Effects of microparticles on deformation and microstructural evolution of fine-grained ice

Abstract

We investigated the effects of microparticles and grain size on the microstructural evolutions and mechanical properties of polycrystalline ice. Uniaxial compression tests were conducted using fine-grained pure ice and silica-dispersed ice under various conditions. Deformation behavior of fine-grained ice was found to be characterized by stress exponent n ≈ 2 and activation energy Q ≈ 60 kJ mol−1. The derived strain rates of fine-grained ice were ≈ 1 order of magnitude larger than those of coarse-grained ice obtained in previous studies, and they were found to be independent of particle dispersion and dependent on the mean grain size of ice, with grain size exponent p ≈ 1.4. Work hardening was observed in dislocation creep, while the strain rate continued to decrease. These results indicate that the deformation mechanism of fine-grained ice is different from typical dislocation creep, often associated with n = 3. Although microparticles restricted grain growth, there was little direct effect on the deformation of fine-grained ice. Microstructural observations of the ice samples indicated that the grain boundaries were straight and that the subgrain boundary densities increased after deformation. Our experiments suggest that grain size and boundaries play important roles in the deformation processes of polycrystalline ice.

Using a composite flow law to model deformation in the NEEM deep ice core, Greenland – Part 1: The role of grain size and grain size distribution on deformation of the upper 2207 m

“…At low equivalent stresses (< 1 MPa) and with the finest-grained samples, Goldsby and Kohlstedt found a third creep regime, which was again grain size independent but with n = 2.4. Recent studies (Saruya et al, 2019) have confirmed the occurrence of GSS creep in fine-grained ice, with n = 2 and p = 1.4. According to Goldsby and Kohlstedt, the GSI regime with n = 4 is governed by dislocation creep using the easy slip systems in ice (basal slip) rate-limited by the hard slip systems (i.e., nonbasal slip).…”

“…The recent study by Saruya et al (2019) confirms the occurrence of GSS creep in fine-grained ice and proposes a different mechanism, where grain boundaries enhance creep by acting as sinks for dislocations. We emphasize that even though the exact mechanisms involved in GSS flow are not understood in detail, there is good experimental evidence for GSS creep in ice and the available flow laws provide a reasonable basis to investigate the role of these mechanisms in polar ice sheets.…”

“…The amount of ice available for calving and melting depends on the flow of ice from the interior towards the margins of the ice sheet. This flow of ice is controlled by two processes: sliding of the ice over the bedrock, which includes various subglacial processes (Zwally et al, 2002;Vaughan and Arth-ern, 2007;Thoma et al, 2010;Wolovick and Creyts, 2016), and the internal deformation of the polycrystalline ice, which is governed by various processes like dislocation creep, grain boundary migration (strain-induced grain boundary migration, SIBM, using the terminology of Faria et al, 2014b) and grain boundary sliding (GBS) (e.g., Duval et al, 1983;Alley, 1992;Goldsby andKohlstedt, 1997, 2001;Duval, 2000, 2004;Schulson and Duval, 2009;Faria et al, 2014a).…”

“…This forms a possible explanation for a lack of accuracy in calculating ice strain rates when using Glen's flow law with fixed n = 3 at very low stress (e.g., Peltier et al, 2000;Durham et al, 2010). For instance, Schulson and Duval (2009) claim that the n = 4 regime applies to tertiary creep, which was the same stress exponent found in the high-pressure experiments (Durham et al, 1983;Kirby et al, 1987). Analysis also indicates that a power law with a stress exponent of n = 4 best describes the observed state of the northern part of the Greenland ice sheet (Bons et al, 2018).…”

“…In contrast, Goldsby and Kohlstedt (2002) interpreted the occurrence of straight grain boundaries and equant grains in Glacial ice from the GRIP ice core as evidence for GBS; however, this interpretation was not accepted by most members of the ice community. More recently, Faria et al (2014a, b) noted that if strain is high in impurity-rich, fine-grained cloudy bands in the Glacial ice, flow is likely to be accommodated by GSS deformation, and Saruya et al (2019) have suggested that ice sheet microstructures with straight grain boundaries could deform by GSS creep with stress exponent of n = 2 and grain size exponent p = 1.4, while microstructures with irregular grain boundaries and high subgrain density deform by GSI dislocation creep with n = 3.…”