RESTRAINTS ON THIN SECTION ANALYSIS OF GRAIN GROWTH IN UNSTRAINED POLYCRYSTALLINE ICE

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RESTRAINTS ON THIN SECTION ANALYSIS OF GRAIN GROWTH IN UNSTRAINED

POLYCRYSTALLINE ICE

A. Gow

To cite this version:

A. Gow. RESTRAINTS ON THIN SECTION ANALYSIS OF GRAIN GROWTH IN UNSTRAINED POLYCRYSTALLINE ICE. Journal de Physique Colloques, 1987, 48 (C1), pp.C1-277-C1-281.

�10.1051/jphyscol:1987139�. �jpa-00226285�

RESTRAINTS ON THIN SECTION ANALYSIS OF GRAIN GROWTH IN UNSTRAINED POLYCRYSTALLINE ICE

A.J. GOW

U.S. Army Cold Regions Research and Engineering Laboratory, 72, Lyme Road, Hanover, NH 03755-1290, U . S . A .

RESUME. Des essais ont 6t6 r6dLis6s h -l°C pour &valuer les effets de la surface libre et de 116paisseur de lames minces sur la croissance des grains dans le cas dU6chantillons trbs poreux et non d6forn6s de glace polycristalline h petits grains.

Les resultats montrent que la croissance des grains est Ggligeable quand la taille moyenne des grains est superieure h 1,5 h 2 fois 1'6paisseur de la lame. La croissance des grains pour des sections plus 6paisses montre que la migration des joints de grains, conduisant h une augmentation de 3-4 fois la taille moyenne des grains, est virtuellement non affect6e par la presence dlun grand nombre de bulles dans la glace. L1accumulation de bulles le long des joints de grains nla pas non plus Qt6 mise en Qvidence. LUattaque en surface aux joints de grains est un p h 6 d n e caractikistique se poursuivant pendant la croissance et mis en 6vidence par le d6placement des traces de joints de grains pendant toute la croissance. La longueur totale de ces traces decroit avec llaccroissement de taille des grains indiquant un &anisme de rdsorption pendant le ph6dne.

Les mesures de croissance volumique du grain sur des 6chantillons slaccordent avec celles obtenues sur des lames minces dont 116paisseur est 2 h 3 fois plus importantes que le didtre myen du grain.

ABSTRACT. Tests were performed at -lOc to evaluate the effects of a free surface and the thickness dimensions of thin sections on the growth of grains in fine-grained, pore-rich, strain-free polycrystalline ice. Results show that negligible growth of grains occurs when the mean size of grains is more than 1.5 to 2 times the section thickness. Grain growth in thicker sections was significant for the fact that grain boundary migration, leading to 3-4 fold increases in average grain size, was virtually unaffected by the presence of large numbers of bubbles in the ice. Nor was there any evidence to indicate any concentrating of bubbles along migrating boundaries. Grain boundary grooving was a characteristic feature of most sections undergoing grain growth. This implies actual migration of grooves during grain growth. The fact that the total length of grooves decreased with increasing grain size also implies some process of groove consumption during grain growth. Three-dimensional grain growth measurements in bulk samples compared favorably with those obtained from sections two to three times thicker than the mean grain diameter.

INTRODUCTION

The purpose of this paper is to describe tests to evaluate the effects of a free surface and,the thickness dimensions of thin sections on grain growth in strain-free polycrystalline ice. Ice, because of its relative softness and trans- parency, is especially well suited to the preparation of thin sections, and in which, the sizes and shapes of crystals are readily revealed when a section is examined between crossed polaroids. As a result, thin sections have been widely used for grain size measurements, both in naturally occurring ice and in the labo- ratory. Experimental observations of grain growth include those of Jellinek and Gouda [I] and Roos [ 2 ] . Jellinek and Gouda, studying the growth characteristics

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987139

JOURNAL DE PHYSIQUE

of microcrystalline ice frozen onto glass slides, determined that the growth be- havior of ice grains under these conditions closely resembled that observed for metals. At about the same time, Gow [ 3 ] and Stephenson [4] independently determined that the same growth relationships also held for crystal growth in the firn and ice in the top hundred meters or so of polar ice sheets. However, the WOrK reported in (1) and (2) has raised questions regarding the likely two-dimensional nature of crystal growth when grain sizes begin to exceed the thickness dimensions of the thin sections themselves. In short, a potential disadvantage of using thin sections to measure grain growth in unstrained polycrystalline ice is that growth may be inhibited by free surface effects so that phenomena observed under these conditions may not accurately reflect grain growth as it occurs in a bulk sample of ice undergoing three-dimensional grain growth. In metals, for example, growth of grains may cease altogether when the mean diameter of grains is greater than about two times the thickness of the section or slice [5].

EXPERIMENTAL TECHNIQUES

Samples of porous, isotropic, fine-grained polycrystalline ice were obtained from cylindrical specimens prepared in molds by Mr. Donald Haynes of USA CRREL.

The general method of preparing those specimens was to pack ice grains into a mold, saturate them with distilled, degassed water, and freeze the resultant mix- ture. This technique (See Haynes [61 for details) removes most but not all air from the molded specimen. Horizontal slices measuring 2.54 cm in diameter were taken from the center of the molded specimen, frozen onto glass plates and then microtomed to the desired thickness. This thickness varied from one third to three times the mean diameter of grains which in this series of Sam les averaged about 0.7 mm. Bulk densities of the molded ice averaged 0.904 Mg/m

5

As soon as the sections were prepared, they were placed in liquid-tight plastic bags inside containers of water-saturated kerosene in a thermally insulated box maintained at a temperature of -1.0'

*

RESULTS AND DISCUSSION

Results obtained with two sections measuring 0.35mm and 1.20mm, in thickness respectively are presented in Figures 1 and 2. At the outset of the tests, both sections consisted of strain-free, equidimensional crystals averaging less than lmm in diameter.

In the thinner of the two sections, where the mean grain diameter was 0.7mm, or about two times the section thickness, grain size as measured by the line inter- cept method showed less than a 10% increase in the mean size of crystals after 19 days when the test was terminated (See Fig. 1). Since the driving force for grain growth in unstrained polycrystalline ice is the reduction of the total grain grain boundary area, it would appear in this particular instance the thinness of the section had minimized vertical grain boundary curvature sufficiently to inhibit grain growth even after sustained exposure to temperatures within 1°c of the melting point. Notable in this and other test sections was the extent of thermal grooving of grain boundaries as the tests progressed. The only sig- nificant signs of grain coarsening appear associated with the elimination of small grain clusters, for example at the site indicated with an X in the photograph taken on the thirteenth day of the test. Increasing the thickness of the section to 0.5mm did not lead to any significant grain growth.

However, increasing the section thickness to 1.2mm (grain size now only 0.6 times the section thickness) resulted in a three-to-four fold increase in grain diameter after 17 days (See Fig. 2). Very little growth was observed during the

that 1) most of the increase in grain size occurred between 6 and 11 days with little change thereafter 2 ) this very substantial boundary migration appears virtu- ally unimpeded by the presence in the ice of large numbers of air bubbles which continue to retain their original positions in the ice despite being swept through

Fig. 1 Example of minimal growth in a thin section (0.35mm thick) of ice consisting of crystals averaging 0.7mm in cross-sectional diameter.

Subdivisions on scale measure lmm. Thin section was main- tained at -l°C for 19 days.

All photographs are of the same area of thin section.

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Fig. 2 Example of vigorous grain growth in a thin section meas- uring 1.20mm thick. Initial

size of crystals was 0.7mm.

Thin section was maintained at -l°C for 17 days.

by the migrating boundaries 3 ) grain outlines are also clearly delineated by groov- ing (etching) of their boundaries at all stages of the growth process; an obvious reduction in the total grooved boundary length with increasing grain size implies active consumption of grooves across the surface of the section 4 ) though little additional growth occured between 11 and 17 days many grains still exhibited sig- nificant curvature of their boundaries implying perhaps that surface tension effects in the vertical boundaries had become too small (because of minimized curvature) to drive growth. Increasing the section thickness to 2.25m led to even larger sized grains after 17 days.

This is contrary to observations on grain growth in metals, for example tungsten, in which grain boundary pinning by vapor-filled micropores appears to be well established [7]. According to Bhatia and Cahn 171 isolated pores can also migrate as well as retard grain boundary migration. However, in their own work on re- crystallization of porous copper compacts Bhatia and Cahn [7] were not able to resolve whether or not moving grain boundaries can sweep pores along with them.

In the few instances where we could determine the direction of grain boundary migration, the boundaries appeared to move away from their centers of curvature.

This is the reverse of what is normally observed in grain growth in metals, that is, for grain coarsening where the driving force is the interfacial energy of the grain boundaries.

Time lapse movie technfques were also used to monitor crystal growth in greater detail. In addition, we have examined grain growth in thin sections taken at intervals from bulk samples exposed to near-melting temperatures for extended periods of time and confirmed the fact that viable measurements of grain growth can be made directly from a single section if its thickness exceeds the initial mean grain diameter by two to three times.

While noting the restraints that thin section dimensions can impose on grain growth measurements in the laboratory, there are however, instances where the preparation of sections, thin with respect to the average crystal size, is essen- tial to the preservation of original crystal textures and orientations. This situation is especially true of highly deformed ice extracted from glaciers, for example, drilled cores from deep within the Antarctic ice sheet which were found to have undergone extensive recrystallization after several years' storage at temperatures of -15 to -20'~ [8]. Such recrystallization would not have been discovered if thin sections of the cores had not been prepared and photographed within an hour or two of pulling the cores to the surface. These sections showed no detectable changes in texture and orientation when they were re-examined ten and sixteen years later, and it was only after new sections made from the stored cores were compared with the original sections that the full extent of this re- crystallization was determined.

REFERENCES

[I] Jellinek, H. H. G., and Gouda, V.K., Phys. Acta. Sol., 31, 1969, 413-423.

[2] Roos, D. V. D. S., J. Glaciol., 6 , 1966, 411-420 [3] Gow, A. J., J. Glaciol., 8, 1969, 241-252.

[4] Stephenson, P. J., In Physics of Snow and Ice (ed. H. ~ura), Inst. Low Temp.

Science., 1, pt. 2, 1967, 725-740.

[ 5 ] Beck, P. A., Kremer, J. C., Demer, L. J., and Holzworth, M. L., Trans. Am.

Inst. Min. Metall. Engrs., 175, 1948, 372-380.

[61 Haynes, F. D., US Army Cold Regions Research and Engineering Laboratory Res. Report 312, 1973, 1-21.

[7] Bhatia, M. M., and Cahn, R. W., Proc. R. Soc. Lond. 362, 1978, 341-360.

[81 Gow, A. J., In Glaciological Data, Report GD-8, World Data Center A for Glaciology, University of Colorado, 1980, 1-139.