Microstructural evolution of polycrystalline ice during confined creep testing

Breton, Daniel J.; Baker, Ian; Cole, David M.

doi:10.1016/j.coldregions.2016.03.009

Cited by 10 publications

(20 citation statements)

References 63 publications

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“…To date there is no direct evidence for slip on other non-basal planes in natural polycrystalline ice, in spite of the shortage of independent slip systems to deform ice only by basal slip (Hutchinson, 1977). Previous studies are mainly based on experimental deformation carried out under laboratory conditions on polycrystalline (Barrette and Sinha, 1994;Bryant and Mason, 1960;Breton et al, 2016) and/or single crystal ice (Montagnat et al, 2003(Montagnat et al, , 2001. This study presents the first step in an ongoing study using naturally deformed ice from a single depth level of two deep ice cores from the large ice sheets.…”

Section: Introductionmentioning

confidence: 99%

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: Introductionmentioning

confidence: 99%

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

et al. 2017

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“…Breton et al (2016) report grain dissection in deformation experiments on laboratory-prepared ice and suspect a grain-size-reducing character. So far, it is assumed that stable grain sizes in ice are achieved by an interplay of dynamic graingrowth, polygonisation and potentially nucleation processes (Alley et al, 1995;De La Chapelle et al, 1998;Montagnat and Duval, 2000;Mathiesen et al, 2004;Roessiger et al, 2011;Chauve et al, 2017;Hidas et al, 2017).…”

Section: Introductionmentioning

confidence: 99%

The Relevance of Grain Dissection for Grain Size Reduction in Polar Ice: Insights from Numerical Models and Ice Core Microstructure Analysis

et al. 2017

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The flow of ice depends on the properties of the aggregate of individual ice crystals, such as grain size or lattice orientation distributions. Therefore, an understanding of the processes controlling ice micro-dynamics is needed to ultimately develop a physically based macroscopic ice flow law. We investigated the relevance of the process of grain dissection as a grain-size-modifying process in natural ice. For that purpose, we performed numerical multi-process microstructure modeling and analyzed microstructure and crystallographic orientation maps from natural deep ice-core samples from the North Greenland Eemian Ice Drilling (NEEM) project. Full crystallographic orientations measured by electron backscatter diffraction (EBSD) have been used together with c-axis orientations using an optical technique (Fabric Analyser). Grain dissection is a feature of strain-induced grain boundary migration. During grain dissection, grain boundaries bulge into a neighboring grain in an area of high dislocation energy and merge with the opposite grain boundary. This splits the high dislocation-energy grain into two parts, effectively decreasing the local grain size. Currently, grain size reduction in ice is thought to be achieved by either the progressive transformation from dislocation walls into new high-angle grain boundaries, called subgrain rotation or polygonisation, or bulging nucleation that is assisted by subgrain rotation. Both our time-resolved numerical modeling and NEEM ice core samples show that grain dissection is a common mechanism during ice deformation and can provide an efficient process to reduce grain sizes and counter-act dynamic grain-growth in addition to polygonisation or bulging nucleation. Thus, our results show that solely strain-induced boundary migration, in absence of subgrain rotation, can reduce grain sizes in polar ice, in particular if strain energy gradients are high. We describe the microstructural characteristics that can be used to identify grain dissection in natural microstructures.

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“…With the scanning electron microscope in low‐vacuum (67 Pa) mode, samples were kept between −60 and −130°C with a temperature controlled cold stage. To automate beam scanning and data collection, an Oxford‐HKL Technology electron backscatter diffraction (EBSD) system was used to control stage motion, data collection, and analysis (Breton et al, ; Obbard et al, ). Because ice grains were typically large (~0.5–2 mm) in comparison to the maximum resolution of the EBSD system (0.5 μm), a 100 μm step size was chosen.…”

Section: Experimental Methodsmentioning

confidence: 99%

The Effects of H₂SO₄ on the Mechanical Behavior and Microstructural Evolution of Polycrystalline Ice

Hammonds

Baker

2018

JGR Earth Surface

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The Earth's large continental ice sheets contain a variety of naturally occurring impurities, both soluble and insoluble. Understanding how these impurities affect the rheology, intrinsic thermodynamic properties, and fate of these ice sheets is not well understood. To investigate the effects that trace amounts of H2SO4 have on the flow and ductility of polycrystalline ice, a series of mechanical tests were conducted at −6, −10, −12.5, and −20°C using laboratory‐prepared specimens of polycrystalline ice doped with 1–15 ppm of H2SO4. Parallel tests were performed on identical but undoped specimens of polycrystalline ice. Mechanical testing included constant‐load tensile creep tests at an initial stress of 0.75 MPa and compression tests at constant displacement rates with initial strain rates ranging from 1 × 10−6 to 1 × 10−4 s−1. It was found that H2SO4‐doped specimens of ice exhibited faster creep rates in tension and significantly lower peak stresses in compression, when compared to the undoped ice. Postmortem microstructural analyses were performed using cross‐polarized light thin section imaging, X‐ray computed microtomography, Raman spectroscopy, and electron backscatter diffraction. These analyses showed that H2SO4‐doped specimens had a larger grain size at strains ≤15%, and an earlier onset of microcracking at lower strain rates than the undoped ice. Strain‐induced grain boundary migration was found to be the predominant mechanism of recrystallization in both doped and undoped specimens. Further, an aqueous phase of H2SO4 was found to exist at the grain boundaries and triple junctions of the doped ice, which is thought to have significantly contributed toward its reduced viscosity.

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Microstructural evolution of polycrystalline ice during confined creep testing

Cited by 10 publications

References 63 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

The Relevance of Grain Dissection for Grain Size Reduction in Polar Ice: Insights from Numerical Models and Ice Core Microstructure Analysis

The Effects of H₂SO₄ on the Mechanical Behavior and Microstructural Evolution of Polycrystalline Ice

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Microstructural evolution of polycrystalline ice during confined creep testing

Cited by 10 publications

References 63 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

The Relevance of Grain Dissection for Grain Size Reduction in Polar Ice: Insights from Numerical Models and Ice Core Microstructure Analysis

The Effects of H2SO4 on the Mechanical Behavior and Microstructural Evolution of Polycrystalline Ice

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The Effects of H₂SO₄ on the Mechanical Behavior and Microstructural Evolution of Polycrystalline Ice