Abstract
Abstract. Torsion experiments were performed in polycrystalline ice at high temperature (0.97 Tm) to reproduce the simple shear kinematics that are believed to dominate in ice streams and at the base of fast-flowing glaciers. As clearly documented more than 30 years ago, under simple shear ice develops a two-maxima c axis crystallographic preferred orientation (CPO), which evolves rapidly into a single cluster CPO with a c axis perpendicular to the shear plane. Dynamic recrystallization mechanisms that occur in both laboratory conditions and naturally deformed ice are likely candidates to explain the observed CPO evolution. In this study, we use electron backscatter 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 of dislocations with a [c]-component Burgers vector, indicating that strong local stress heterogeneity develops, in particular, close to grain boundaries, even at high temperature and high finite shear strain. Based on these observations, we propose that nucleation by bulging, assisted by sub-grain boundary formation and followed by grain growth, is a very likely candidate to explain the progressive disappearance of the c axis CPO 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 polycrystal plasticity models limiting dislocation slip on non-basal slip systems and allowing for efficient accommodation of strain incompatibilities by an association of bulging and formation of sub-grain boundaries with a significant [c] component.
Highlights
Ice deforms by shear in many natural conditions such as glaciers and ice sheets, and in particular along ice streams
By using state-of-the-art analytical techniques (i.e., automatic ice texture analyzer (AITA), Electron backscatter diffraction (EBSD) and weighted Burgers vector analysis), we were able to characterize the crystallographic preferred orientation (CPO), microstructure and geometrically necessary dislocation structures in ice deformed under torsion in the laboratory at an unprecedented resolution
The experiments, performed at high temperature, up to shear strains of 2, favored dynamic recrystallization observed in natural conditions with slower strain rates such as cold glaciers, ice streams and some deep ice core areas
Summary
Ice deforms by shear in many natural conditions such as glaciers and ice sheets, and in particular along ice streams. Simple shear occurs in the deeper portions of these cores. The strongest simple shear occurs in fastflowing glaciers and ice streams (see review by Hudleston, 2015). During large-scale ice flow, deformation induces development of strong crystallographic preferred orientation (CPO) or texture. Since the late 1970s a growing number of studies have provided measurements of increasing accuracy of the CPO evolution along ice cores (refer to Gow and Williamson, 1976; Alley, 1988; Lipenkov et al, 1989, for pioneer work, and Faria et al, 2014b, for a review).
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