When the shape of a metal is changed by the application of forces, deformation takes place in two stages as is shown during a tensile test. At ﬁ rst, crystals within the metal are distorted in an elastic manner and this distortion increases proportionally with the increase in stress. If the stress is removed during this stage, the metal returns to its original shape, illustrating the elastic nature of the deformation. On the other hand, if the stress is increased further, a point is reached (the yield point) where forces which bind together the atoms in the lattice structure are overcome to the extent that layers or planes of atoms begin to slide over each other. This process of ‘ slip ’ , as metallurgists call it, is not reversible; so, if the stress is now removed, permanent deformation remains in the metal ( Figure 6.1 ). This type of deformation is termed plastic .