Effect of High-Temperature Structure and Diffusion on Grain-Boundary Diffusion Creep in fcc Metals |
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Authors: | P Keblinski V Yamakov |
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Institution: | (1) Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180, USA;(2) Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA |
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Abstract: | Molecular dynamics simulations of high-energy twist and tilt bicrystals of fcc palladium reveal a universal, liquid-like, isotropic high-temperature diffusion mechanism, characterized by a rather low self-diffusion activation energy that is independent of the boundary type or misorientation. Medium-energy grain boundaries exhibit the same behavior at the highest temperatures; however, at lower temperatures the diffusion mechanism becomes anisotropic, with a higher, misorientation-dependent activation energy. Our simulations demonstrate that the lower activation energy at elevated temperatures is caused by a structural transition, from a solid boundary structure at low temperatures to a liquid-like structure at high temperatures. We demonstrate that the existence of such a transition has important consequences for diffusion creep in nanocrystalline fcc metals. In particular, our simulations reveal that in the absence of grain growth, nanocrystalline microstructures containing only high-energy grain boundaries exhibit steady-state diffusion creep with a creep rate that agrees quantitatively with that given by the Coble-creep formula. Remarkably, the activation energy for the high-temperature creep rate is the same as that characterizing the universal high-temperature diffusion in high-energy energy bicrystalline grain boundaries. |
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Keywords: | grain boundary diffusion diffusion creep plasticity molecular-dynamics simulation |
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