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. In order to better understand ice deformation mechanisms,we document the microstructural evolution of ice with increasing strain. Weinclude data from experiments at relatively low temperatures ( −20 and −30 ∘ C), where the microstructural evolution with axial strain hasnever before been documented. Polycrystalline pure water ice was deformedunder 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 backscattereddiffraction (cryo-EBSD) analyses. All deformed samples contain subgrain(low-angle misorientations) structures with misorientation axes that liedominantly in the basal plane, suggesting the activity of dislocation creep(glide primarily on the basal plane), recovery and subgrain rotation. Grainboundaries are lobate in all experiments, suggesting the operation ofstrain-induced grain boundary migration (GBM). Deformed ice samples arecharacterized by interlocking big and small grains and are, on average,finer grained than undeformed samples. Misorientation analyses betweennearby grains in 2-D EBSD maps are consistent with some 2-D grains beingdifferent limbs of the same irregular grain in the 3-D volume. Theproportion of repeated (i.e. interconnected) grains is greater in thehigher-temperature experiments suggesting that grains have more irregularshapes, probably because GBM is more widespread at higher temperatures. Thenumber of grains per unit area (accounting for multiple occurrences of thesame 3-D grain) is higher in deformed samples than undeformed samples, andit increases with strain, suggesting that nucleation is involved inrecrystallization. “Core-and-mantle” structures (rings of small grainssurrounding big grains) occur in −20 and −30 ∘ C experiments,suggesting that subgrain rotation recrystallization is active. Attemperatures warmer than −20 ∘ C, c  axes develop a crystallographicpreferred orientation (CPO) characterized by a cone (i.e. small circle)around the compression axis. We suggest the c -axis cone forms via theselective 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 balancedby grain rotation. Furthermore, the opening angle of the c -axis conedecreases with temperature. At −30 ∘ C, the c -axis CPO changes froma 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 activegrain rotation together with a less effective GBM. We suggest that latticerotation, facilitated by intracrystalline dislocation glide on the basalplane, is the dominant mechanism controlling grain rotation. Low-angleneighbour-pair misorientations, relating to subgrain boundaries, are moreextensive and extend to higher misorientation angles at lower temperaturesand higher strains supporting a relative increase in the importance ofdislocation activity. As the temperature decreases, the overall CPOintensity decreases, primarily because the CPO of small grains is weaker.High-angle grain boundaries between small grains have misorientation axesthat have distributed crystallographic orientations. This implies that, incontrast to subgrain boundaries, grain boundary misorientation is notcontrolled by crystallography. Nucleation during recrystallization cannot beexplained by subgrain rotation recrystallization alone. Grain boundarysliding of finer grains or a different nucleation mechanism that generatesgrains with random orientations could explain the weaker CPO of thefine-grained fraction and the lack of crystallographic control on high-anglegrain boundaries.

the cryosphere

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