Grain boundaries
Commonly used crystalline materials, with a few exceptions such as semiconductors and superalloys used to manufacture turbine blades for the aeronautical industry, are generally polycrystalline. They are not formed from a single grain (single crystal) but from a set of grains of variable size (from \({1}{\rm \, \mu m}\) to several centimetres depending on manufacturing routes and thermo-mechanical treatments applied to the material). These grains are juxtaposed and the regions where the different grains are in contact are called grain boundaries. These regions are transition zones characterized by more or less disrupted structures that permit the geometric and crystallographic accommodation of the polycrystal's constituent grains (Fig. below).
Polycrystals are generally isotropic when constituted of grains with random crystallographic orientation, called equiaxe grains. Conversely, single crystals are strongly anisotropic. Properties measured in a polycrystal generally result from an average of all grains. For example, the Young's modulus of polycrystalline copper is around \({110}{\rm \, GPa}\) but it varies by \({67}{\rm \, GPa}\) in directions \(<100>\) to \({192}{\rm \, GPa}\) in direction \(<111>\) in single crystals.
As a rule, grain boundaries are both sources and traps for point defects and dislocations. During quenching, excess vacancies eliminate more rapidly at grain boundaries. Grain boundaries also play an important role in plastic deformation as they can induce dislocations under the action of a stress and constitute as well obstacles to the movement of dislocations.