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

Chao Qi, David J. Prior, Lisa Craw, Sheng Fan, Maria-Gema Llorens, Albert Griera, Marianne Negrini, Paul D. Bons, David L. Goldsby
2019 The Cryosphere  
<p><strong>Abstract.</strong> Synthetic polycrystalline ice was sheared at temperatures of <span class="inline-formula">−5</span>, <span class="inline-formula">−20</span> and <span class="inline-formula">−30</span><span class="thinspace"></span><span class="inline-formula"><sup>∘</sup></span>C, to different shear strains, up to <span class="inline-formula"><i>γ</i>=2.6</span>, equivalent to a maximum stretch of 2.94 (final line length is 2.94 times the original length). Cryo-electron
more » ... electron backscatter diffraction (EBSD) analysis shows that basal intracrystalline slip planes become preferentially oriented parallel to the shear plane in all experiments, with a primary cluster of crystal <span class="inline-formula"><i>c</i></span> axes (the <span class="inline-formula"><i>c</i></span> axis is perpendicular to the basal plane) perpendicular to the shear plane. In all except the two highest-strain experiments at <span class="inline-formula">−30</span><span class="thinspace"></span><span class="inline-formula"><sup>∘</sup></span>C, a secondary cluster of <span class="inline-formula"><i>c</i></span> axes is observed, at an angle to the primary cluster. With increasing strain, the primary <span class="inline-formula"><i>c</i></span>-axis cluster strengthens. With increasing temperature, both clusters strengthen. In the <span class="inline-formula">−5</span><span class="thinspace"></span><span class="inline-formula"><sup>∘</sup></span>C experiments, the angle between the two clusters reduces with strain. The <span class="inline-formula"><i>c</i></span>-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 curved boundaries decreases with increasing shear strain. Subgrains are observed in all samples. 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.</p>
doi:10.5194/tc-13-351-2019 fatcat:nui6bk32yfa4rkpp4ws4nnk4fi