Microstructural Evolution of Carrara Marble During Semi-Brittle Deformation

Fifteen marble samples were subjected to semi-brittle deformation through triaxial compression experiments, reaching axial strains of about 0.5%, 1.0%, 2.0%, 4.0%, or 7.5% at temperatures of 20°C, 200°C, or 350°C, under a confining pressure of 400 MPa. Deformation twins, lattice curvature, and intragranular microfractures in the samples were quantitatively characterized using forescattered electron images and electron backscatter diffraction. Microstructural analyses revealed that twins accommodate most of the shortening during the first 2% strain, whereas lattice curvature associated with geometrically necessary dislocations predominantly develops with more strain. Intragranular fracture intensity exhibits an almost linear correlation with strain during the first 2% strain but increases more gradually with strain thereafter. The mechanical data indicate a strong decrease of yield stress with temperature increasing between 20°C and 200°C, consistent with the temperature dependence of the critical resolved shear stress for dislocation glide. The post-yield strain hardening is likely caused by progressively increasing intensity of interactions among dislocations and between dislocations and twin boundaries. Based on the microstructural data and interpreted hardening mechanisms, we propose a phenomenological model, with state variables representing microstructural elements that hinder dislocation glide, as a step toward development of a microphysical constitutive model of semi-brittle deformation.