Lecture note 17 (11-01-2011)

Lecture note 17 (11-01-2011) - MAE 581 Advanced Materials...

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1 MAE 581 Advanced Materials Science Lecture note 17 Crystal defects Types of imperfection Real solids invariably contain structural discontinuities and localized regions of disorder. This heterogeneity can exist on both microscopic and macroscopic scales, with defects or imperfections ranging in size from missing or misplaced atoms to features that are visible to the naked eye. The majority of materials used for engineering components and structures are made up from a large number of small interlocking grains or crystals. It is therefore immediately appropriate to regard the grain boundary surfaces of such polycrystalline aggregates as a type of imperfection. Other relatively large defects, such as shrinkage pores, gas bubbles, inclusions of foreign matter and cracks, may be found dispersed throughout the grains of a metal or ceramic material. In general, however, these large-scale defects are very much influenced by the processing of the material and are less fundamental to the basic material. More attention will thus be given to the atomic-scale defects in materials. Within each grain, atoms are regularly arranged according to the basic crystal structure but a variety of imperfections, classified generally as crystal defects, may also occur. A schematic diagram of these basic defects is shown in Figure 3.1. These take the form of: Point defects, such as vacant atomic sites (or simply vacancies) and interstitial atoms (or simply interstitials), where an atom sits in an interstice rather than a normal lattice site Line defects, such as dislocations Planar defects, such as stacking faults and twin boundaries Volume defects, such as voids, gas bubbles and cavities.
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2 Figure 3.1 (a) Vacancy–interstitial. (b) Dislocation. (c) Stacking fault. (d) Void. Point defects
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Lecture note 17 (11-01-2011) - MAE 581 Advanced Materials...

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