The relaxation times for the molecular processes in gases and in a majority of liquids are so short, that molecules/atoms are almost always in a well-defined state of complete equilibrium. Consequently, the structure of a gas or liquid does not depend on its past history. In contrast, the relaxation times for some of the significant atomic processes in crystals are so long, that a state of equilibrium is rarely, if ever, achieved. It is for this reason that metals in general (and ceramics and polymers, under special conditions) show the usually desirable characteristic of work-hardening with straining, or strain-hardening. In other words, plastic deformation distorts the atoms from their equilibrium positions, and this manifests itself subsequently in hardening.
In fact, hardening by plastic deformation (rolling, drawing, etc.) is one of the most important methods of strengthening metals, in general. Figure 6.1 shows a few deformation-processing techniques in which metals are work-hardened. These industrial processes are used in the fabrication of parts and enable the shape of metals to be changed. The figure is self-explanatory. Rolling is used to produce flat products such as plates, sheets, and also more complicated shapes (with special rolling cylinders). In forging, the top hammer comes down, and the part is pushed into a die (closed-die forging) or is simply compressed. Extrusion uses a principle similar to that in the use of a tube of toothpaste. The material is squeezed through a die, and its diameter is reduced.