10 Apr 2024
 | 10 Apr 2024
Status: this preprint is open for discussion.

Evolution of crystallographic preferred orientations of ice sheared to high strains by equal-channel angular pressing

Qinyu Wang, Sheng Fan, Daniel H. Richards, Rachel Worthington, David J. Prior, and Chao Qi

Abstract. Plastic deformation of polycrystalline ice 1 h induces crystallographic preferred orientations (CPOs), which give rise to anisotropy in the viscosity of ice, thereby exerting a strong influence on the flow of glaciers and ice sheets. The development of CPOs is governed by two pivotal mechanisms: recrystallization dominated by subgrain/lattice rotation and by strain-induced grain boundary migration (GBM). To examine the impact of strain on the transition of the dominant mechanism, synthetic ice (doped with ∼1 vol.% graphite) was deformed using equal-channel angular pressing technique, enabling multiple passes to accumulate substantial shear strains. Nominal shear strains up to 6.2, equivalent to a nominal von Mises strain of ε′ ≈ 3.6, were achieved in samples at a temperature of −5 °C. Cryo-electron backscatter diffraction analysis reveals a primary cluster of crystal c axes perpendicular to the shear plane in all samples, accompanied by a secondary cluster of c axes at an oblique angle to the primary cluster antithetic to the shear direction. With increasing strain, the primary c-axis cluster strengthens, while the secondary cluster weakens. The angle between the clusters remains within the range of 45° to 60°. The c-axis clusters are elongated perpendicular to the shear direction, with this elongation intensifying as strain increases. Subsequent annealing of the highest-strain sample reveals the same CPO patterns as observed prior to annealing, albeit slightly weaker. A synthesis of various experimental data suggest that the CPO pattern, including the orientation of the secondary cluster, results from a balance of two competing mechanisms: lattice rotation due to dislocation slip, which fortifies the primary cluster while rotating and weakening the secondary one, and grain growth by strain-induced GBM, which reinforces both clusters while rotating the secondary cluster in the opposite direction. As strain increases, GBM contributes progressively less. This investigation supports the previous hypothesis that a single cluster of c axes could be generated in high-strain experiments, while further refining our comprehension of CPO development in ice.

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Qinyu Wang, Sheng Fan, Daniel H. Richards, Rachel Worthington, David J. Prior, and Chao Qi

Status: open (until 26 Jun 2024)

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Qinyu Wang, Sheng Fan, Daniel H. Richards, Rachel Worthington, David J. Prior, and Chao Qi
Qinyu Wang, Sheng Fan, Daniel H. Richards, Rachel Worthington, David J. Prior, and Chao Qi


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Short summary
To examine if the single cluster fabric in natural ice is formed due to high strains, we deformed synthetic ice to large strains using a unique technique. A shear strain of 6.2 was achieved in laboratory. We explored how the two mechanisms, which control microstructure and fabric evolution, evolve with strain, and established a fabric development model. These results will help understanding the fabrics in natural ice and further comprehending glacier and ice sheet flow dynamics.