Nanocrystalline materials have a proportion of atoms at grain boundaries that can be as high as 50% and are thus expected to have properties quite different from those of bulk or large-grained polycrystalline materials. In this article, we study the influence of the presence of grain boundaries on thermal expansion, mean-square amplitudes of vibration (MSAV’s), and hydrogen diffusion using classical molecular-dynamics simulations with embedded-atom-method potentials, for the particular case of the special [100] $\Sigma =5,$ 13, and 17 twist grain boundaries. We find that in the presence of grain boundaries, thermal expansion increases only slightly while both hydrogen diffusion and MSAV’s are enhanced significantly, in accord with neutron-diffraction measurements. The presence of pores, or voids, likewise, seem to have very little effect on thermal expansion. The diffusion of hydrogen is found to proceed mainly in the plane of the grain boundaries, where the electron density is lowest.

• Received 21 April 1999

DOI:https://doi.org/10.1103/PhysRevB.60.10107