Strength evolution and the development of crystallographic preferred orientation during deformation of two-phase marbles

• We report mechanical tests of synthetic marbles containing glassy carbon particles.
• Triaxial compression strength increases with increasing carbon content.
• Samples with spherical particles, loaded in torsion, are weaker than all others.
• Preferred orientation produced in torsion varies with particle shape.
• Deformation mechanisms may vary with particle shape and concentration.

We conducted conventional triaxial compression and confined torsion tests on calcite rocks at 1023 K and equivalent strain rates between 10− 6 and 10− 3 s− 1, to investigate the influence of variations in volume fraction and geometry of a second phase on the evolution of strength and CPO. Nominally dense, two-phase marbles were made by hot-isostatic pressing of mixtures of calcite and amorphous carbon. The carbon grains were either spheres or splinters with known size distributions, were chemically inert, and are nearly rigid. Adding 1 vol.% spheres caused only small changes in strength or CPO. Strengths were consistent with the activation of slip on f101¯1, the hardest system in calcite. Adding 10 vol.% splinters increased the strength at low strains and strain rates but had smaller effects at high strains or rates. The CPO of rocks containing splinters were similar to those in pure samples, but with reduced intensity. When 10 vol.% spheres were added, the aggregate strength under confined torsion was less than that of the pure material. Distinct and intense CPOs developed. The reduced stresses were consistent with those necessary for slip on systems weaker than f  . Viscoplastic self-consistent calculations used to model the texture evolution required slip on f101¯1 and c1¯21¯0 to match the CPO of pure samples; results were consistent with the inclusion of slip on r1¯21¯0. In samples containing 10 vol.% spheres deformed in torsion, slip activity on the harder systems was decreased. The exact cause of the strength reduction in these experiments is not understood, but it is possible that adding smooth spheres enhanced grain-boundary sliding, thereby decreasing activity on f101¯1. Thus, shape and volume fraction of second phases may be important in determining CPO and strength; and it is important to consider the loading geometry.