Effect of pre-existing crystallographic preferred orientation on the rheology of Carrara marble

Abstract

Localized deformation during plastic flow generally reflects mechanical weakening which is believed to be caused by grain size reduction and, in some cases, the development of a crystallographic preferred orientation (CPO). This study aims to investigate experimentally the contribution of CPO development to the drop in flow strength seen in Carrara marble samples during large strain torsion experiments at temperatures in the range 600 to 800oC and strain rates of 3x10-4s-1 to 1x10-3s-1 at constant confining pressure (300 MPa). The starting material shows little or no CPO. Samples were (1) deformed to achieve a well-developed CPO, (2) annealed at 727°C for five hours in order to allow the grains to grow to approximately their original size and shape while maintaining a reasonably strong CPO, and finally (3) deformed again, under the same conditions as step (1). At constant experimental conditions, peak and large-strain flow stresses measured for annealed samples with a pre-existing CPO were always found to be lower than those of the undeformed samples deformed without a pre-existing CPO. The contribution of CPO to the total reduction of the flow strength in Carrara marble because of the pre-existing CPO is a function of temperature and varies from 33% at 727oC to 67% at 800oC. The different contributions at different temperatures are interpreted to be due to the following temperature-dependent processes: (a) activation of different slip systems, followed by development of CPO of a variable type and strength after step (1), (b) differing extents of dynamic recrystallization, producing different grain size distributions, (c) varying amounts of annealing after step (2), resulting in a microstructure with some parts retained from the first deformation phase. In contrast to the effect of temperature, however, strain rate did not seem to have any effect on the total weakening, nor on the contribution of CPO to this weakening. These results implicate that the development of a CPO is an important, or in some cases even the most important, process that contributes to strain weakening in calcite under the investigated conditions.