Azuma, N.

Hamann, Ilka

Kipfstuhl, Sepp

English

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Figure 5: sGB density evolution of creep tests with total strain. Mean of at least 4 measure points.Microstructural deformation features in artificially deformed iceand in polar ice cores (EDML, Antarctica)Ilka Hamann1,2, Nobuhiko Azuma1, Sepp Kipfstuhl21Nagaoka University of Technology, Nagaoka, Japan2Alfred-Wegener-Institute of Polar and Marine Research, Bremerhaven, GermanySummary and ConclusionsAcknowledgementsReferencesIntroductionExperimental resultsIce of polar ice sheets is an important information source for studying the past climate. For a correct interpretation of the age ofthe ice knowledge on flow and deformational processes following the deposition is essential.Recently obtained data on sub-grain boundaries (sGB) of the deep ice core from EPICA-Dronning Maudland (Hamann et al. inprep., Figure 1 and 2) reveal that an onset of polygonization/sub-grain rotation recrystallization or an onset of migrationrecrystallization cannot be found. Indeed this study shows that sub-grain formation is active in any depth of EDML ice core,which indicates that the classical tripartition of recrystallization regimes (1. Grain growth, 2. Polygonization/sub-grain rotationrecrystallization, 3. Migration recrystallization), which is the standard conception (e.g. Duval 2000), is not easily applicable hereand has to be reconsidered.In order to understand the sGB evolution with ongoing deformation under controlled conditions, creep experiments followingstandard procedures have been conducted and creep test samples have been prepared as thin sections to investigatemicrostructures (Kipfstuhl et al. submitted).Bons, P.D., Jessell, M.W. 1999. Micro-shear zones in experimentally deformed OCP. J. Struct. Geol. 21, 323-334.Duval, P. 2000. Deformation and dynamic recrystallization of ice in polar ice sheets. in: Physics of ice core records. p. 103-113. ed. T. Hondoh. Hokkaido University Press.Sapporo.Föll, H. 2000. Defects in Crystals. Hyperscript. www.techfak.uni-kiel.de/matwis/amat/ def_en/index.htmlHamann, I., Kipfstuhl, S., Faria, S.H., Azuma, N. Sub-grain boundaries in EPICA Dronning Maudland (EDML) deep ice core. in preparation.Humphreys, F.J. and Hatherly, M. 2004. Recrystallization and Related Annealing Phenomena. Pergamon.Kipfstuhl, S., Hamann, I., Lambrecht, A., Freitag, J., Faria, S. H., Grigoriev, D., Azuma,N. 2006. Microstructure mapping a new method for imaging deformation-induced microstructural features of ice on the grain scale, submitted to J. Glaciol.Wang, Y., Kipfstuhl, S., Azuma, N., Thorsteinsson, Th., Miller, H. 2003. Ice-fabrics study in the upper 1500 m of Dome C (East Antarctica) deep ice core. Ann. Glaciol.  37, 97-104.Sub-grain boundarydensity evolutionFigure 1: Grain size evolution during creep tests and concurrent annealing tests. A: at -5°C. Green lineare data of grain growth experiment conducted by Nishimura. B: at -20°C. Linear fits are added after theparabolic growth law R=k*t1/n (A=a+b*t) (Humphreys and Hatherly 2004), to compare conditions at -5°Cand -20°C.ABExperimental conditionsallow grain growth(Figure 1). Increaseunder creep is slowerthan under pure graingrowth conditions (greendata) from olderexperiments.Concurrently conductedannealing experimentssurprisingly showapproximately the sameincrease with time ascreep tests, maybe dueto sample size.A significant differencewas recognized withdifferent temperatures.Figure 2: Photomicrographs after 8 days experimentat -4.8°C. A: Annealing only. B: Creep test with0.6MPa stress and 10.7% total strain.Grain size evolution Grain shape evolutionGrain shapes are significantly more irregular in samples exposed to creepcompared to those from annealing experiments (Figure 2). This is most obvious inhigher strained samples, but measurable in all (Figure 4), b a newly inventedparameter using the ratio of the real perimeter and the convex perimeter (Figure 3).Figure 4: Mean perimeter ratio evolution with strain. A value of 1 would result from a material with no grainswith inward bulges (e.g. ideal hexagons filling a surface). Red: -5°C experiments. Blue: -20°C.Figure 3: Schematic illustration of the definitionof convex perimeter and real perimetermeasured by Image Pro. This parameter isindependent of size and elongation of objectsand is used here to measure the regularity ofgrains.sGB density definition: sum of all sGB lengths per area (ca. 5 mm by 6 mm of a sample surface).It increases between 0.5 to 2% strain and reaches a steady state with approximately 3.5 mm-1 (Figure 5).At highest strains it increases again. With increasing strain the variability of sGB densities inside thesample increases as well. Due to slow experiment at low temperatures only few experiments could beconducted so far. However, they follow the same trend.Sub-grain boundary typesThe appearance of sGB is manifold. Variations occur in shapes and intensities (grey values). Similar to previous study on Antarctic ice sheets different types could bedistinguished (Figure 6 and 7).1 (z). Irregular (usually zigzag- or step-shaped) often appearing in networks with one reticule direction at high angle and one parallel to the basal plane trace2 (c). Regular, straight and orthogonal or at high angle to basal plane occurring solely or in groups of few with similar orientation but at most sub-parallel3 (p). Regular, straight and parallel to basal plane occurring in swarms of many exactly parallel sGB in one grain.Regarding their orientation with the crystals c-axes and basal planes (Figure 7) type 2 is distinguished as tilt boundary (see Figure 8) representing the classicalevolution of sGB by polygonization, type 3 as twist boundary or micro-shear zone (Bons & Jessel 1999) and type 1 seems to represent a mixture of both, e.g.alternating parts of a tilt and a twist boundary.Not all sGB can be doubtlessly classified especially if they show no distinct shape.Figure 6: Examplefor sGB types inphotomicrographof vertical sectioncut from creep testsample (-4.8°C,0.6MPa, 10.7%total strain).GB=grainboundary,p=parallel sGB,z=zigzag typesGB, C=classicalpolygonizationtype sGB,?=bowed, notclassified type sGBFigure 7: Example of microstructure mapping and etch pit method combination. Photomicrographs of vertical section from creeptest sample (-4.8°C, 0.4MPa, 1.27% total strain.) A: Sublimated surface showing grain boundaries (GB) and different types ofsGB (indication see Figure 6). B: Same sector as a, between crossed polarizers. As section is ≈3mm thick grain boundaries lieoblique in section, changing interference colours are visible along them. However clear change in orientation is visible acrossGB, but not across sGB. C: Same sector as a and b after application of etch pit method. Numbers indicate higher magnificationdetail images shown in e,d,f. White bar indicates trace of basal plane in cutting surface according to etch pit shape. (Scale:a,b,c width ~1.14mm; e,d,f width ~305µm).Figure 8: Schematicillustration of sGBtypes. A: Simplifiedshape. B & C:Theoretical types(after Föll, H. 2000).a b cDue to its eye-catching and easily recognizable nature, statistic on parallel sGB has been done. The fast decreasing evolution of their frequency can be explainedin combination with the total sGB density. As sGB density is increasing with strains of 0.5 to 3 %, the fraction of parallel type is decreasing with a similar slope.This finding suggests, that mainly other types are produced in the conducted experiments. Since new other sGB are introduced the fraction of parallel ones isdepleted.Figure 9: Frequency of parallel type of sGB in length percent with respect to all versus strain.Observations of microstructures from samples deformed by creep experiments reveal information on the formation of thesefeatures. During experiments under conditions, which allow grain growth, the grain shape changes significantly with increasingstrain. The grains become more irregular with bulging grain boundaries, indicating that grain boundary migration is at least partlycaused by internal strain energy differences. This is the case in experiments conducted at high temperature (-5°C) as well asunder low temperatures (-20°C), where migration recrystallisation was not expected according to standard conceptions.With increasing strain from 0.5 to 2% sub-grain boundary density increases, reaching a stable value at approximately 3% strain. As strainrates reached a steady state at ca. 2 to 3% during these experiments, an explanation might be the following. The decreasing trend of thecreep rate corresponds to the evolution of the substructure of the crystal, namely the production of dislocation walls and sub-grainboundaries. During primary transient creep the strain rate decreases continuously, which means that the deformation speed slows down, i.e.the material hardens. Taking into account that the deformation is to a large extent accomplished by introduction and movement ofdislocations, one cause for this decrease of deformation rate is the production of obstacles, which hinder the motion of dislocations. Mostcommon obstacles for dislocation, apart from impurities (which are absent here as pure ice is used) are dislocation walls and sub-grainboundaries. As the production of such obstacles continues the deformation rate keeps decreasing until a steady amount of obstacles isreached. This probably falls together with the achievement of maximum value of the sub-grain boundary density. As dislocations cannotmove freely stress must be accumulating around obstacles. Due to the fact that deformation rate increases again at some point a newprocess must take over the main deformation activity. This might be rotation of crystals or grain boundary migration. That grain boundarymigration intensifies and starts to dominate can be observed by the grain shape parameter.This study was done during a one-year research stay in Azuma-Laboratory at NagaokaUniversity of Technology in Japan. I wish to express appreciation to the laboratory members (Dr. M. Takada, A. Shigekuni, Y. Oba, T.Kokure,T. Nakamura, K. Oba, K. Anno, H. Kobayashi and many others), who helped me in so manifold ways. Technical and life support were invaluable. I thank Ch.Weikusat for patience and staying power during conducting the mainpart of the image analysis.Figure 1: Examples ofmicrostructures in EDMLice core. Grainboundaries (GB), sliplines (SL) and sub-grainboundaries (sGB) areindicated. Lower rightcorner shows the depthof the samples. Differentgrey values of featuresin the same picture aredue to size of the etchgroove produced bysublimation dependingon number of bondingneighbours of atoms(viz. order of the region,number of dislocations).The different grey valueshave been interpreted asdifferent stages of sGBformation (light-young,dark-old) by Wang et al.2003. Note that sGBoccur in many differentdepth ranges and evenappear very similar inshape and distribution.Figure 2: sGB frequencies with depth in EDML ice core. A:Frequencies of grains with sGB with respect to all grains. B: As sGBstatistic on exact length of of sGB was not possible numbers of sGBper grain have been counted. Mean values are given in B. Note:frequencies of sGB-bearing grains and mean number of sGB pergrain do not change with depth. Therefore an onset of polygonizationby increased production of sGB could not be proofen.

Microstructural deformation features in artificially deformed ice and in polar ice cores (EDML, Antarctica)

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