CRYSTAL GROWTH & DESIGN
Examination of Dislocations in Ice
2002 VOL. 2, NO. 2 127-134
I. Baker Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03755 Received October 26, 2001
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〈112 h 0〉 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. Introduction Understanding both the structure and dynamics of the dislocations in a material is fundamental to understanding its mechanical properties. Over the past 40 years, the structure and mechanical properties of both laboratory-grown and natural ice Ih have received considerable attention, see, for example, refs 1-3 for reviews. Adjacent (0001) glide planes in hexagonal ice Ih are connected through randomly oriented hydrogen bonds. Glen4 first pointed out that protonic disorder in ice presents the major obstacle to the glide of the a/3〈112 h 0〉 dislocations. For a dislocation to move, the hydrogen bonds have to be appropriately reoriented. This bond reorientation occurs by the movement of ions (H3O+ and OH-) and Bjerrum defects (D type and L type); see Figure 1. That the dislocation mobility is related to protonic rearrangement was also suggested by Petrenko and Schulson5 who used an electrical technique to extract point defects, which appeared to cause the ice to become harder. However, this “hardening” was later reinterpreted to arise from suppression of sliding at the specimen/platen interface.3 There have been several attempts to relate the mechanical properties of ice to its internal defect structure.6-13 Ice has a similar crystal structure to wurtzite ZnS in which there are two types of (0001) planes, the so-called shuffle set and the glide set;9 see Figure 1. Models have been produced that attempt to relate dislocation velocities in single crystals to the applied stresses6-10 based on knowledge of the dislocations and slip planes. An outcome of this modeling is that the measured dislocation velocities can be more accurately modeled if the dislocations are assumed to move on the glide set of planes [where they can, in principle, dissociate into pairs of a/3〈011 h 0〉 partials connected by a stacking fault14]. While understanding single crystals is important, naturally occurring ice is polycrystalline, although the
Figure 1. Section in the (11 h 00) plane showing both ionic (H3O+, OH-) and Bjerrum (L, D) point defects and a dislocation (⊥) on the shuffle set of (0001) basal planes in the ice Ih structure. The oxygen atoms are indicated by the large open circles and the hydrogen atoms are indicated by the small black circles. The short unconnected lines containing hydrogen atoms indicate out-of-plane bonds. The Burgers’ vector of the dislocation is a/3〈112 h 0〉. Courtesy of E. M. Schulson.
grain size can be very large (from 1 to 1000 mm). Only recently have studies attempting to correlate the internal defect structure with the mechanical properties of polycrystalline of ice been successfully initiated.15-17 Of particular importance in polycrystals are both the mechanisms of grain boundary generation of dislocations and the interaction of gliding dislocations with grain boundaries. For example, the model used to explain the “brittle-to-ductile transition” in ice12,13 is based on dislocation pile-ups, which have only recently been observed unambiguously.17 The purpose of this paper is to discuss the three techniques, etch pitting-replication, transmission electron microscopy, and X-ray topography, which have been used to examine dislocations in ice. The advantages and disadvantages of each technique are described, and some of the more significant observations are outlined. Etch Pitting Etch pitting, which has been used for dislocation examination in a large number of materials, involves coating the surface of ice with a Formvar solution. This
10.1021/cg0100282 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/19/2002
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Figure 2. Optical micrograph of etch pits in two grains, GI and GII, on either side of a grain boundary (GB) in ice. Note that the lower grain is being viewed almost perpendicular to the basal plane.
Figure 3. Schematic demonstrating how etch-pitting replication cannot reveal the true dislocation distribution and density in a material. A dislocation is shown expanding under a shear stress, τ, but the etch pit reveals only a single pit regardless of the length of the dislocation. Further, etch-pitting used to reveal dynamic behavior would probably cause the dislocation to be pinned at the surface as indicated. The dislocation loop would not be observable at all using etch-pitting since it does not intersect a surface.
produces a pit where a dislocation intersects the surface, and forms a replica of the pit, see Figure 2. Either the etch-pit itself or the replica can then be examined using either an optical microscope or a scanning electron microscope. There have been a number of studies using this technique,18-25 which have revealed valuable information about dislocations in ice, such as their density at zero or very low strains and the fact that dislocations play a role in the plastic flow of ice. Etch pitting has been extensively used to examine nonbasal dislocations in ice.18-21 However, basal dislocations, which play the dominant role in viscoelastic flow of ice, are difficult to observe. Etch tracks or grooves corresponding to moving dislocations can be observed, allowing estimates of dislocation velocities, but again these are produced by gliding nonbasal dislocations.21 In addition to the difficulty of “imaging” basal dislocations, the etch-pitting technique has four major problems. (1) The technique suffers from the fundamental problem that three-dimensional dislocation arrangements are characterized solely by surface examination; see Figure 3. Thus, a dislocation expanding under a shear stress will produce only a single etch pit regardless of the length of the dislocation. Also, a dislocation loop that does not intersect a surface would be unobservable.
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(2) Sinha21 has pointed out that poor surface preparation can produce changes in the surface defect structure. In any case, the near-surface region is likely to be quite different from the bulk, and, thus, bulk dislocation densities and distributions cannot be observed, a feature confirmed in X-ray synchrotron-based topographic observations17 and evident by simple inspection of the schematic in Figure 3. (3) The size of the etch pits, typically >3 µm, places an upper limit on the dislocation density that can be examined of 1 × 1011 m-2. (4) Interpretation is often difficult. For example, features around a deformed notch where no etch-pits are present have been interpreted as being either dislocation free zones or having a very high dislocation density!22,23 Also, etch pitting-replication studies purportedly showing dislocation pile-ups against grain boundaries23 may have actually revealed emission of dislocations from the grain boundaries. Finally, etchpitting used to reveal dynamic behavior probably causes the dislocation to be pinned at the surface; see Figure 3. Sinha19,21 refined the etch-pitting technique so that “whiskers” of Formvar corresponding to the dislocation core are produced at the bottom and the sides of etch pits. This whisker technique reveals both basal and nonbasal dislocations and shows the three-dimensional structure of the dislocations. It also allows higher dislocation densities to be examined, for 0.25 µm whiskers, up to, say, 1 × 1013 m-2, which is a very high dislocation density for ice. However, this refined technique is still limited to revealing dislocations that intersect a surface and complex dislocation arrangements are difficult to observe. Transmission Electron Microscopy There have been a number of attempts to use transmission electron microscopy (TEM) to study the internal defect structure of ice.26-35 TEM is a practical application of Braggs’ law for imaging crystals and utilizes a single energy or wavelength of electrons incident on the specimen. The resulting micrographs are two-dimensional projections of the three-dimensional structure of a material. The attraction of the technique is its high resolution. For bright field imaging, image widths are typically 10-20 nm, but with weak-beam imaging dislocation image widths of ∼1 nm can be obtained. Using the latter technique, if dislocations dissociate into partials, the partials can thus be resolved if their separation is greater than ∼1.5 nm. This is important since, as noted earlier, in ice, there are two types of basal planes in ice on which slip could occur, the socalled shuffle set of planes and the glide set of planes; see Figure 1. The planes of the shuffle set are more widely spaced, but the packing of the oxygen atoms in the glide set resembles that in a close-packed hexagonal metal. Dislocations on the glide set can potentially lower their self-energy by dissociating into two partials of a/3〈011 h 0〉 type. For example, the a/3〈21 h1 h 0〉 screw dislocation could dissociate into two 30° partials, whereas the a/3〈21 h1 h 0〉 60° dislocation would dissociate into an edge partial and a 30° partial. Fukuda et al.26 have estimated that the separation of these partials is around 20 nm. Thus, the particular appeal of TEM for ice is
Examination of Dislocations in Ice
Figure 4. The Lang setup for conventional X-ray topography. The crystal is set to the Bragg angle for a particular set of planes and translated back and forth through a monochromatic X-ray beam. The X-ray beam is a vertical line collimated using slit S1. Slit S2 remains fixed, while the photographic plate and specimen are translated together.
that the dissociation of a a/3〈21 h1 h 0〉 dislocation into a/3〈01 h 10〉 partials would be clearly observable, and, hence, slip on the glide set of planes would be confirmed.
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Other fine structure of dislocations would also be easily observable with TEM. While the use of a TEM is appealing, there are three intrinsic problems associated with its application to the study of ice: (1) It is difficult to transfer ice specimens to the TEM and keep them there without either sublimation or frost formation. However, these problems can be ameliorated using cold stages and cryotransfer equipment. (2) Ice is very sensitive to ionization damage by the electron beam. For example, during dislocation analysis of ice, which requires the observation of the dislocations in many different diffraction conditions, Falls et al.27 found that by the time the analysis was complete numerous small voids had formed and the dislocation arrangements had changed substantially. This problem can also be ameliorated, but not overcome, by setting up diffraction conditions away from the area of interest and imaging the area only for the time required to take an image. This technique has been automated on some modern TEMs. (3) It is extremely difficult to prepare thin foils of ice from bulk samples. Another issue is that since electrons interact strongly with matter, the maximum usable thickness of a specimen is ∼1 µm. There is some question whether TEM specimens this thin can be representative of bulk ice.
Figure 5. Dislocations (dark lines) and stacking faults (gray areas) in a single crystal of ice imaged using the conventional Lang method. Courtesy of T. Hondoh.
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TEM studies of ice have revealed27,28 dislocations, stacking faults, and brine pockets (in ice grown from saltwater). However, these features have been observed in samples produced either by condensing water vapor onto a cold substrate in the TEM or by making a thin ice film from water rapidly frozen between polymer filmcovered grids. There have been no published attempts of the study of dislocation structures from bulk material. X-ray Topography Similar to the TEM, X-ray topography (XT) is also a practical application of Braggs’ law, and, like TEM images, X-ray topographs are two-dimensional projections of the three-dimensional defect structure of a material. Ice is a particularly suitable for X-ray topography for three reasons: (1) It has a relatively low molecular weight, and, hence, a low X-ray absorption coefficient, allowing thick specimens to be used and bulk structures to be observed. (2) Crystals can be grown with a sufficiently low dislocation density that the motion of individual dislocations can be followed. (3) Dislocations can be made to move slowly enough that their motion under stress can be observed. While XT has many advantages, it suffers from the disadvantage that X-ray extinction distances are large. Thus, images of dislocations in ice are typically 20 µm in width (although image widths as small as 1 µm are possible using weak-beam imaging on a synchrotron36,37). This means that the technique is usually confined to high quality crystals that have been carefully grown and handled to ensure a very low dislocation density of
