tailieunhanh - Mechanical Properties of Engineered Materials P2

Examples of point defects: (a)] vacancy and interstitial elements; (b) substitutional element and interstitial impurity element; (c) pairs of ions and vacancies. [(a) and (c) are adapted from Shackleford, 1996—reprinted with permission from Prentice-Hall; (b) is adapted from Hull and Bacon, 1984. Reprinted with permission from Pergamon Press.] | Figure Examples of point defects a vacancy and interstitial elements b substitutional element and interstitial impurity element c pairs of ions and vacancies. a and c are adapted from Shackleford 1996 reprinted with permission from Prentice-Hall b is adapted from Hull and Bacon 1984. Reprinted with permission from Pergamon Press. Defects are imperfections in the structure. They may be one-dimensional point defects Fig. line defects Fig. two-dimensional plane defects Fig. or three-dimensional volume defects such as inclusions or porosity Fig. d . The different types of defects are described briefly in this section. One-Dimensional Point Defects One-dimensional point defects Fig. may include vacancies Fig. a interstitials Figs a and b solid solution elements Fig. b and pairs or clusters of the foregoing Fig. c . Pairs of ions Frenkel defects or vacancies Schottky defects are often required to maintain charge neutrality Fig. c . Point defects can diffuse through a lattice especially at temperatures above approximately of the absolute melting temperature. If the movement of point defects produces a net state change it causes thermally activated stress-induced deformation such as creep. The diffusion of point defects such as vacancies may also lead to the growth of grains in a polycrystalline material. Line Defects Line defects consist primarily of dislocations typically at the edges of patches where part of a crystallographic plane has slipped by one lattice Copyright 2003 Marcel Dekker Inc. Figure Examples of line defects a edge dislocations b screw dislocations c mixed dislocations. Adapted from Hull and Bacon 1980. Reprinted with permission from Pergamon Press. spacing Fig. . The two pure types of dislocations are edge and screw Figs a and b . Edge dislocations have slip Burgers vectors perpendicular to the dislocation line Fig. while screw dislocations have translation vectors