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 polefigure 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 (e-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.

http://iopscience.iop.org/article/10.1088/0034-4885/67/8/R02/pdf

Texture and anisotropy

The distribution and orientation of platelet-shaped particles of α-alumina in a fine-grained alumina matrix is shown to template texture development via anisotropic grain growth. The textured microstructure ranges from 4 wt% oriented platelet particles in calcined samples to nearly 100% oriented α-Al2O3 grains after sintering at 1400°C. A CaO + SiO2 liquid phase creates favorable thermodynamic and kinetic conditions for anisotropic grain growth and grain reorientation during sintering. Important criteria for templated grain growth include (1) anisotropic crystal structure and growth, (2) high thermodynamic driving force for template grain growth, and (3) modification of diffusion in the system to continuously provide material to the anisotropically growing template grains.

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Texture Development by Templated Grain Growth in Liquid-Phase-Sintered α-Alumina

Fracture toughness of ZrO2-toughened alumina could he increased by macroscopic interfaces, such as those existing in laminated composites. In this work, tape casting was used to produce A/A or A/B laminates, where A and B can be Al2O3, Al2O3/5 vol% ZrO2, and Al2O3/l0 vol% ZrO2. An increase of toughness is observed, even in the Al2O3/Al2O3 laminates.

Tape Casting of Al2O3/ZrO2 Laminated Composites

The use of the average or maximum grain size, G, in G-tensile strength, σ, relations and the resultant character of the σ-G−1/2 plots have been considered. Experimental and literature data (mainly at 22°C) show that using the size of exaggerated grains in Al2O3, β-Al2O3, B4C, SiC, Si3N4 and probably TiO2 (i.e. mainly non-cubic materials), is appropriate, provided such grains are the source of failure and are larger than or equal to the flaw size, which is most probable at intermediate G, otherwise an average G, Ga, is appropriate. Larger grains or clusters of larger grains do not always act as fracture origins, because associated flaws, mainly cracks (e.g. from machining) or pores are required. Ga was found to be appropriate for materials (e.g. MgO, Y2O3, ZrO2 and MgAl2O4) not typically having exaggerated grains. The slopes of the larger and finer G branches of the σ-G-1/2 plots are shown to be less than for polycrystalline KIC and more than 0, respectively, contrary to assertions of other workers using maximum G, Gm. The latter slope indicates that the R-curve effects and related mechanisms have much more limited effects on σ than is commonly recognized. Better measurement and characterization needs are identified, i.e. (1) basic consistency between σ and G values (violated in the past use of Gm), and (2) addressing the spatial distribution of larger grains and other defects, e.g. via fractography. Use of an average (possibly weighted) grain diameter for G instead of a linear intercept, is recommended.

Ceramic tensile strength-grain size relations: grain sizes, slopes, and branch intersections

Aqueous suspensions of alumina and other powders containing no visible morphological anisotropy were tape cast under different conditions to study the role of processing parameters and powder characteristics in the origin of in-plane sintering shrinkage anisotropy. High anisotropic shrinkage, with the typical shrinkage in the transverse direction exceeding that in the casting direction by more than 10%, was observed for tape-cast alumina cast at high solids loading and high shear rates. Tape casting without any binder also resulted in similar anisotropy. Lower solids loading with increased agglomeration resulted in a drastic reduction of shrinkage anisotropy. Under similar casting conditions, BaTiO3 did not show any shrinkage anisotropy unlike alumina. Cubic zinc ferrite and titania powders also showed some consistent shrinkage anisotropy. A correlation was found between in-plane particle orientation and anisotropic shrinkage proving that powder characteristics, particularly particle shape, control anisotropic shrinkage even in apparently equiaxed powders.

Anisotropic shrinkage in tape-cast alumina : Role of processing parameters and particle shape

The degree of preferred orientation (texture) of doctor-bladed polycrystalline 99.5% Al2O3 substrates from 3 sources was evaluated by qualitative optical and quantitative X-ray analyses. The substrates exhibit a fiber texture in which the basal planes lie preferentially in the plane of the substrate. The same texture is weakly evident in the unfired materials. The strength of the texture in fired substrates varies from lot to lot and among suppliers, but its character remains essentially the same. Measurements of dielectric constant, flexural strength, and thermal expansivity are consistent with the observed texture. The possible origins of the texture and its importance in substrate applications are discussed.

P. L. Key

Papers

Preferred Orientation in Al2O3 Substrates

Vorzugsorientierung in al2o3-substraten