Dieter Wolf
Materials Science Division, Argonne National Laboratory
Argonne, IL 60439, USA
A novel multiscale approach for the simulation of microstructural evolution in nanocrystalline materials incorporating all the relevant time and length scales of the problem, ranging from the atomic level to the continuum, is described, using grain growth as a simple case study. The molecular-dynamics simulations reveal the presence of two growth mechanisms, namely (a) the conventional curvature-driven migration of the grain boundaries and (b) grain rotations with subsequent grain coalescence. These insights can be captured quantitatively, in the form of a theory of diffusion-accommodated grain rotation. This enables mesoscale simulations in which the objects evolving in space and time are the grain boundaries and grain junctions rather than the atoms, thus allowing the growth topology and long-time growth kinetics to be determined for a system containing a very large number of grains of arbitrary size. However, whereas the motion of the atoms follows Newton’s law, the grain-boundary motion is viscous, involving a dissipative force law. This law of motion can be incorporated into a novel mesoscopic simulation approach based on the variational functional for the dissipated power. In each time step this functional can be minimized using a velocity Monte-Carlo algorithm. As an outlook, we describe how the effects of applied stress can be incorporated, by meshing the grain interiors such that the grain-interior nodes link up with the already discretized grain boundaries delimiting each grain.
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*Work supported by the U.S. Department of Energy, Basic Energy Sciences-Materials Sciences, under Contract W-3l-l09-Eng-38.
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