Inelastic deformation can occur in crystalline materials by plastic ‘flow’. This behavior can lead to large permanent strains, in some cases, at rapid strain rates. In spite of the large strains, the materials retain crystallinity during the deformation process. Surface observations on single crystals often show the presence of lines and steps, such that it appears one portion of the crystal has slipped over another, as shown schematically in Fig. 6.1 (a). The slip occurs on specific crystallographic planes in well-defined directions. Clearly, it is important to understand the mechanisms involved in such deformations and identify structural means to control this process. Permanent deformation can also be accomplished by twinning (Fig. 6.1(b)) but the emphasis in this book will be on plastic deformation by glide (slip).

Theoretical shear strength

Figure 6.2 shows one possible way in which crystal glide could occur, with one plane of atoms being sheared past an adjacent plane. In the perfect crystal, the atoms are assumed to lie directly above each other with a planar spacing d. Clearly, as the atoms are displaced, the stress will rise and pass through a maximum. Once the displacement u reaches a value of b/2, i.e., at the mid-shear position, the atoms would be equally as likely to complete the displacement (u=b) as to return to their original positions.