Texture and Anisotropy
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INSTITUTE OF PHYSICS PUBLISHING REPORTS ON PROGRESS IN PHYSICS Rep. Prog. Phys. 67 (2004) 1367–1428 PII: S0034-4885(04)25222-8 Texture and anisotropy H-R Wenk1 and P Van Houtte2 1 Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA 2 Department of MTM, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium E-mail: [email protected] Received 17 February 2004 Published 5 July 2004 Online at stacks.iop.org/RoPP/67/1367 doi:10.1088/0034-4885/67/8/R02 Abstract A large number of polycrystalline materials, both manmade and natural, display preferred orientation of crystallites. Such alignment has a profound effect on anisotropy of physical properties. Preferred orientation or texture forms during growth or deformation and is modified during recrystallization or phase transformations and theories exist to predict its origin. Different methods are applied to characterize orientation patterns and determine the orientation distribution, most of them relying on diffraction. Conventionally x-ray pole- figure goniometers are used. More recently single orientation measurements are performed with electron microscopes, both SEM and TEM. For special applications, particularly texture analysis at non-ambient conditions, neutron diffraction and synchrotron x-rays have distinct advantages. The review emphasizes such new possibilities. A second section surveys important texture types in a variety of materials with emphasis on technologically important systems and in rocks that contribute to anisotropy in the earth. In the former group are metals, structural ceramics and thin films. Seismic anisotropy is present in the crust (mainly due to phyllosilicate alignment), the upper mantle (olivine), the lower mantle (perovskite and magnesiowuestite) and the inner core (ε-iron) and due to alignment by plastic deformation. There is new interest in the texturing of biological materials such as bones and shells. Preferred orientation is not restricted to inorganic substances but is also present in polymers that are not discussed in this review. 0034-4885/04/081367+62$90.00 © 2004 IOP Publishing Ltd Printed in the UK 1367 1368 H-R Wenk and P Van Houtte Contents Page 1. Introduction 1370 2. Measurements of textures 1370 2.1. Overview 1370 2.2. X-ray pole-figure goniometer 1371 2.3. Synchrotron x-rays 1371 2.4. Neutron diffraction 1374 2.5. Transmission electron microscope 1374 2.6. Scanning electron microscope 1375 2.7. Comparison of methods 1375 3. Data analysis 1376 3.1. Orientation distributions and texture representations 1376 3.2. From pole figures to ODF 1378 3.3. Use of diffraction spectra 1378 3.4. Statistical considerations of single orientation measurements 1379 3.5. From textures to elastic anisotropy 1380 4. Polycrystal plasticity simulations 1380 4.1. General comments 1380 4.2. Deformation 1381 4.3. Recrystallization 1383 5. Important texture types in metals 1385 5.1. Fcc metals 1385 5.2. Bcc metals 1389 5.3. Hcp metals 1390 5.4. Phase transformations 1391 6. Ceramic textures 1394 6.1. Bulk ceramics 1394 6.1.1. α-alumina (Al2O3) 1394 6.1.2. Silicon nitride (Si3N4) 1394 6.1.3. Zirconia (ZrO2) 1394 6.1.4. Ceramic matrix composites 1395 6.1.5. Bulk high-temperature superconductors 1395 6.2. Thin films and coatings 1396 6.2.1. Silicon and diamond 1396 6.2.2. Nitride, carbide and oxide coatings 1397 6.2.3. Epitaxial films 1398 7. Textures in minerals and rocks 1399 7.1. Calcite (CaCO3) 1400 7.2. Quartz (SiO2) 1402 7.3. Olivine (Mg2SiO4) 1404 7.4. Sheet silicates 1406 Texture and anisotropy 1369 7.5. Ice (H2O) 1407 7.6. Halite (NaCl) and periclase (MgO) 1408 7.7. Polymineralic rocks 1409 7.8. Cement minerals 1411 7.9. Earth structure 1412 7.10. Textures as indicators of strain history 1413 7.11. Anisotropy in the deep earth 1415 8. Textures in mineralized biological materials 1419 8.1. Nacre of mollusc shells (aragonite) 1419 8.2. Bones (apatite) 1419 9. Conclusions 1421 References 1422 1370 H-R Wenk and P Van Houtte 1. Introduction Preferred orientation of crystallites (or texture) is an intrinsic feature of metals, ceramics, polymers and rocks and has an influence on physical properties such as strength, electrical conductivity, piezoelectricity, magnetic susceptibility, light refraction and wave propagation, particularly in the anisotropy of these properties. The directional characteristics of many polycrystalline materials were first recognized not in metals but in rocks and were described as ‘texture’ (d’Halloy 1833). In the 20th century texture research was largely pursued by metallurgists but recently it has gained importance in ceramics (e.g. high temperature superconductors), polymers, and regained interest in the earth sciences. The reason for the latter is that seismologists have discovered anisotropic wave propagation in large sectors of the earth’s interior and a likely cause is preferred orientation of crystals that developed by deformation during the earth’s long history. This review will highlight some aspects of textures with focus on new approaches and methods, as well as relevant problems. Metallurgists and ceramicists are engaged in texture research to develop materials with favourable properties. In contrast, geologists and geophysicists are using textures to interpret the past. The rationale is thus reversed. In metallurgy specimens are readily available for analysis, and theories can be tested with experiments. Deep-earth materials do not occur on the surface and many are unstable at ambient conditions. Also, many geological conditions are outside the realm of experiments, particularly the slow strain rates and highly heterogeneous nature of rock formations. Yet, in spite of these differences, methods and approaches are remarkably similar, even though the objects of interest vary greatly in dimension. This review is intended to provide a brief introduction for physical scientists, not for texture experts. Some of the important issues are highlighted with examples, and we refer new researchers in the field of texture and anisotropy to important publications. Since the classic books on metallurgy (e.g. Wassermann and Grewen (1962), Dillamore and Roberts (1965), Hatherly and Hutchinson (1979)) and geology (e.g. Sander (1950), Turner and Weiss (1963)), there have been newer books (e.g. Bunge (1982), Wenk (1985), Kocks et al (2000)), numerous journal articles and particularly research papers in the tri-annual proceedings of the International Conferences of Textures of Materials (ICOTOM). These publications need to be consulted for details. Textures in polymers, though important, are only mentioned peripherally (see, e.g. G’Sell et al (1999)). While preparing this review we noticed that almost half of the references are in physics journals and fifteen in Nature and Science, illustrating that texture and anisotropy are subjects of core physics as well as of general interest. We try to give a balanced account of recent progress in texture research; however, in this broad field it was hard to avoid some emphasis on our own specialities, particularly in the selection of examples which were more readily available. We do not suggest that these are in any way more important. 2. Measurements of textures 2.1. Overview Interpretation of textures has to rely on a quantitative description of orientation characteristics. Two types of preferred orientations are distinguished: the lattice preferred orientation (LPO) or ‘texture’ (also ‘preferred crystallographic orientation’) and the shape preferred orientation (or ‘preferred morphological orientation’). Both can be correlated, such as in sheet silicates with a flaky morphology in schists, or fibres in fibre-reinforced ceramics. In many cases they Texture and anisotropy 1371 are not. In a rolled cubic metal the grain shape depends on the deformation rather than on the crystallography. Many methods have been used to determine preferred orientation. Optical methods have been extensively applied by geologists, using the petrographic microscope equipped with a universal stage to measure the orientation of morphological and optical directions in individual grains (e.g. Phillips (1971)). Metallurgists have used a reflected light microscope to determine the orientation of cleavages and etch pits (e.g. Nauer-Gerhardt and Bunge (1986)). With advances in image analysis, shape preferred orientation can be determined quantitatively and automatically with stereological techniques. Optical methods of LPO measurements of some minerals have also been automated (Heilbronner and Pauli 1993). Today diffraction techniques are most widely used to measure crystallographic preferred orientation (e.g. Bunge (1986), Kocks et al (2000)). X-ray diffraction with a pole-figure goniometer is a routine method. For some applications synchrotron x-rays provide unique opportunities. Neutron diffraction offers some distinct advantages, particularly for large bulk samples. Electron diffraction using the transmission (TEM) or scanning electron microscope (SEM) is gaining interest, because it permits one to correlate microstructures, neighbour relations and texture. There are two distinct ways to measure orientations. One way is to average over a large volume of a polycrystalline aggregate. A pole figure collects signals from many crystals and spatial information is lost (e.g. misorientations with neighbours), but also some orientation relations (such as how x, y, and z-axes of individual crystals correlate). The second method is to measure orientations of individual crystals. In that case orientations