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Polycrystalline materials are composites of crystalline particles or "grains" separated by thin "amorphous" grain boundaries (GBs). Although GB5 have been exhaustively investigated at low temperatures, at which these regions are relatively ordered, much less is known about them at higher temperatures, where they exhibit significant mobility and structural disorder and characterization methods are limited. The time and spatial scales accessible to molecular dynamics (MD) simulation are appropriate for investigating the dynamical and structural properties of GBs at elevated temperatures, and we exploit MD to explore basic aspects of GB dynamics as a function of temperature. It has long been hypothesized that GB5 have features in common with glass-forming liquids based on the processing characteristics of polycrystalline materials. We find remarkable support for this suggestion, as evidenced by string-like collective atomic motion and transient caging of atomic motion, and a non-Arrhenius GB mobility describing the average rate of large-scale GB displacement. [ABSTRACT FROM AUTHOR]

Abstract: We generalize a previous study of the atomic motions governing grain boundary migration to consider arbitrary misorientations of [001] tilt boundaries. Our examination of the nature of atomic motions employed three statistical measures of atomic motion: the non-Gaussian parameter, the “dynamic entropy” and the van Hove correlation function. These metrics were previously shown to provide a useful characterization of atomic motions both in glass-forming liquids and strained polycrystalline materials. As before, we find highly cooperative, string-like motion of atoms, but the grain boundary migration itself is a longer timescale process in which atoms move across the grain boundary. These observations are consistent with our previous results for Σ5 [001] tilt boundaries. It is evident from our work that the grain boundary structure and misorientation have a significant influence on the rate of grain boundary migration. [Copyright &y& Elsevier]

Abstract: The atomistic mechanism by which grain boundaries migrate is important for understanding the relationship between their structures and dynamical properties. We perform a series of three-dimensional molecular dynamic simulations of the migration of Σ5 tilt grain boundaries of different inclinations in nickel. We identify the migration mechanism through frequent quenches and analysis of the atomic displacements, local and global excess volume, and stress. The migration mechanism has the following components: local volume fluctuations precede the displacements of 3–4 linear atomic clusters in the direction parallel to the tilt axis which, in turn, are followed by individual atomic hops that are primarily perpendicular to the boundary plane. Excess volume is key to both the volume fluctuations and the atomic hops perpendicular to the boundary plane. The linear, or string-like, atomic motion parallel to the tilt axis also gives rise to a strong anisotropy in the grain boundary self-diffusivity. [Copyright &y& Elsevier]