Plastic flow of two‐phase marbles

To quantify the effect of rigid inclusions on the flow behavior of a creeping matrix, we fabricated a suite of two‐phase marbles by hot isostatically pressing mixtures of calcite and quartz powders and subsequently deformed these synthetic rocks at 300 MPa confining pressure, up to 100 MPa pore pressure, temperatures ranging from 873 to 1073 K, strain rates and stresses ranging from ∼10 −7 s −1 to ∼10 −3 s −1 and ∼4 MPa to 190 MPa, respectively. In constant displacement rate tests performed to axial strains of up to about 30%, we observed deformation at approximately constant stress for samples with 20% or less quartz. Even small additions of second phase significantly strengthen the aggregate, although the magnitude of the strengthening varies significantly with temperature and imposed strain rate. Investigation of the microstructure of starting samples reveals that initial porosity directly varies with quartz content and that matrix grain size inversely varies with the amount of second phase. Strength decreases modestly with increasing porosity, and consequently, strength increases as the samples compact. The effect of matrix grain size on the aggregate strength depends on the deformation conditions. The majority of tests exhibit an inverse relation between strength and matrix grain size (i.e., similar to a Hall‐Petch relation), but fine‐grained samples deformed at the highest temperatures exhibit decreasing strength with decreasing grain size. The addition of the quartz particles seems to result in both structural strengthening owing to decreased grain size and to transfer of load from the flowing matrix to the rigid quartz particles.