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
Authors: Fan Sheng , Hager Travis F. , Prior David J. , Cross Andrew J. , Goldsby David L. , Qi Chao , Negrini Marianne , Wheeler John 
Published: November 2020

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.