Microstructure and texture evolution in polycrystalline ice during hot torsion. Impact of intragranular strain and recrystallization processes

Abstract

Abstract. Torsion experiments were performed in polycrystalline ice at high temperature (0.97 ⋅ Tm) to reproduce simple shear conditions close to those encountered in ice streams and at the base of fast flowing glaciers. As well documented more than 30 years ago (Hudleston, 1977; Bouchez and Duval, 1982), under simple shear ice develops a two-maxima c-axis texture, which evolves rapidly into a single cluster texture with c-axis perpendicular to the shear plane. This evolution still lacks a physical explanation. Current viscoplastic modeling approaches on ice involving dislocation slip on multiple slip systems (basal pyramidal, and prismatic) fail to reproduce it. Dynamic recrystallization mechanisms that occur in both laboratory conditions and in natural setups are likely candidates to explain the texture evolution observed. In this study, we use Electron BackScattering Diffraction (EBSD) and Automatic Ice Texture Analyzer (AITA) to characterize the mechanisms accommodating deformation, the stress and strain heterogeneities that form under torsion of an initially isotropic polycrystalline ice sample at high temperature, and the role of dynamic recrystallization in accommodating these heterogeneities. These analyses highlight an interlocking microstructure, which results from heterogeneity-driven serrated grain boundary migration, and sub-grain boundaries composed by dislocations with [c]-component Burgers vector, indicating that strong local stress heterogeneity develops, even at high temperature and high finite shear strain. Based on these observations, we propose that that nucleation by bulging, assisted by sub-grain boundary formation, is a very likely candidate to explain the progressive disappearance of the texture cluster at low angle to the shear plane and the stability of the one normal to it. We therefore strongly support the development of new models limiting dislocation slip on non-basal slip system and allowing for efficient polygonization by an association of bulging and formation of sub-grain boundaries with a significant [c]-component.

Crystallographic preferred orientations of ice deformed in direct-shear experiments at low temperatures

“…All c-axis clusters in our experiments are elongated in the direction sub-perpendicular to the shear direction. This elongation has been observed in many previous studies (Kamb, 1972;Bouchez and Duval, 1982;Li et al, 2000;Wilson and Peternell, 2012). Li et al (2000) attributed this elongation to extensional deformation in the shear plane normal to Step size is 5 µm (see Table 2, columns 5-7).…”

“…In our experiments, the primary cluster of c axes is normal to the shear plane in all deformed samples. This statement is true for most other ice samples deformed dominantly by simple shear in the laboratory, for which CPOs with significant numbers of measured grains are published (Kamb, 1972;Bouchez and Duval, 1982;Li et al, 2000). Wilson and Peternell (2012) reported the primary cluster being slightly oblique to the imposed shear plane, but the sample images (Fig.…”

“…All other samples have uniform thicknesses. Since the CPOs of sheared ice are often characterized by double clusters of c axes (e.g., Kamb, 1972;Hudleston, 1977;Bouchez and Duval, 1982;Jackson, 1999), an angle ϕ was used to quantify the relative orientation between the two clusters. We adopted the same approach as previously described by Bouchez and Duval (1982).…”

“…Polycrystalline ice 1 h deformed in the laboratory (e.g., Kamb, 1972;Li et al, 2000;Wilson et al, 2014;Qi et al, 2017) and in nature (e.g., Gow and Williamson, 1976;Hudleston, 1977;Thorsteinsson et al, 1999;Treverrow et al, 2016;Weikusat et al, 2017) develops strong crystallographic preferred orientations (CPOs, often called crystal orientation fabric, COF, in the glaciological literature), usually presented as the preferred orientation of ice [0001] axes, i.e., c axes. As single crystals of ice are most easily deformed by glide on the (0001) plane, i.e., the basal plane (Nakaya, 1958;Wakahama, 1967;Duval et al, 1983), the manner in which the c axes are aligned affects the flow strength for the given applied deformation kinematics, for example, simple shear versus uniaxial compression (e.g., Shoji and Langway, 1988;Azuma, 1995;Li et al, 1996;Duval et al, 2010;Budd et al, 2013).…”

“…However, most deformation in ice sheets and glaciers is dominantly simple shear (Cuffey and Paterson, 2010) and CPOs developed in compression and simple shear are not equivalent (Alley, 1992). Shear experiments on polycrystalline ice are limited to relatively high temperatures and mostly relatively low shear strains (Kamb, 1972;Bouchez and Duval, 1982;Li et al, 2000;Wilson and Peternell, 2012). Only four published shear experiments (Bouchez and Duval, 1982;Wilson and Peternell, 2012) achieve shear strains greater than 0.5 at temperatures ≤ −10 • C, with the coldest experiment conducted at −15 • C. Shear experiments at colder conditions (Wilson and Peternell, 2012) have not achieved shear strains > 0.12.…”