OF SHEAR ZONES IN POLYCRYSTALLINE

Transcription

1 Tectonophysics, 78 (1981) Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands THE DEVELOPMENT CAMPHOR OF SHEAR ZONES IN POLYCRYSTALLINE J.L. URAI and F.J. HUMPHREYS Instituut voor Aardwetenschappen, Budapestlaan 4, Utrecht (The Netherlands) Department of Metallurgy and Materials Science, Imperial College, London SW7 (Great Britain) (Received January 26, 1981) ABSTRACT Urai, J.L. and Humphreys, F.J., The development of shear zones in polycrystalline camphor. In: G.S. Lister, H.-J. Behr, K. Weber and H.J. Zwart (Editors), The Effect of Deformation on Rocks. Tectonophysics, 78: Thin polycrystalline specimens of rhombohedral camphor have been deformed in pure shear in the temperature range K. The deformation processes, dynamic recrystallization and the development of microstructure were followed by transmission polarized light microscopy, When the stress distribution in the specimens was favourable, shear zones were initiated. The development of microstructure changed drastically above K: the shape of the dynamically recrystallized grains which was equiaxed below this temperature became strongly elongated. This change, probably due to the development of a marked anisotropy in grain boundary mobilities, and the activation of new slip systems made the development of shear zones a more frequently occurring phenomenon above K. INTRODUCTION In-situ observations of the development of microstructure during the deformation of polycrystalline analogue materials can be a useful aid to the interpretation of the microstructures of deformed minerals. Results have been reported for ice (Wilson, 1979), paradichlorobenzene (Means, 1980), and camphor (Urai et al., 1980). Camphor (C10H160) occurs in several forms, the actual transformation temperatures being dependent both on the purity of the material and the proportions of d- and l-isomers present (Urai et al., 1980): 452K 365i7K 243K Liquid + f.c.c. T==- Rhombohedral Rhombohedral II In a previous publication (Urai et al., 1980) we have described the experimental methods by which the deformation in compression of thin ( Elsevier Scientific Publishing Company

2 678 Pm) specimens of Rhombohedral I camphor could be observed in transmitted polarized light, and have reported our observation of slip, twinning, and dynamic recrystallization which occur in the temperature range K. The present paper reports an investigation into the high strain behaviour of this material where the heterogeneous distribution of the deformation results in the formation of shear zones, based on the same set of experiments as Urai et al. (1980). The heterogeneous nature of the deformation is governed by three types of factors : (1) The material properties (strongly temperature dependent), such as the critical stresses for slip and twinning, and for grain boundary sliding related to diffusivity and grain boundary energies. (These are the most important factors determining the change in deformation behaviour above K) (2) The anisotropy of the grain structure (shap.e and lattice orientation) which was particularly important in our experiments in which the specimens of size 7 X 7 X 0.15 mm contained elongated grains up to 2 mm long. (3) The geometry of the test. The friction between specimen and confining glass plate and the constraints due to the deformation ram also influence the stress distribution in the specimen. However, this was a rather unpredictable process and its presence could only be deduced from deviations from the bulk strain, predicted on base of the grain structure alone. The deformation behaviour of camphor changes markedly above K and the results of experiments carried out in the two temperature regimes are reported separately. LOW TEMPERATURE REGIME (below 315?r 5K) The initial microstructure is shown schematically in Fig. 1. Grains oriented with their (001) plane parallel (A) or perpendicular (B) to the specimen plate deformed in different ways. Slip in this temperature range only occurred on the (001) plane, and thus only grains of orientation B could undergo substantial glide. Grains such as A deformed predominantly by twinning. At low strains, small equiaxed grains formed by dynamic recrystallization at regions of strain heterogeneity such as kink boundaries, twins and grains boundaries. At higher strains, the deformation was localized in shear zones which made angles of 45 to the compression direction. The initiation of these zones could occur in different ways, as illustrated in Fig. 2 which is a sequence of stills from a time lapse tine record of the experiments. Grain B (Fig. 2a) favourable oriented for slip (like B in Fig. 1) constitutes a plane of weakness in a direction og high shear stress. Stress concentration at the end of this grain results in the initiation of a shear zone, at C (Figs. 2a and 2b).

3 679 Fig. 1. Sketch of the strongly orientation dependent deformation behaviour of the individual grains, caused by the confining glass plates. Grains A and B show the behaviour in the low temperature regime, where (001) is the only slip system operating: grain A is unfavourably oriented for slip and will deform by twinning, while grain B deforms by (001)~slip and develops kinks. In the high temperature regime shown by grains C and D there is evidence for the initiation of a non-(001) slip system (dotted lines); therefore twinning is absent. In grain A (like A in Fig. 1) unfavourably oriented for slip, a narrow movement zone is rapidly initiated at D, leading to the formation of a shear zone. The initial rapid movement could have been an intragranular fracture, but this could not be unambiguously identified. The small grains formed by dynamic recrystallization at C and along D appear to be initially of random orientation. As deformation proceeds, these grains are continuously renewed by recrystallization, maintaining a grain size which is a function of temperature and strain rate (Urai et al., 1980). A preferred orientation, one of easy glide, gradually develops. Deformation then becomes easier in these regions, and shear zones develop (Fig. 2~). Mature shear zones consisted of fine grains (-10 pm) due to the relatively high strain rate in the shear zone, with a strong preferred orientation. Often a darker contrast was observed along the centre line (E in Fig. 2d). This may have been due to a change of orientation. HIGH TEMPERATURE REGIME ( K K) Shear zones were more easily initiated. This is illustrated by a sequence of micrographs (Fig. 3). In this high temperature range twinning is absent and the dynamically recrystallized grains are strongly elongated. In Urai et al. (1980) we tentatively suggested that the growth of elongated grains into apparently undeformed grains could be due to elastic energy effects. More recent results and the measurement of grain shape changes within the specimens indicate the likehood of slip occurring in grains such as C in Fig. 1 along planes other than (001). In Fig. 3 it can be seen that grains like A, which are favourably oriented for slip on (OOl), grow rapidly along planes of high shear stress into the dark green (B) in which (001) slip cannot take place.

4 Fig. 2. Shear zone development in the low temperature regime. T= 300K e - 10e4 S-. Time lapse between the photographs is approx. 40 min. Grain B, favourably oriented for (001)slip, forms a line of weakness in grain A, and a conjugate set of two shear zones develops (CD and BC). Note the displacement of two halves of shear zone BC in a later stage by a new shear zone FG (2d). Compression axis parallel to long edge of photographs. Crossed polarizers. Specimen thickness 0.15 mm.

5 681 We now suggest that undetected slip occurred in the grains like B along planes of high shear stress, and that the dynamically recrystallizing grain is consuming these deformed regions. This explanation, shown schematically in Fig. 1 (grains c and d) is supported by the micrographs of Fig. 3, where they show the shape change of the contact between shear zone AC and grain B, and keeping in mind that the specimen is only 0.15 mm thick, this can only be explained by slip in a direction parallel to the specimen plane. Thus, as shown in Fig. 3, regions which are essentially single crystals of easy glide orientations propagate through a differently oriented crystal, strain concentrates there, and a shear zone is formed. The interaction of shear zones was frequently observed and this is also seen in Fig. 3. At the intersection of AC and DE in Fig. 3c and 3d much smaller dynamically recrystallized grains are formed. The tine films of such processes clearly show the way in which shear zones may become inactive because of intersection by another. This process is beginning in Fig. 3e, where at G a new zone starts growing towards A, cutting off branch HJ which has came into an unfavourable position. Static recrystallization of the specimens deformed in the high temperature regime produced grains with a large variety of sizes and elongated shape (Fig. 3f). The elongate shape of the new grains, resulting from both dynamic and static recrystallization, is a remarkable feature of the high temperature regime, probably due to the development of a marked anisotropy in the grain boundary mobilities. DISCUSSION In both the low and high temperature range of our investigation the origin and survival of the shear zones is due to the favoured growth of grains of easy glide orientation. More complicated deformation processes are suggested by the presence of small regions of other orientations with the zones. In all cases when the specimens were allowed to statistically recrystallize, a microstructure consisting of large grains with an apparently poor preferred Fig. 3 (pp ). Shear zone development and interaction in the high temperature regime. T = 323K; E - 10m5 s-i. Time lapse between photographs: (a) undeformed specimen, (b) after 52 min; (c) after 148 min; (d) 181 min; (e) 220 min; (f) after 12 hrs of static recrystallization. Scale bar is 0.5 mm, specimen thickness 0.15 mm. Crossed polarizers, compression axis parallel to long edge of the photograph. 3a-d. Development of shear zone AC by growth of A into grain B. This grain (B) has its optic axis parallel to the line of sight and thus will stay dark when deformed by non- (001) slip. 3d and 3e. Development of shear zone DE, which is displacing HJ of shear zone AC. In Fig. 3e HJ has arrived into such an unfavourable orientation that a new shear zone is initiated by the growth of grain G towards A. See text for further discussion.

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7 Fig. 3. (for legend see p. 681). 683

8 684 orientation resulted (see fig. 6a in Urai et al., 1980) and it was impossible to recognize regions of former shear zones. Thus the geometric softening is an essential characteristic of these zones. It is important in forming the zones and in maintaining them. Regions of non-easy glide orientations are presumably formed during the deformation, but are preferentially consumed. When the deformation is stopped, grains of all orientations are able to grow, thus forming a texture with an apparently poor preferred orientation. This latter observation, whereby static annealing effectively obliterates the microstructure characteristic of the deforming specimen, could well be of significance with regard to the interpretation of the microstructure of naturally deformed minerals. The shear zones formed in this temperature regime are somewhat similar to those formed in deformed magnesium (Burrows et al., 1979) and geometric softening processes are also thought to be important in the development of shear zones in quartzites (White et al., 1980). CONCLUSIONS (1) The deformation of polycrystalline specimens of rhombohedral camphor is strongly dependent on the temperature. (2) Shear zones develop at ail temperatures investigated, by geometric softening accomplished by process of dynamic recrystallization. (3) At low temperatures the dynamically recrystallized grains are equiaxed whereas at high temperatures they are elongated. (4) Static recrystallization after deformation may not only change the grain microstructure in the shear zones, but also the grain lattice orientation distribution. ACKNOWLEDGEMENTS This work was supported by the Science Research Council and by a grant to J.L.U. form the Molengraaff foundation. The authors would like to acknowledge valuable discussions with their colleagues in the Geology Department at Imperial College, in particular Dr S.H. White. One of the authors (J.L.U.) wants to thank P.F. Williams for valuable suggestions on the initiation of non-(001) slip in camphor. REFERENCES Burrows, S.E., Humphreys, F.J. and White, S.T., Dynamic recrystallisation and textural development in magnesium deformed in compression at elevated temperatures. Proc. 5th Int. Conf. on Strength of Metals and Alloys, I, p Means, W.D., High temperature simple shearing fabrics : a new experimental approach. J. Struct. Geol., 2:

9 Urai, J.L., Humphreys, F.J. and Burrows, S.E., In situ studies of the deformation and dynamic recrystallisation of rhombohedrai camphor. J. Mater. Sci., 15: White, S.H., Burrows, S.E., Carreras, J., Shaw, N.D. and Humphreys, F.J., J. Struct. Geol., 2: Wilson, C.J.L., Boundary structures and grain shape in deformed multilayered poiycrystalline ice. Tectonophysics, 57: T19-T25.