First-Principles Investigation of Mechanical and Interfacial Properties of Fe–Al Intermetallic Compounds

Fe–Al intermetallic compounds are promising candidates for hydrogen permeation barrier coatings owing to their excellent oxidation stability and inherent resistance to hydrogen embrittlement. However, the mechanical properties and interfacial behavior of different Fe–Al phases, particularly at Fe/Fe–Al interfaces, remain insufficiently understood, limiting their reliable application in hydrogen-containing environments. In this work, density functional theory (DFT) calculations were employed to systematically evaluate the bulk mechanical moduli, surface energetics, and interfacial adhesion of FeAl, Fe3Al, and Fe2Al5. The results reveal that FeAl exhibits the highest elastic and shear moduli due to its B2-ordered structure and directional bonding, while Fe2Al5 shows pronounced anisotropy and the lowest strength as a consequence of its low-symmetry structure. Surface energy analysis indicates that Fe2Al5 possesses relatively stable facets, whereas interfacial adhesion calculations demonstrate that FeAl/Fe and Fe3Al/Fe interfaces provide significantly stronger bonding compared to Fe2Al5/Fe. Charge density and electron localization function (ELF) analyses confirm that Fe–Fe bonds are dominated by metallic character with delocalized electrons, whereas Al-rich regions display enhanced localization, leading to weaker interfacial adhesion in Fe2Al5/Fe. These findings clarify the fundamental mechanisms governing Fe–Al mechanical and interfacial properties and provide theoretical guidance for the design of robust Fe–Al-based hydrogen barrier coatings.