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Crystallographic preferred orientations of ice deformed in direct-shear experiments at low temperatures

This paper is available in a repository.
This paper is available in a repository.

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Abstract

We sheared synthetic polycrystalline ice at temperatures of −5, −20 and −30 °C, to different shear strains, up to γ = 2.6 (equivalent strain of 1.5). Cryo-electron backscatter diffraction (EBSD) shows that basal intra-crystalline slip planes become preferentially oriented parallel to the shear plane, in all experiments. This is visualized as a primary cluster of crystal c-axes (the c-axis is perpendicular to the basal plane) perpendicular to the shear plane. In all except the two highest-strain experiments at −30 °C, a secondary cluster of c-axes is observed, at an angle to the primary cluster. With increasing strain, the primary c-axis cluster strengthens. With increasing temperature, both clusters strengthen. In the −5 °C experiments, the angle between the two clusters reduces with increasing strain. The c-axis clusters are elongated perpendicular to the shear direction. This elongation increases with increasing shear strain and with decreasing temperature. Highly curved grain boundaries are more prevalent in samples sheared at higher temperatures. At each temperature, the proportion of and irregularity of curved boundaries decreases with increasing shear strain. Subgrains are observed in all samples. Recrystallized grains and subgrains are similar in size and are both smaller than the original grains. Microstructural interpretations and comparisons of the data from experimentally sheared samples with numerical models suggest that the observed crystallographic orientation patterns result from a balance of the rates of lattice rotation (during dislocation creep) and growth of grains by strain-induced grain boundary migration (GBM). GBM is faster at higher temperatures and becomes less important as shear strain increases. These observations and interpretations provide a hypothesis to be tested in further experiments and using numerical models, with the ultimate goal of aiding the interpretation of crystallographic preferred orientations in naturally deformed ice.

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