Point Defects
For single-atom structures, a number of point defects arethere Shown are a vacancy
(an absent atom); an interstitial atom, occupying a normally unoccupied site; and two types of
impurities, one in an interstitial site and the other substituting for an atom. a number of
point defects are shown for an ionic compound AB. Substitutional ions, vacancies, and impurity ions
are shown. In ionic compounds, because charges must be balanced, when a cation is removed, an
anion is also removed. The resulting vacancy and interstitial point defects are called a Schottky pair.
A Frenkel defect occurs when an ion is removed from its normal site and is placed in an interstitial
site. The presence of defects—interstitials and vacancies—is necessary for diffusion to occur in many
Dislocations
Two basic types of dislocations exist in solids—edge and screw dislocations. An edge dislocation
consists of an extra plane of atoms, It is represented by the symbol -L- and
has associated compression and tension. A screw dislocation is formed by the atom planes spiraling Combinations of screw and edge dislocations also exist, which are called
mixed dislocations.Dislocations are important because of their effect on the properties, in particular the mechanicalproperties, of engineering materials. The slip of a metal is the result of the movement of dislocations;plastic deformation is the result of the generation of dislocations; the increased strength and brittleness as a result of cold working is due to a generation and pileup of dislocations; and creep in a material is the result of dislocation climb.
For single-atom structures, a number of point defects arethere Shown are a vacancy
(an absent atom); an interstitial atom, occupying a normally unoccupied site; and two types of
impurities, one in an interstitial site and the other substituting for an atom. a number of
point defects are shown for an ionic compound AB. Substitutional ions, vacancies, and impurity ions
are shown. In ionic compounds, because charges must be balanced, when a cation is removed, an
anion is also removed. The resulting vacancy and interstitial point defects are called a Schottky pair.
A Frenkel defect occurs when an ion is removed from its normal site and is placed in an interstitial
site. The presence of defects—interstitials and vacancies—is necessary for diffusion to occur in many
Dislocations
Two basic types of dislocations exist in solids—edge and screw dislocations. An edge dislocation
consists of an extra plane of atoms, It is represented by the symbol -L- and
has associated compression and tension. A screw dislocation is formed by the atom planes spiraling Combinations of screw and edge dislocations also exist, which are called
mixed dislocations.Dislocations are important because of their effect on the properties, in particular the mechanicalproperties, of engineering materials. The slip of a metal is the result of the movement of dislocations;plastic deformation is the result of the generation of dislocations; the increased strength and brittleness as a result of cold working is due to a generation and pileup of dislocations; and creep in a material is the result of dislocation climb.
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