Strength characteristics of the r, f, and c slip systems in calcite

Numerical modelling of crystallographic preferred orientation or texture development in plastically deformed materials requires specification of all potential slip and twinning systems and their relative strengths or critical resolved shear stresses (CRRS). It is widely believed that the most important intracrystalline deformation systems in calcite are twinning on <e{(1)over bar 018}<< 40(4)over bar 1 >(+)>>, slip on <r{10(1)over bar 4}<<(2)over bar 021 >(+/-)>> and slip on <f{(1)over bar 012}<< 2(2)over bar 01 >(+/-)> the superscripted signs indicating slip sense. It is these systems which have been used in modelling texture development in calcite polycrystals to date. However, modelling has been limited by insufficient and partly conflicting data on the (relative) strengths of the various slip and twinning systems in calcite. Deformation experiments on calcite single crystals, performed by us in recent years, taken together provide new data for a more complete and consistent quantification of the strength properties of the calcite glide systems. In our experiments, single crystals of calcite were compressed in both the <[40(4)over bar 1]> and <[40(4)over bar 1]> orientations, at temperatures and constant strain rates in the range 300-800 degrees C and 3 x 10(-4)-3 x 10(-8) s(-1), respectively, The stress-strain curves obtained mostly show two yield points. Slip line analysis shows that these yield points are related to the onset of macroscopically observable slip, first on an <r <(2)over bar 021 >(+/-)> system and then on an <f < 10(1)over bar 1 >(+/-)> or c < a > system. The latter two systems dominate at higher temperature (> 600 degrees C at strain rate 3 x 10(-5) s(-1)) and have not been taken into account in any texture modelling so far. With increasing absolute temperature (T-abs), calculated CRSS values (tau(C)) for the r, f and c systems decrease in a manner which can be described by empirical power law functions of the type tau(C) = A(T-abs)(b), where b < 0. No evidence has been found for a difference in strength between positive and negative slip on the <r <(2)over bar 021 >> and <f < 10(1)over bar 1 >> systems. The yield stresses for <r <(2)over bar 021 >(+)> and <f < 10(1)over bar 1 >(+)> slip were found to be rather insensitive to strain rate, with conventional power law stress exponents > 12. Comparing the present results with previous data, we conclude that two regimes of slip system activity exist, namely a low-temperature regime involving e twinning, slip on <r <(2)over bar 021 >(+/-)> and on <f < 2(2)over bar 01 >(-)> (positive direction not reported), and a high-temperature regime with <r <(2)over bar 021 >(+/-)>, <f < 10(1)over bar 1 >(+/-)> and c < a > slip. This slip system transition occurs at T similar to 400 degrees C at strain rate 10(-4)-10(-5) s(-1), and is expected to be associated with one or more texture transitions. The results point to a clear need for renewed texture modelling of calcite polycrystals.