The evolution of deformation mechanisms during the mechanical weakening of polycrystalline ice: a quantitative microstructural study
Flowing glaciers and ice sheets play key roles in shaping planetary surfaces, and form important feedbacks with climate, both on Earth and elsewhere in the solar system. Ice becomes mechanically weaker during the deformation, and it is correlated with microstructural changes such as grain size reduction and a development of crystallographic preferred orientation (CPO). Weakening manifests as strain rate enhancement after minimum strain rate (under constant load) or a stress drop after peak stress (under constant displacement rate). To understand better the evolution of deformation mechanisms and its control on ice microstructural changes, we conducted a series of ice deformation experiments, mostly under uniaxial compression. Ice microstructural data were generated from cryogenic electron backscattered diffraction (cryo-EBSD) analyses. Deformed ice samples are characterised by small grains interlocking with big grains. After deformation, the number of small grains increases, suggesting an activation of nucleation. Big grains have irregular grain boundaries, suggesting the operation of strain-induced grain boundary migration (GBM). We quantify grain boundary irregularity using a sphericity parameter, Ψ, defined as the ratio of grain area and grain perimeter, divided by grain radius. The Ψ decreases with grain size up to a threshold grain size, above which Ψ define a plateau (low temperatures) or a less steep slope (high temperatures). We used the threshold grain size to segregate recrystallized grains, which grow via strain-induced GBM with boundaries becoming more irregular, from remnant grains, which initially have similar boundary irregularities and become more irregular at similar rates due to GBM. The average grain size change rates associated with GBM quantified from the threshold grain size are similar at high and low temperatures, suggesting similar GBM rates. The balance between boundary mobility and driving force is likely the cause of similar GBM rates at high and low temperatures. Intragranular boundaries are widely developed after deformation, suggesting the activity of dislocation creep, recovery and subgrain rotation. Low- (4°-10°) and high-angle (> 10°, up to ~30°) components of intragranular boundaries all have misorientation axes that lie dominantly within the basal plane. This observation suggests (1) intracrystalline dislocation glide on the basal plane is the dominant mechanism controlling grain rotation, and (2) subgrain rotation can proceed to very high misorientation angles and the resultant boundary remains strongly crystallographically controlled. Many intragranular boundaries with misorientation axes and primary Burgers vectors within the basal plane show a smearing of Burgers vectors to non-basal directions, suggesting a general importance of basal edge dislocations coupled with non-basal dislocations during the development of intragranular boundaries. At high temperatures, c-axes align in a cone (small circle) around the compression axis, consistent with the operation of strain-induced GBM and grain rotation. The opening-angle of the c-axis cone decreases with an increasing strain and with a decreasing temperature, suggesting a more active grain rotation. As the temperature decreases, the overall CPO intensity decreases, primarily because the CPO of small grains is weaker. Grain boundaries between small grains have misorientation axes that have distributed crystallographic orientations. This observation can be explained by grain boundary sliding and/or nucleation with random orientation. Many grains have high aspect ratios (>3), and they have boundary misorientation axes and slip directions within the basal plane, suggesting an activation of kinking in the grain segmentation. These grains have basal planes generally sub-parallel to the compression axis. This is because at these orientations, ice basal planes are fundamentally unstable: any rotation will re-orient ice basal planes to orientations with an increased shear stress and thus promote kinking. We estimated the contribution of CPO development and grain size reduction to the weakening, which starts from strains of 1-3%. The magnitude of weakening is insensitive to temperature. However, grain size reduction is a more effective weakening mechanism at low temperature, whilst the CPO development is more effective at high temperature. We suggest the balance between CPO development and grain size reduction as well as with other mechanisms (e.g. strain energy reduction led by dynamic recrystallization) is likely to give a weakening effect that is independent of temperature.
Advisor: Prior, David; Huble, Christina
Degree Name: Doctor of Philosophy
Degree Discipline: Department of Geology
Publisher: University of Otago
Keywords: high temperature deformation; ice; electron backscatter diffraction; mechanical weakening; dynamic recrystallization; deformation mechanism
Research Type: Thesis