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

Abstract

Abstract. Synthetic polycrystalline ice was sheared at temperatures of −5, −20 and −30 ∘C, to different shear strains, up to γ=2.6, equivalent to a maximum stretch of 2.94 (final line length is 2.94 times the original length). Cryo-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 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 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 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.

Temperature and strain controls on ice deformation mechanisms: insights from the microstructures of samples deformed to progressively higher strains at −10, −20 and −30 °C

“…1) at all temperatures first rise to the peak stresses and then relax to approach near-constant stresses with strains. This pattern matches published constant-displacement-rate experiments (Mellor and Cole, 1982;Durham et al, 1983;Durham et al, 1992;Piazolo et al, 2013;Vaughan et al, 2017;Qi et al, 2017;Craw et al, 2018;Qi et al, 2019), and is comparable to the constant-load experiments (Budd and Jacka, 1989;Jacka and Li, 2000;Treverrow et al, 2012;Wilson and Peternell, 2012) where strain rate first decreases to a minimum and then increases to approach a near-constant strain rate.…”

“…3(d For each sample, we calculated the mean grain diameter ( B ), the peak grain diameter ( CDEF ) and square mean root diameter Table 3. While the mean diameter, B , is commonly used for grain size analyses in the literature (e.g., Jacka, 1994;Piazolo et al, 2013;Vaughan et al, 2017;Qi et al, 2017;Qi et al, 2019), Lopez-Sanchez and Llana-Fúnez (2015) showed that the frequency peak ( CDEF ) of a grain size distribution provides the most robust measure of the recrystallized grain size. However, the population of grains smaller than CDEF is too small to provide representative CPO information for most of the data sets in this study.…”

“…This can include porosity loss (Vaughan et al, 2017) and the intergranular stress redistribution used to explain primary creep in constant load experiments (Duval et al, 1983). The drop of stress after peak correlates with dynamic recrystallization driven grain size reduction and CPO development (Jacka and Maccagnan, 1984;Vaughan et al, 2017;Qi et al, 2019). Experiments with initial grain size as a https://doi.org/10.5194/tc-2020-2 Preprint.…”

“…Enhancement correlates with the development of a crystallographic preferred orientation (CPO) (Jacka and Maccagnan, 1984;Vaughan et al, 2017) and also with other microstructural changes, in particular grain size reduction (Craw et al, 2018;Qi et al, 2019). Understanding the deformation and recrystallization mechanisms responsible for ice microstructure and CPO development is therefore essential for quantifying how different mechanisms contribute to ice creep enhancement in nature.…”

Recrystallization processes, microstructure and crystallographic preferred orientation evolution in polycrystalline ice during high-temperature simple shear

“…M max is the calculated torque, τ max is the maximum calculated shear stress, ε max is the maximum measured strain, andγ f andε f are the final measures shear strain rate and strain rate. ∼ 1 × 2 cm 2 , but is able to recover the full orientation of the crystal (all crystallographic axes) with an expected angular resolution better than 0.7 • (Randle, 1992) and with a practical spatial resolution for ice down to 5 µm. Sections tangential to the experimental cylinders were cut out for both AITA and EBSD analyses.…”