Week 3: Dislocations. Lecture 5 and 6, Stacking faults. Frenkel and Schottky disorders

A perfect crystal is an idealization; there is no such thing in nature. Atom arrangements in real materials do not always follow perfect crystalline patterns. Even then, most of the materials that are useful in engineering are crystalline to a very good approximation. There are also some fundamental physical reason for this. The preferred structures of solids at low temperature are those that minimize the energy of that solid material. The low-energy atomic configurations are almost invariably crystalline since the regular pattern of the crystal lattice repeats whatever local configuration is most favorable for bonding. There is also a fundamental physical reason why the crystal is imperfect. Sometime nature makes materials perfect by creating imperfections in it. While a perfect crystalline structure may be preferred energetically, at least in the limit of low temperature, atoms are relatively immobile in solids and it is, therefore, difficult to eliminate whatever imperfections are introduced into the crystal during its growth, processing or use. In this Chapter we shall identify and describe the various defects that are found in crystalline solids.

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