HETEROGENEOUS DIFFUSION EFFECTS IN POLYCRYSTALLINE MICROSTRUCTURES

Diffusion in polycrystalline microstructures is influenced by two fact ors that are not considered in a simple continuum material description . These are kinetic effects, where the diffusivities of grain boundari es differ from those of the grains, and free-energy effects, where the driving force for diffusion is dependent on a free energy function th at includes the phase of the diffusion medium as well as its compositi on. Simulation studies of kinetic diffusion effects in two-dimensional polycrystalline microstructures obtained from the Potts model have sh own that the compositional gradients resulting from these effects are transitory in nature, and that average diffusivities are dependent on microstructural features such grain size, diffusion bottlenecks in gra in boundary paths, and model lattice artifacts. Similar studies of fre e-energy effects using the phase-field model show that metastable grai n boundary segregation of components occurs, and that the rate and nat ure of grain growth can be influenced by an imposed composition gradie nt. Collectively, these studies show that the two simulation technique s, finite-difference solutions of fast grain boundary diffusion in the polycrystalline microstructures obtained from the Potts-model and pha se-field model solutions of diffusion in evolved and evolving microstr uctures with spatially dependent chemical potentials, are complementar y in studying diffusional effects in polycrystalline microstructures. (C) 1997 Elsevier Science S.A.