A material is worked by utilizing plastic deformation to give it a shape suitable for its application. In this process, a change occurs not only in the visible shape and size, but also at the atomic level in the interior of the material. Plastic deformation of a metal occurs by slipping of atoms on specific planes of a crystal. This slipping of atoms does not occur at any one time over the entire crystal plane. In fact, it occurs by the movement of linear lattice defects called dislocations. The figure shows the atomic structure of an edge dislocation and the process by which plastic deformation occurs when the dislocation moves on the slip plane. Materials in which dislocations can move easily are those which tend to be soft and subject to easy plastic deformation. On the other hand, hard and strong materials can be obtained if it is difficult for dislocations to move. Factors that make it difficult for dislocations to move include foreign atoms in solid solutions and precipitated particles. Hardening by these factors is referred to as solid solution hardening and precipitation hardening, respectively. As plastic deformation proceeds, many dislocations accumulate in a crystal, which interact with each other and prevent movement of the dislocation. Therefore, a material becomes harder as plastic deformation continues. This is called work hardening. A work hardened material returns to its former soft state when the accumulated dislocations disappear. When the work-hardened material is heated in an annealing process, a large number of dislocations disappear through the diffusion of atoms. During hot rolling, the as-rolled material is soft because both work hardening and annealing occur simultaneously. However, during cold rolling, only a work hardening occurs and, therefore, cold-rolled material is hard and brittle. Another change in the crystal as a result of plastic deformation is the rotation of a crystal, which occurs when plastic deformation is caused only on a specific plane and direction of slipping. The rotation of a crystal forms a texture in which the crystal grains are oriented in the direction of the mechanical working. In cold rolling with a large amount of deformation, the crystal grains become elongated. If a material with elongated grains is heated after being subjected to plastic deformation above the critical value, new equiaxed grains with fewer dislocations nucleate and grow, and the material returns to the soft state that existed before deformation. This phenomenon, which is called recrystallization, is used for the refining and softening of crystal grains.