Strength is an important property of a material and is very sensitive to the micro­structure of the material. Therefore, understanding the ways in which strength of a material can be improved is of great technological significance. To be specific, we will discuss the strengthening mechanisms in terms of their effect on the yield stress of the material (defined as the stress at which gross plastic deformation begins). However, in order to accomplish that objective, we need to understand the science of the strengthening mechanisms. Dislocations are a common factor in almost all the important strengthening mechanisms we will discuss here. If dislo­cations are able to move in a crystal with relative ease, it means the material does

not intrinsically offer any resistance to the dislocation movement and thus would be less strong. But if the microstructure is laden with various obstacles, dislocation movement will be effectively impeded and this movement obstruction will be trans­lated into the increase of the strength of the materials. This principle is applicable to a wide array of materials with crystalline structures. The obstacle to dislocation movement could be dislocations themselves (strain hardening), grain boundaries (Hall-Petch or grain size strengthening), solute atoms (solid solution strengthen­ing), precipitates (precipitate strengthening), and dispersions of fine stable particles (dispersion strengthening). There are a few other strengthening mechanisms like texture strengthening, composite strengthening, and so on, which are not directly but indirectly related to dislocations. However, they will not be discussed here for brevity.

4.4.1