carbon and nitrogen) on the cohesive energy of <110> symmetrical tilt grain boundaries (STGBs) in bcc Fe under a gaseous hydrogen environment by using density functional theory. We found a good correlation among GB energy, GB free volume, and the hydrogen concentration at GBs under hydrogen environments: high-energy GBs have large gaps, and many hydrogen atoms are captured at these spaces. Thus, higher-energy GBs are more influenced by hydrogen. It is also shown that the binding energy between hydrogen and a GB is negligible when nitrogen or carbon atoms exist at the GB at their solubility limit. Therefore, carbon and nitrogen atoms exclude hydrogen atoms from GBs and improve the cohesive energy of GBs under hydrogen environments. (author)