Using grain boundary irregularity to quantify dynamic recrystallization in ice

Dynamic recrystallization is an important mechanical weakening mechanism during the deformation of ice, yet we currently lack robust quantitative tools for identifying recrystallized grains in the “migration” recrystallization regime that dominates ice deformation at temperatures close to the ice melting point. Here, we propose grain boundary irregularity as a quantitative means for discriminating between recrystallized (high sphericity, low irregularity) and remnant (low sphericity, high irregularity) grains. To this end, we analysed cryogenic electron backscatter diffraction (cryo-EBSD) data of deformed polycrystalline ice, to quantify dynamic recrystallization using grain boundary irregularity statistics. Grain boundary irregularity has an inverse relationship with a sphericity parameter, Ψ, defined as the ratio of grain area and grain perimeter, divided by grain radius in 2-D so that the measurement is grain size independent. Sphericity (Ψ) typically decreases with increasing grain size, up to a threshold grain size, above which Ψ either plateaus (at temperature, T< -10°C) or increases much more gradually (at T≥ -10°C). There is no apparent relationship between grain boundary sphericity and grain c-axis orientation even at very high temperatures (-4 and -5°C), where GBM dominants, suggesting little crystallographic control on the activity of grain boundary migration (GBM). Decreasing sphericity up to the threshold grain size can be explained by newly-formed, small, spherical recrystallized grains growing via strain-induced GBM and thereby developing increasingly irregular grain boundaries. We suggest that the plateau (or gradual decrease) in sphericity at larger sizes represents a population of original grains (i.e., remnant grains) that becomes increasingly irregular (at similar rates) due to the balance between GBM and nucleation. In this interpretation, the threshold grain size represents the largest grain size reached by a growing recrystallized grain by the end of an experiment. Thus, grain boundary irregularity provides a means for discriminating between recrystallized and remnant grains—a capability that is potentially useful for evaluating dynamic recrystallization processes in ice deformed at temperatures close to the melting point. The threshold grain size and experiment duration can be used to calculate the rates of recrystallization and grain size evolution associated with GBM. Grain size evolution rates are similar at high and low temperatures, suggesting similar GBM rates. Previous studies show that grain boundary mobility decreases with decreasing temperature. The driving force of GBM, on the other hand, has a positive correlation with stress, which increases with a decreasing temperature if strain rate remains unchanged. The balance between boundary mobility and driving force is likely the cause of similar GBM rates between high and low temperatures.