Examination of Dislocations in Ice

Abstract

Three techniques have been used to study dislocations in ice: etch pitting-replication, transmission electron microscopy, and X-ray topography (XT). Each is considered, and it is shown that the most useful is XT. This is because ice has low absorption of X-rays and can be produced with a low dislocation density, thus, allowing relatively thick specimens to be studied. The many useful observations that have been made with conventional XT are presented. However, the introduction of high-intensity synchrotron radiation showed that conventional XT observations are of dislocations that have undergone recovery. Thus, the important dynamic observations and measurements that have been made using synchrotron XT are also outlined. In single crystals, it has been shown that slip mainly occurs by the movement of screw and 60° a/3〈112̄0〉 dislocations on the basal plane. In addition, the operation of Frank-Read sources has been clearly demonstrated, and dislocation velocities have been measured. In contrast, in polycrystals, dislocation generation has been observed to occur at stress concentrations at grain boundaries, and this completely overwhelms any lattice dislocation generation mechanisms. The nature of faulted dislocation loops has been determined in both polycrystals and single crystals.

An evaluation of the rate-controlling flow process in Newtonian creep of polycrystalline ice

“…Directly observing dislocations and semi-quantifying the dislocation density can use X-ray topography [37,38]. However, because of its low resolution, X-ray topography cannot give useful information when the dislocation density is higher than ∼1 × 10 9 m −2 (using TEM to observe dislocations in ice is very difficult).…”

Imaging dislocations in ice

“…The dislocation loop would not be observable at all using etch-pitting since it does not intersect a surface. Reproduced from Baker (2002) with permission of the publisher. crystals either from a temperate glacier or that have been carefully grown and handled to ensure a dislocation density of Ͻ1 ϫ 10 9 m Ϫ2 .…”

“…Slit S2 remains fixed, while the photographic plate and specimen are translated together. Reproduced from Baker (2002) with permission of the publisher. Fig.…”

Creep of granular ice with and without dispersed particles

“…Although many studies emphasize the importance of dislocations, dislocation-based mechanisms have not been fully incorporated into constitutive models of ice because of the difficulty in quantifying the dislocation density. Some studies (Baker, 1997(Baker, , 2002 using X-ray topography provide a way to directly observe dislocations and semi-quantify the dislocation density. However, because of its low resolution, X-ray topography cannot give useful information when the dislocation density is higher than $1 Â 10 9 m -2 (Baker, 2002), nor does it give any information on the mobile fraction of dislocations.…”

“…Some studies (Baker, 1997(Baker, , 2002 using X-ray topography provide a way to directly observe dislocations and semi-quantify the dislocation density. However, because of its low resolution, X-ray topography cannot give useful information when the dislocation density is higher than $1 Â 10 9 m -2 (Baker, 2002), nor does it give any information on the mobile fraction of dislocations. However, a methodology (Cole, 1995;Cole and Durell, 2001) has been developed to calculate the effective mobile dislocation density from a specimen's cyclic response and this, in turn, supports a quantitative assessment of dislocation processes during ice flow.…”