First principles DFT analysis on the diffusion kinetics of hydrogen isotopes through bcc iron (Fe): Role of temperature and surface coverage

The diffusion of interstitial H, D, and T in Fe metal at different temperature are evaluated using ab-initio density functional theory and transition state theory. The thermal expansion coefficient, Helmholtz free energy of activation, jump factor of diffusion was determined by use of activation energy and phonon calculations. The calculated diffusion coefficient was well described by a constant activation energy, (D = D0exp (-Ea)/kT)) with Ea = 0.016, 0.041, and 0.050 eV and D0 = 1.042 × 10−7, 0.736 × 10−7 and 0.572x10−7 m2.s−1for H, D, and T, respectively using harmonic transition state theory (hTST) with temperature correction. The calculated permeability and solubility also followed the Arrhenius relation.The earlier experimentally reported higher diffusivity of H atom at lower temperatures than that at higher temperature (200–600 K) was well explained by Wigner + hTST model with temperature correction and semi-classical transition state theory (SC-TST). The present computed results followed the similar trend of experimental findings. Further, the ideal fracture energy was evaluated using the Born-Haber thermodynamic cycle for varied coverage of H, D and T to account for decohesion-based embrittlement. The lighter H atom was seen to cause more decohesion-based embrittlement compared to D and T due to reduced cohesion. Additionally, the effect of temperature on ideal fracture energy was evaluated for H, D and T in Fe. The decohesion-based embrittlement was increased with increase in the temperature for H, D and T up to certain temperature (for H lower than 500 K, for D around 500 K and for T above 500 K) but with further increase in the temperature, the ideal fracture energy was reduced due to decrease in the adsorption energy and increase in the solution enthalpy of H isotopes. Further, the path and kinetics of dissociation and reassociation of H2 molecule were established on Fe (100) surface at 0.5 and 2.0 ML coverage. At lower concentration (0.5 ML), dissociation of H2 molecule on Fe (100) surface was favored, whereas at high concentration (2.0 ML), reassociation of H2 molecule was favored.