TABLE OF CONTENTS The mechanical properties of materials are often limited by their imperfections. The theoretical cleavage and shear strengths of materials are given by (see Section 4.2):
where E and G are the Young s and shear moduli, respectively; a is the interatomic spacing, and ? is the surface energy of the material. These equations predict exceedingly high strengths (on the order of GPas), and few materials reach such strengths. (See Chapter 1.) Indeed, this is somehow the Holy Grail of materials science: If materials were perfect, those values could be reached. However, all materials contain imperfections, either by design or inadvertently produced during processing. We review these in this and subsequent chapters. They are classified, according to their dimensions, into four kinds, each discussed in a separate section as follows:
Point (atomic or electronic) defects (Section 4.3)
Line (or one-dimensional) defects (Section 4.3)
Interfacial (or two-dimensional) defects (Chapter 5, Section 5.1)
Volume (or three-dimensional) defects (Chapter 5, Section 5.2).
Cracks are discussed in chapters 7 and 8, on fracture.
Imperfections determine the mechanical response of materials, and the manner in which the response is used to enhance performance in a material will be analyzed in considerable detail in chapters 5 through 9. Note that the dimensional scale of defects covers a wide spectrum, 10 ? 1 4 m, as shown schematically in Figure 4.1. Electronic point defects do not affect mechanical properties significantly and will therefore not be discussed in this...
