High Temperature Structure and Properties of Grain Boundaries -- Insights Obtained from Atomic Level Simulations |
| P. Keblinski Materials Science and Engineering Department Rensselaer Polytechnic Institute, Troy, NY, USA |
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| Molecular-dynamics simulations of grain boundaries in Si and fcc metals reveal that high-energy boundaries are disordered, even at low temperatures, with their local atomic structure very similar to that of bulk amorphous material. By contrast, low energy grain boundaries are crystalline all the way up to the melting point. Upon heating intergranular "confined amorphous" structures of high-energy grain boundaries exhibit reversible transition into universal, highly confined, liquid-like structure. High-temperature properties, such as the grain boundary diffusion therefore involve liquid-like mechanisms, with activation energies related to the diffusion process in the melt. By contrast to Si and fcc metals, high-energy diamond grain boundaries are more ordered structurally, but contain coordination disorder resulting from ability of carbon to change its hybridization from sp3 to sp2 character. Based on the insights obtained from our simulation of individual grain boundaries we were able to design nanocrystalline 3D microstructures, which allow to study grain boundary diffusion creep process by molecular-dynamics simulations. In order to prevent grain growth and thus to enable steady-state diffusion creep to be observed on a time scale accessible to molecular-dynamics simulations (of typically 10-9s, our input microstructures were tailored to (i) have a uniform grain shape and a uniform grain size of nm dimensions and (ii) contain only high-energy grain boundaries that, consistently with our studies of individual, bicrystalline grain boundaries, exhibit rather fast, liquid-like self-diffusion at high temperatures. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state diffusion creep that is homogeneous with a strain rate that agrees quantitatively with that given by the Coble-creep formula. |
| DOI: 10.12693/APhysPolA.102.123 PACS numbers: 61.43.Dq, 61.72.Mm |