Non-basal dislocations should be accounted for in simulating ice mass flow

Chauve, Thomas; Montagnat, Mâurine; Piazolo, Sandra; Journaux, Baptiste; Wheeler, John; Barou, Fabrice; Mainprice, David; Tommasi, Andréa

doi:10.1016/j.epsl.2017.06.020

Cited by 28 publications

(57 citation statements)

References 51 publications

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“…Although somewhat unexpected with respect to the macroscopic behaviour of ice, this result is in accordance with earlier Xray work on a limited number of measurements in polar ice . This is in agreement with the recent findings of Chauve et al (2017) who find up to 35 % nonbasal slip in experimentally deformed artificial ice. The activation of non-basal slip suggests that, locally within grains, stresses were high enough to activate the harder slip systems.…”

Section: P-type Twist Boundaries: P[c]supporting

confidence: 82%

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

et al. 2017

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Abstract. Ice has a very high plastic anisotropy with easy dislocation glide on basal planes, while glide on non-basal planes is much harder. Basal glide involves dislocations with the Burgers vector b = a , while glide on non-basal planes can involve dislocations with b = a , b = [c], and b = c + a . During the natural ductile flow of polar ice sheets, most of the deformation is expected to occur by basal slip accommodated by other processes, including non-basal slip and grain boundary processes. However, the importance of different accommodating processes is controversial. The recent application of micro-diffraction analysis methods to ice, such as X-ray Laue diffraction and electron backscattered diffraction (EBSD), has demonstrated that subgrain boundaries indicative of non-basal slip are present in naturally deformed ice, although so far the available data sets are limited. In this study we present an analysis of a large number of subgrain boundaries in ice core samples from one depth level from two deep ice cores from Antarctica (EPICA-DML deep ice core at 656 m of depth) and Greenland (NEEM deep ice core at 719 m of depth).EBSD provides information for the characterization of subgrain boundary types and on the dislocations that are likely to be present along the boundary. EBSD analyses, in combination with light microscopy measurements, are presented and interpreted in terms of the dislocation slip systems. The most common subgrain boundaries are indicative of basal a slip with an almost equal occurrence of subgrain boundaries indicative of prism [c] or c + a slip on prism and/or pyramidal planes. A few subgrain boundaries are indicative of prism a slip or slip of a screw dislocations on the basal plane. In addition to these classical polygonization processes that involve the recovery of dislocations into boundaries, alternative mechanisms are discussed for the formation of subgrain boundaries that are not related to the crystallography of the host grain.The finding that subgrain boundaries indicative of nonbasal slip are as frequent as those indicating basal slip is surprising. Our evidence of frequent non-basal slip in naturally deformed polar ice core samples has important implications for discussions on ice about plasticity descriptions, rate-controlling processes which accommodate basal glide, and anisotropic ice flow descriptions of large ice masses with the wider perspective of sea level evolution.

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Section: P-type Twist Boundaries: P[c]supporting

confidence: 82%

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

et al. 2017

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“…One sample (TGI0.71), with a γ max of 0.71, was annealed for 72 hours at -7 • to study the effect of annealing on both texture and microstructure. The microstructure of one of these samples has been partially described in previous work of our team to compare the statistical representation of WBV (or Burger vector) with [c]-component in GNDs in samples submitted to uniaxial unconfined compression and to torsion (Chauve et al, 2017b).…”

Section: Resultsmentioning

confidence: 99%

“…This means that if a • 10 −4 µm −1 ) was imposed for the misorientation analysis to filter the noise resulting from the angular resolution of the EBSD data. We suggest to the interested readers to refer the to Appendix A of Chauve et al (2017b) for more details about this analysis. Nevertheless, it should be reminded that the WBV analysis does not directly provide information about mobile dislocations, responsible for the majority of the plastic deformation.…”

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confidence: 99%

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

Journaux

Chauve

Montagnat

et al. 2018

Preprint

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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.

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“…Grains may rotate to high Schmid factor orientations where they are then able to grow by GBM. Grains which are in hard basal slip orientations will experience higher stress and attempt to activate nonbasal slip systems [ Chauve et al , ], storing greater magnitudes of internal strain. This is supported by the observed progressive loss of grains in hard slip orientations with increasing strain.…”

Section: Discussionmentioning

confidence: 99%

Insights into anisotropy development and weakening of ice from in situ P wave velocity monitoring during laboratory creep

et al. 2017

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Polycrystalline ice weakens significantly after a few percent strain, during high homologous temperature deformation. Weakening is correlated broadly with the development of a crystallographic preferred orientation (CPO). We deformed synthetic polycrystalline ice at −5°C under uniaxial compression, while measuring ultrasonic P wave velocities along several raypaths through the sample. Changes in measured P wave velocities (Vp) and in the velocities calculated from microstructural measurements of CPO (by cryo‐electron backscatter diffraction) both show that velocities along trajectories parallel and perpendicular to shortening decrease with increasing strain, while velocities on diagonal trajectories increase. Thus, in these experiments, velocity data provide a continuous measurement of CPO evolution in creeping ice. Samples reach peak stresses after 1% shortening. Weakening corresponds to the start of CPO development, as indicated by divergence of P wave velocity changes for different raypaths, and initiates at ≈3% shortening. Selective growth by strain‐induced grain boundary migration (GBM) of grains favorably oriented for basal slip may initiate weakening through the formation of an interconnected network of these grains by 3% shortening. After weakening initiates, CPO continues to develop by GBM and nucleation processes. The resultant CPO has an open cone (small circle) configuration, with the cone axis parallel to shortening. The development of this CPO causes significant weakening under uniaxial compression, where the shear stresses resolved on the basal planes (Schmid factors) are high.

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Non-basal dislocations should be accounted for in simulating ice mass flow

Cited by 28 publications

References 51 publications

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

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

Insights into anisotropy development and weakening of ice from in situ P wave velocity monitoring during laboratory creep

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