THOMAS YOUNG CENTRE
THE LONDON CENTRE FOR THE THEORY AND SIMULATION OF MATERIALS
Professor Hajime Kimizuka
Department of Mechanical Science and Bioengineering, Osaka University
Understanding the H diffusion and transport in Fe and Fe alloys is extremely important because H severely degrades ductility and causes H embrittlement, which has been a serious materials problem. In this study, we investigate the H diffusivity and the H trapping effect around the various crystalline defects, including the vacancy, dislocation, and grain boundary in bcc-Fe, by employing the path-integral molecular dynamics (PIMD) modeling and adopting an ab-initio-based empirical potential; in our calculations, we took quantum effects into consideration at finite temperatures.
To evaluate both quantum and thermal effects on the lattice diffusivity of H, time evolutions of mean-square displacements of H atoms in the bulk Fe are sampled at temperatures of 100-1000 K using path-integral centroid molecular dynamics (CMD) method. The obtained diffusion coefficients and activation energies are in excellent agreement with experimental measurements over a wide temperature range. The H diffusion process in Fe is found to be accelerated even at ambient temperatures by taking into account of quantum effects due to the significant reduction of the apparent activation energy for H migration. Also, the PIMD-based free-energy profiles for H diffusion around a vacancy, dislocation and grain boundary are evaluated to characterize the H trapping effect around such crystalline defects. We confirm that the H atom is strongly trapped in the vicinity of the lattice defects; moreover, the quantum nature of H is found to enhance the H trapping effect in the vicinity of the defects as temperature decreases. Our results indicate that the H diffusivity both across and along the linear and planar defects is significantly low as compared to lattice diffusion. Thus the so-called "fast" dislocation-pipe or grain-boundary diffusion does not occur for H in bcc-Fe.
 H. Kimizuka, H. Mori, and S. Ogata, Phys. Rev. B 83, 094110 (2011).
 H. Kimizuka and S. Ogata, Phys. Rev. B 84, 024116 (2011).
 T. Yoshikawa, T. Takayanagi, H. Kimizuka, and M. Shiga, J. Phys. Chem. C 116, 23113 (2012).