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

Abstract

Abstract. In order to better understand ice deformation mechanisms, we document the microstructural evolution of ice with increasing strain. We include data from experiments at relatively low temperatures (−20 and −30 ∘C), where the microstructural evolution with axial strain has never before been documented. Polycrystalline pure water ice was deformed under a constant displacement rate (strain rate ∼1.0×10-5 s−1) to progressively higher strains (∼ 3 %, 5 %, 8 %, 12 % and 20 %) at temperatures of −10, −20 and −30 ∘C. Microstructural data were generated from cryogenic electron backscattered diffraction (cryo-EBSD) analyses. All deformed samples contain subgrain (low-angle misorientations) structures with misorientation axes that lie dominantly in the basal plane, suggesting the activity of dislocation creep (glide primarily on the basal plane), recovery and subgrain rotation. Grain boundaries are lobate in all experiments, suggesting the operation of strain-induced grain boundary migration (GBM). Deformed ice samples are characterized by interlocking big and small grains and are, on average, finer grained than undeformed samples. Misorientation analyses between nearby grains in 2-D EBSD maps are consistent with some 2-D grains being different limbs of the same irregular grain in the 3-D volume. The proportion of repeated (i.e. interconnected) grains is greater in the higher-temperature experiments suggesting that grains have more irregular shapes, probably because GBM is more widespread at higher temperatures. The number of grains per unit area (accounting for multiple occurrences of the same 3-D grain) is higher in deformed samples than undeformed samples, and it increases with strain, suggesting that nucleation is involved in recrystallization. “Core-and-mantle” structures (rings of small grains surrounding big grains) occur in −20 and −30 ∘C experiments, suggesting that subgrain rotation recrystallization is active. At temperatures warmer than −20 ∘C, c axes develop a crystallographic preferred orientation (CPO) characterized by a cone (i.e. small circle) around the compression axis. We suggest the c-axis cone forms via the selective growth of grains in easy slip orientations (i.e. ∼ 45∘ to shortening direction) by GBM. The opening angle of the c-axis cone decreases with strain, suggesting strain-induced GBM is balanced by grain rotation. Furthermore, the opening angle of the c-axis cone decreases with temperature. At −30 ∘C, the c-axis CPO changes from a narrow cone to a cluster, parallel to compression, with increasing strain. This closure of the c-axis cone is interpreted as the result of a more active grain rotation together with a less effective GBM. We suggest that lattice rotation, facilitated by intracrystalline dislocation glide on the basal plane, is the dominant mechanism controlling grain rotation. Low-angle neighbour-pair misorientations, relating to subgrain boundaries, are more extensive and extend to higher misorientation angles at lower temperatures and higher strains supporting a relative increase in the importance of dislocation activity. As the temperature decreases, the overall CPO intensity decreases, primarily because the CPO of small grains is weaker. High-angle grain boundaries between small grains have misorientation axes that have distributed crystallographic orientations. This implies that, in contrast to subgrain boundaries, grain boundary misorientation is not controlled by crystallography. Nucleation during recrystallization cannot be explained by subgrain rotation recrystallization alone. Grain boundary sliding of finer grains or a different nucleation mechanism that generates grains with random orientations could explain the weaker CPO of the fine-grained fraction and the lack of crystallographic control on high-angle grain boundaries.

Kinking facilitates grain nucleation and modifies crystallographic preferred orientations during high-stress ice deformation

“…2(g, i), 3(g, i), 4(g, i)), matching kink band observations from previous studies (Piazolo et al, 2015;Seidemann et al, 2020). Most (~60%) kink bands have misorientation angles higher than 5-10°, which is commonly used to distinguish subgrain boundaries from grain boundaries in ice (Fan et al, 2020;Qi et al, 2017), and a considerable proportion (>20%) of kink bands has misorientation angles higher than 20° (Fig. 5(b)).…”

“…3.3.3). Weak recrystallized grain CPOs have been reported in previous studies on experimentally deformed ice (Fan et al, 2020) as well as naturally and experimentally deformed rock (Bestmann and Prior, 2003). Published models that attempt to explain CPO weakening…”

“…Undeformed coarse-grained ice samples exhibit a homogeneous microstructure with slightly irregular grain boundaries and a small number of intragranular boundaries (mostly low angle) (Figs. 1(a, g))-compared with fine-grained (~300 μm) ice samples fabricated via the same "flood-freeze" method (Fan et al, 2021(Fan et al, , 2020. Grain sizes follow a slightly left-skewed log-linear distribution, with a peak at ~1300 µm, and a tail extending down to ~300 µm (Fig.…”

Modeling Ice‐Crystal Fabric as a Proxy for Ice‐Stream Stability

“…Temperatures in ice‐stream margins are generally warmer than their surroundings, and simplified models suggest that they can sometimes approach C through much of the ice column (e.g., Hunter et al., 2021). While laboratory measurements suggest recrystallization is important at stresses and strains seen in ice streams, perhaps even at relatively low temperatures (Fan et al., 2020; Journaux et al., 2019), further measurements are needed to confirm the role of recrystallization in natural settings. Here, we assume that the tripartite framework holds and interpret results only in areas colder than C.…”

Effect of an orientation-dependent non-linear grain fluidity on bulk directional enhancement factors

“…The viscosity of polycrystalline ice depends on physical properties such as the orientation distribution of individual grains (orientation fabric) (Shoji and Langway, 1985, 1988), grain sizes (Goldsby and Kohlstedt, 1997; Kuiper and others, 2020 a , 2020 b ; Fan and others, 2020), and hardening effects due to crystal lattice defects (Faria and others (2014) and references therein). The orientation fabric is particularly important because ice monocrystals deform by dislocation slip (shear) along crystallographic planes (Fig.…”