Texture development of calcite by deformation and dynamic recrystallization at 1000 K during torsion experiments of marble to large strains

Torsion deformation experiments were performed on solid cylinders of Carrar a marble at high temperature (1000 K) and constant twist rate (about 4 x 10 (-4) rad s(-1)) to large twist angles (between 80 degrees and 840 degrees). These conditions correspond to simple shear deformation at constant shear strain rate (3 X 10(-4) s (-1)) and variable amounts of shear strain gamma of 1, 2, 5 and 11 at the outer mantle of the sample cylinders. Lattice pref erred orientations (LPO) were measured on polished thin sections using auto mated electron backscatter diffraction (EBSD) to analyze the microtextures in orientation imaging micrographs (OIM), Shear deformation produced first a microstructure of elongated grains with shape and orientation close to the finite strain ellipse of the imposed she ar strain (gamma = 1 and 2), A deformation texture developed with monoclini c sample symmetry and an oblique c-axis distribution relative to the shear plane. With increasing shear, the marble recrystallized by subgrain rotatio n and nucleation of small grains, starting along grain boundaries, which ul timately replaced the whole deformation microstructure. A steady-state recr ystallization microstructure evolved with nearly equant grains of about 10 mum in size (gamma = 5 and 11), The LPO changed completely from the monocli nic deformation texture into a distribution with a sharp single orientation component with r{10(1) over bar 4} parallel to the shear plane and a[(1) o ver bar2 (1) over bar0] parallel to the shear direction, This recrystalliza tion texture: has orthorhombic symmetry with respect to shear plane and she ar direction. During the transition, the applied Row stress decreased. Simple shear deformation of calcite was modeled with the self-consistent po lycrystal plasticity theory. It included a model for dynamic recrystallizat ion based on a balance between growth and nucleation. Model results agree w ith experimental data if slip on r{10(1) over bar 4 }a[(1) over bar2 (1) ov er bar0] is introduced as a potential slip system for calcite. In that case , the steady-state recrystallization texture has an "easy slip" preferred o rientation. The hypothesis remains speculative until this slip system is ac tually confirmed by direct observation. (C) 2001 Elsevier Science B.V. All rights reserved.