Texture and anisotropy of polycrystalline materials - Diffraction of X-rays, synchrotron radiation and neutrons

Most materials used technologically have a polycrystalline structure. Their properties depend on both the structure of the polycrystalline aggregate and the properties of the constituent crystals. The properties of the crystals are essentially determined by the choice of materials. However, the structure of the aggregate, and therefore its influence on the properties of the resulting materials, depends on the treatment the materials have undergone. Because of the anisotropy of crystalline properties, the orientation statistics of individual crystallites and the pairwise correlations of these orientations (correlation functions) play a predominant role among the aggregate parameters of polycrystalline material. Texture and related quantities are therefore important structural parameters from two points of view:

• they influence material properties;

• they evolve as materials are processed.

Distributions of crystallite orientations and correlations of orientation pairs can be described quantitatively by mathematical models.

Technological standards define the maximum variation margins allowed for material properties. These margins become narrower as the quality of the materials increases; they are particularly narrow for so-called high-tech materials. For this reason, basic material properties such as composition and purity must be controlled. As soon as the uncertainty in properties is less than the uncertainty due to crystallographic anisotropy, texture and related quantities become the dominant structural parameters; as control of basic quantities is constantly being improved, this will sooner or later apply to all polycrystalline materials. Texture has long been studied and controlled in metallurgy, but is less common in other types of material, for two main reasons:

• technological standards do not really require texture control for non-metallic materials;

• non-metallic materials have more complex crystalline structures, making texture analysis of these materials more difficult, if not virtually impossible, using conventional techniques.

Both are constantly evolving, so that texture studies and controls are becoming important for all technological materials. This article defines textures and related quantities, and presents the experimental determination of textures by X-ray diffraction, synchrotron radiation and neutron diffraction.