THE PLASTIC-DEFORMATION OF POLYCRYSTALLINE DOLOMITE - COMPARISON OF EXPERIMENTAL RESULTS WITH THEORETICAL PREDICTIONS

The trigonal carbonate mineral dolomite is plastically highly anisotropic with some orientations much more favored for slip over others. The plastic behaviour for polycrystalline dolomite in compression is modelled using a model of single slip, the Taylor theory of homogeneous strain and the viscoplastic self-consistent theory of Molinari et al. The theoretical results are compared with the findings of optical and TEM microstructural analyses of 175 individual grains in polycrystals of Crevola dolomite that had been deformed in the temperature range 25-900-degrees-C. There is broad agreement between experiment and theory from some viewpoints (e.g. the expected mechanisms are operative and the stress axes of grains move in the senses which are predicted), but discrepancies also exist at both the macroscopic and microscopic level. Taking into account all grains that can be analyzed, the number of plastic deformation mechanisms observed averages to 1.9 per grain, which is below the value of 4.9 (for high temperature) and 4.3 (for low temperature) predicted from a viscoplastic Taylor model, counting those systems which contribute at least 10% to the total shear in each grain. It is closer to predictions from a viscoplastic self-consistent model with an average of 1.7 and 3.1 systems respectively. The heterogeneity which is caused by such a small number of slip systems, is partly expressed by brittle and grain boundary effects. In addition it is manifest as a sub-grainscale variability of slip activity and dislocation density that is at least equally important. The deformation mechanisms operative do not always agree with predictions based on Schmid factors which is mainly due to local interaction between grains and their neighbors.