Defects

Abstract

Over time materials change. A material changes towards thermodynamic equilibrium with its environment, or away from equilibrium if it is subjected to external influences such as mechanical deformation, irradiation and chemical attack. In crystalline materials defects are the agents of change. Point defects are agents of diffusion, line defects called dislocations are agents of plastic (permanent) deformation, and planar defects called grain boundaries are agents of recrystallisation and many other processes. In metals diffusion occurs primarily through the motion of vacancies. There is a population of such vacancies in thermodynamic equilibrium. Experimental evidence for their existence and their free energy of formation is presented. The ease of movement of dislocations governs the strength and ductility of crystalline materials. In insulators defects may be electrically charged. Many properties of crystalline materials are governed by defects and their interactions.