First principles assessment of ideal fracture energies of materials with mobile impurities: implications for hydrogen embrittlement of metals

We propose that the ideal fracture energy of a material with mobile bulk impurities can be obtained within the framework of a Born-Haber thermodynamic cycle. We show that such a definition has the advantage of initial and final states at equilibrium, connected by well-defined and measurable energetic quantities, which can also be calculated from first principles. Using this approach, we calculate the ideal fracture energy of metals (Fe and Al) in the presence of varying amounts of hydrogen, using periodic density functional theory. We find that the metal ideal fracture energy decreases almost linearly with increasing hydrogen coverage, dropping by ∼45% at one-half monolayer of hydrogen, indicating a substantial reduction of metal crystal cohesion in the presence of hydrogen atoms and providing some insight into the cohesion-reduction mechanism of hydrogen embrittlement in metals.