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. Understanding ice deformation mechanisms is crucial for understanding the dynamic evolution of terrestrial and planetary ice flow. To understand better the deformation mechanisms, we document the microstructural evolution of ice with increasing strain. We include data from deformation at relatively low temperature (−20 and −30 °C) where the microstructural evolution has never before been documented. Polycrystalline pure water ice was deformed under a constant displacement rate (equal to the strain rate of ~1.0×10−5 s−1) at temperatures of −10, −20 and −30 °C to progressively higher true axial strains (~ 3, 5, 8, 12 and 20 %). Mechanical data show peak and steady-state stresses are larger at colder temperatures as expected from the temperature dependency of creep. Cryo-electron backscattered diffraction (EBSD) analyses show distinct sub-grain boundaries in all deformed samples, suggesting activation of recovery and subgrain rotation. Deformed ice samples are characterised by big grains interlocking with small grains. For each temperature series, we separated big grains from small grains using a threshold grain size, which equals to the square mean root diameter at ~ 12 % strain. Big grains are more lobate at −10 °C than at colder temperatures, suggesting grain boundary migration (GBM) is more prominent at warmer temperatures. The small grains are smaller than subgrains at −10 °C and they become similar in size at −20 and −30 °C, suggesting bulge nucleation facilitates the recrystallization process at warmer temperature and subgrain rotation recrystallization is the nucleation mechanism at colder temperatures. At temperatures warmer than −15 °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 as a result of selective growth of grains at easy slip orientations (i.e., ~ 45° to shortening direction) by strain-energy driven GBM. This particular finding is consistent with previous works. 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 transits 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. As the temperature decreases, the overall CPO intensity decreases, facilitated by the CPO weakening in small grains. We suggest the grain size sensitivity of grain boundary sliding (GBS) favours a faster strain rate in small grains and leads to the CPO weakening at cold temperatures. CPO development cannot provide a uniform explanation for the mechanical weakening (enhancement) after peak stress. Grain size reduction, which can be observed in all deformed samples, is most likely to cause weakening (enhancement) and should be considered to have a significant control on the rheology of natural ice flow.

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

“…Data availability. Data can be obtained via https://doi.org/10.6084/m9.figshare.12980243 (Fan et al, 2020).…”

Microstructures and Fabric Transitions of Natural Ice from the Styx Glacier, Northern Victoria Land, Antarctica

“…In terms of density, sample SG_50 is classified as firn-ice transition, and the other samples are defined as ice [58,59]. Deformation temperatures between −29.1 to −31.2 • C estimated from the measurements of the bore hole, are colder than the vast majority of published deformation experiments for which CPO and microstructural data have been presented, but are at a similar temperature to −30 • C experiments presented by [17,26,29].…”

“…The rotation axes of the analyzed samples, shown in Figure 8, can be classified into two types: one parallel to the c-axes that is activated at the depths of 50, 140, and 170 m, and another along the basal plane that has been highlighted at all depths except at 140 m. Assuming that only the basal slip system is active, rotation axes parallel to the c-axis can be related to twist walls (screw dislocations) of the basal slip system, while rotation axes along the basal plane can be related to tilt walls (edge dislocations) of the basal slip system. Statistically significant data sets from experiments identify both of these rotation axes [26,71] and subgrain boundary trace analyses, and weighted burgers vector analyses [72] of these same samples (Sheng Fan, personal communication) indicate that the rotation axes in basal plane are clearly related to basal plane dislocations. Other studies have constrained the operation of non-basal dislocations in both experimental [73,74] and natural [70,75] ice and further work is needed to determine the contribution of non-basal dislocations in these samples.…”

“…Single-maxima clusters are observed in uniaxial compression laboratory experiments only at low temperatures and/or fast strain rates [17,26]. The vast majority of laboratory experiments give an open cone (small circle) CPO in uniaxial compression.…”

“…The vast majority of laboratory experiments give an open cone (small circle) CPO in uniaxial compression. Laboratory experiments have shown that a transition from an open cone to a single cluster of c-axes [26][27][28] with increasing stress, decreasing temperature, and increasing strain. This has been interpreted as an increase in pyramidal slip systems by Piazolo et al [27] and to a change in balance between lattice rotation and grain boundary migration recrystallization by Qi et al [28].…”