Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 96

A dislocation is a discontinuity in atomic packing that is associated with several atomic sites along a line in a crystal. The interatomic bonds are highly distorted in the immediate vicinity of a dislocation (Figure 3.18), causing some degree of mechanical strain. Consequently, a crystal that contains dislocations has a higher energy than a similar but dislocation‐free crystal due to the energy associated with the mechanical strain. The higher the number of dislocations per unit volume, the higher the strain energy of the crystal. Because of the ease with which metal ions can break and reform bonds (Chapter 2), the mechanical properties of metals are strongly dependent on the presence of dislocations. The property of ductility, for example, that makes metallic biomaterials such as Ti6Al4V, stainless steel and Co–Cr alloys attractive for use as fracture fixation plates, hip, and knee implants and dental implants is due to the presence of dislocations in these materials.

Figure 3.18 Schematic representation of a dislocation in a crystal and the distortion of the interatomic bonds in its immediate vicinity.

Dislocations can become more numerous when a metal is deformed under appropriate mechanical stresses such as those during rolling or extrusion, thereby enhancing the strain energy of the crystals. Practically, deformation of a metal to increase its strain energy followed by thermal treatment of the highly strained material is one of the common methods used to control the microstructure of a metal and, thus, its mechanical properties (Chapter 6).

In comparison, dislocations are unimportant in the majority of ceramics at ordinary temperatures. A considerably higher amount of energy is required for the creation and migration of dislocations in ceramics due to their strong ionic or covalent bonding, and their composition composed of two or more dissimilar atoms. Consequently, the dislocation density (number of dislocations per unit volume) is low or negligible, and the role of dislocations can be neglected in the majority of ceramics, including those used as biomaterials such as aluminum oxide, hydroxyapatite, and β‐tricalcium phosphate.