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Figure 3.16 Schematic representation of the types of point defects in a crystal.
The majority of metals are not used in their pure state because they often have inadequate properties for most applications. Instead, other metals are commonly added to them in various amounts, forming alloys with an improved combination of properties such as higher strength, fatigue resistance, and corrosion resistance. Atoms of the alloying metal occupy sites in the host metal structure, forming substitutional or interstitial defects. The alloying metal is said to form a solid solution with the host metal. When examined at a scale larger than the atomic scale, a solid solution is homogeneous. It can be visualized as a solid‐state analog of a liquid solution such as a solution formed by dissolving common salt (sodium chloride) in water. The amount of one metal that can be alloyed with another metal to form a solid solution can vary from less than a fraction of a percent by weight (abbreviated wt%) to nearly 100 wt%, depending on the combination of metals. Copper, for example, can dissolve more than 30 wt% zinc to form brass, forming a substitutional solid solution in which the zinc atoms occupy the regular atomic sites of copper. On the other hand, iron can dissolve no more than ~0.007 wt% carbon at room temperature to form an interstitial solid solution known as mild streel. Brass and mild steel, however, are not suitable for use as biomaterials on account of their low corrosion resistance in vivo.
A Ti alloy that is widely used as implants in orthopedic and dental surgery is Ti6Al4V, composed of ~6 wt% aluminum and ~4 wt% vanadium (V), with the remainder (~90 wt%) being Ti (Chapter 6). Although Ti6Al4V has an elastic modulus comparable to Ti, its superior strength, fatigue resistance, and corrosion resistance make it well suited for applications such as implants for total joint replacement, fracture fixation plates and dental implants. Certain stainless steels and cobalt–chromium (Co–Cr) alloys also find considerable use in orthopedic surgery.
Ceramic solid solutions are widely used in technological applications. One solid solution used in biomedical applications is YSZ, formed by adding ~3 mol% of yttria (Y2O3) to zirconia (ZrO2). In this solid solution, the yttrium (Y) atoms predominantly occupy the regular zirconium (Zr) sites in the host ZrO2 structure, resulting in the formation of substitutional defects. Vacant defect sites (vacancies) are also created in the process to compensate for the difference in valence between Y and Zr.
The inorganic constituent of bone and teeth is not pure hydroxyapatite, Ca10(PO4)(OH)2, but a highly substituted hydroxyapatite‐like material. This material has a structure that resembles that of hydroxyapatite but it contains approximately 4–8 wt% of (CO3)2−, often the main substituting species, smaller amounts of ions such as (HPO4)2−, Mg2+ and Na+ (less than ~1 wt%), and trace amounts of ions such as Sr2+, K+, F−, and Cl− (Posner and Betts 1975). While the defect chemistry of hydroxyapatite is complex, (CO3)2− and (HPO4)2− ions typically substitute at the anionic (PO4)3− sites in the crystal structure, whereas ions such Mg2+, Na+, Sr2+, and K+ substitute at the cationic Ca2+ sites, and F− and Cl− ions substitute for OH− ions (Figure 3.17).
Figure 3.17 Major substituting ions and approximated formula of hydroxyapatite‐like constituent of bone.
3.4.2 Line Defects: Dislocations
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.
Types of Dislocations
Dislocations are divided into two basic types, called edge dislocations and screw dislocations (Figure 3.19) but they are often more complex in real crystals, composed of some mixture of these two basic types. In a simple sense, an edge dislocation can be visualized as making a cut in a perfect crystal and inserting an additional half‐plane of atoms. The edge of the half‐plane of atoms within the crystal is the dislocation line, that is, defects along a line of atomic sites. Instead of inserting an extra half‐plane of atoms after making a cut in the crystal, we can displace one part of the crystal relative to the other part in a direction parallel to the bottom of the cut. This gives the other basic type of dislocation, called a screw dislocation because, if traced in the direction of the dislocation line, the planes of atoms form a helical surface or a screw.
Figure 3.19 Schematic representation of part of a perfect crystal (a) and the arrangement of atoms near the two basic types of dislocations, edge dislocation (b) and screw dislocation (c).
Slip or Plastic Deformation Resulting from Dislocation Motion