Evolution of mechanical state in Carrara marble during deformation at 400° to 700°C

Abstract

The evolution of the mechanical state of Carrara marble during deformation up to strains of 0.2 at 400° to 700°C and strain rates of 10−4 to 10−6 s−1 has been investigated by applying Hart's [1976] state variable description of inelastic deformation to the results of isothermal, constant displacement rate experiments performed at these conditions. At T⪕600°C the magnitude of the mechanical state increases over the full range of strains investigated even when the deformation occurs at almost constant stress and strain rate, although the rate of increase at given strain decreases with increasing temperature and decreasing strain rate. The deformation microstructures suggest that under these conditions mechanical twinning controls the deformation behavior, with twin boundary migration becoming significant at T⪖450°C. As a dynamic recovery process, twin boundary migration is as efficient as strain‐induced grain boundary migration. At T>600°C the magnitude of the mechanical states attained is difficult to evaluate reliably using the method employed, but the continuing evolution of the deformation microstructures suggests that a steady state is not attained by strains of 0.2. At these conditions, mechanical twinning is replaced by other intracrystalline slip processes and by grain boundary bulge nucleated dynamic recrystallization. Hart's description of inelastic deformation, as previously evaluated for Carrara marble at T⪕400°C, may be extrapolated to 600°C provided the mechanical state evolution equation is modified to accommodate twin boundary migration and the influence of annealing‐type recovery processes. Extrapolation to higher temperatures may require modifications to the equation of state in that description. Although at the conditions investigated steady state appears to be a poor approximation, steady state flow laws of exponential and power law type do provide a good empirical description of the data. However, the observation that steady state is not attained indicates that any microphysical interpretation of the magnitude of the material parameters in such steady state flow law fits should be treated with caution.