Crystallographic Texture of Materials

Most of the materials are crystalline in the solid state, and majority of them are polycrystalline in nature. In a polycrystalline material, each grain is an individual crystal whose orientation differs from that of its neighbors. Therefore, a polycrystal can be considered as an aggregate of many individual single crystals, which may be differently oriented.

As has been stated already, texture of a rolled sheet material is commonly represented as {hkl} 〈uvw〉, which means that most of the grains in the sheet material are such that their {hkl} planes are nearly parallel to the rolling plane and the 〈uvw〉 directions of the grains are nearly parallel to the rolling direction. In practice, however, it may so happen that in a rolled sheet, a number of grains have their {h ₁ k ₁ l ₁} planes parallel to the rolling plane and their 〈u ₁ v ₁ w ₁〉 directions parallel to the rolling direction; another few grains may have their {h ₂ k ₂ l ₂} planes parallel to the rolling plane and their 〈u ₂ v ₂ w ₂〉 directions parallel to the rolling direction and so on. In that case, we say that the texture of the sheet material has a few components represented by {h ₁ k ₁ l ₁} 〈u ₁ v ₁ w ₁〉, {h ₂ k ₂ l ₂} 〈u ₂ v ₂ w ₂〉···, and so on.

The previous chapter describes how texture of a material can be represented in terms of pole figures and ODFs. These methods of representation require the basic orientation data to be obtained from the crystallites or grains, which constitute the material. This chapter will deal with the different experimental techniques that are employed for this purpose.

Texture evolves at almost every stage of material processing. In fact, it is very difficult to avoid texture completely at any stage of processing. Metal castings have very clear and characteristic textures which depend on material, its purity, as well as on the solidification conditions.

As a result of plastic deformation of a material, there is a change in shape of the constituent grains and the total grain boundary area increases substantially. The mechanism of deformation leads to continuous generation of dislocation, which aids to increase the grain boundary area. This leads to the appearance of internal structure within the grains. Further, the orientations of single crystals and of individual grains of a polycrystalline material change relative to the directions of the applied stresses.

A plastically deformed material, due to its increased content of physical defects, is in a thermodynamically metastable state. On increasing the temperature, the material can lower its free energy by the removal and rearrangement of the lattice defects.

Thin films of metals, ceramics, or polymers find many applications in electronic, magnetic, and optical devices. Thin films are usually characterized by the presence of very sharp crystallographic textures. In fact, it is rather difficult to produce thin films without texture.

The preceding chapters mainly dealt with texture formation in metallic materials. However, textures do form in non-metallic materials also during processing. The mechanism of texture evolution in non-metals is similar to that in metals. Although the method of texture measurement is essentially the same for all crystalline materials, however, non-metallic materials pose some complications in the measurement technique.

Crystallographic texture influences most of the mechanical, physical, and chemical properties of polycrystalline materials. The presence of texture in a material brings in anisotropy in the properties. Although most of the properties are affected by texture, the mechanical properties such as elastic moduli, strength, ductility, fracture toughness, and fatigue are the most studied ones due to their engineering importance. There have been a few reports on texture dependence of high-temperature properties, mainly creep and oxidation resistance.

In the previous chapters, a detailed account of texture formation in different materials has been presented. These materials were representative of a particular structure type that responds to processing in a certain way.