Texture of Solnhofen limestone deformed to high strains in torsion

Solnhofen limestone was deformed in torsion to shear strains (gamma) ranging from 1 to 12, at a temperature of 750 degrees C, 300 MPa confining pressure and a maximum strain-rate of 10(-3) s(-1). These deformation conditions correspond to the intracrystalline power law dislocation creep field close to the boundary to the grain-size-sensitive superplastic creep field. The grain-shape microstructure was observed using orientation contrast by backscattered electrons in the SEM. The grains remain sub-equant with an average grain size of around 4 mu m, even to the highest strains. Lattice preferred orientation was determined using both X ray texture goniometry and automated electron back scatter diffraction. The c-axis preferred orientation develops from two main maxima with a weak sub-maximum, through two maxima to a single maximum perpendicular to the shear plane. The rate of increase of the intensity of the single maximum with increasing strain diminishes, and it appears that there is a tendency to a steady-state texture. The final single c-axis maximum is displaced slightly towards the shortening direction of the applied simple shear. The a-axes tend to a girdle perpendicular to the e-axis maximum. It is proposed that the partitioning of deformation between intra-and inter-crystalline mechanisms results in a pulsating strain state in the grains, contributing to the maintenance of sub-equant grains. It is argued that the lattices of constituent grains rotate continuously with no stable end orientation and that this can lead to a steady-state texture. The experimental preferred orientation compares well with that of natural calcite mylonites in the position of the c-axis maximum and the a-axis girdle. (C) 1998 Elsevier Science Ltd.