Semi-brittle Transient Creep and Twinning-induced Plasticity in Carrara Marble

Abstract:

Observations of micro-structures in carbonate rocks deformed in the laboratory at temperatures, T < 400 °C, indicate that strain occurs by a combination of mechanical twinning, dislocation motion, and dilatant fracturing. To better constrain the systematics of semi-brittle flow, we performed a set of about 70 experiments at eight different temperatures (20 < T < 800 °C) on Carrara marble using a Paterson deformation apparatus. At each T, experiments were done at different confining pressures (50 < P_C < 300 MPa) and strain rates (10^-6 < ε’ <10^-4 s^-1), up to inelastic strains of ε = 0.12. When P_C > 100 MPa, at all T < 650°C and all ε’, transient strain hardening occurs. However, the functional relationships of resulting differential stress σ(T, P_C, ε’) and strain hardening h(T, P_C, ε’) = ∂σ/∂ε differ from each other and vary as conditions are changed. For example, at 23°C, when P_C < 100MPa, the samples weaken with ε. For all P_C at this T, strength at a strain of 5%, (σ|_{ε=5%, fixed ε’}) increases nonlinearly with increasing P_C; at P_C =300 MPa, σ|_ε=5% increases with ε’. However, at 23°C, h|_{ε=5%, fixed ε’} also increases with increasing P_C, but h|_{ε=5%, Pc fixed} decreases with increasing ε’. In contrast, at 600°C, the dependence of σ on P_C is very weak, and h virtually depends on ε’ alone. In the intermediate range 200 < T < 400 °C, the relative variations of σ(P_C, ε’) and h(P_C, ε’) are smaller than the two end conditions. At all T < 650°C (less than half the absolute melting point of calcite), when P_C is greater than 50 MPa, h coefficients are substantial (>1% of the shear modulus). In this way the deformation is similar to steels and hexagonal metals that deform in a regime called twinning induced plasticity (TWIP). During TWIP, deformation proceeds by a combination of “easy” mechanical twinning and dislocation glide on several slip systems whose glide planes are at high angles to the twin plane. In the examined calcite rocks, depending on conditions, the hardening resulting from twinning may be reduced either by dilation owing to brittle processes (at low pressures and temperatures), or by recovery and recrystallization (at higher temperatures or slower strain rates). Both microstructural observations and mechanical deformation data are consistent with this interpretation. We conclude that understanding the nature of the mechanisms that accommodate discontinuous inelastic strain at the termination of twins at grain boundaries is important for interpreting semi-brittle deformation in marble.