Abstract
Chemically induced grain boundary migration (CIGM) was observed in situ in calcite bicrystals in an optical microscope heat-stage. Three high-angle and two low-angle boundaries were observed. The solute was SrCO3. Migration rates and solute concentrations were compared with boundary orientations. The coherency strain driving force was generalized for plane stress in any crystal system and applied to calcite. Because of the high anisotropy of lattice parameter expansion with Sr addition, a strong dependence of Sr concentration and migration driving force on crystallographic orientation was predicted. However, no correlation between SrCO3 concentration and boundary orientation was observed, and little evidence for driving force dependence on orientation was found. Some evidence for a correlation between solute concentration and migration rate was found, but the nature of the correlation changed between bicrystals. Migration rates always decreased with time. Near-surface solute concentrations were depressed beneath values expected for stress-free equilibrium and plane stress modified equilibrium, but without obvious orientation dependence, which suggested that the coherency stress had a hydrostatic component. A PV driving force for migration might be present if solute diffusion fields around grain boundaries caused pressure gradients around the boundaries.
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