Page 143 - Materials Science and Engineering An Introduction

4.5 Dislocations—Linear Defects • 115

C Al A Cu
C Al = * 100
C Al A Cu + C Cu A Al
Tutorial Video: = (97)(63.55 g/mol) * 100
How to (97)(63.55 g/mol) + (3)(26.98 g/mol)
Convert from
Atom Percent = 98.7 at%
to Weight and
Percent
C Cu A Al
C Cu = * 100
C Cu A Al + C Al A Cu
(3)(26.98 g/mol)
= * 100
(3)(26.98 g/mol) + (97)(63.55 g/mol)
= 1.30 at%

Miscellaneous Imperfections

4.5 DISLOCATIONS—LINEAR DEFECTS
A dislocation is a linear or one-dimensional defect around which some of the atoms are
misaligned. One type of dislocation is represented in Figure 4.4: an extra portion of a
plane of atoms, or half-plane, the edge of which terminates within the crystal. This is
edge dislocation termed an edge dislocation; it is a linear defect that centers on the line that is defined
along the end of the extra half-plane of atoms. This is sometimes termed the dislocation
dislocation line
line, which, for the edge dislocation in Figure 4.4, is perpendicular to the plane of the
page. Within the region around the dislocation line there is some localized lattice distor-
tion. The atoms above the dislocation line in Figure 4.4 are squeezed together, and those
below are pulled apart; this is reflected in the slight curvature for the vertical planes of
: VMSE atoms as they bend around this extra half-plane. The magnitude of this distortion de-
Edge creases with distance away from the dislocation line; at positions far removed, the crystal
lattice is virtually perfect. Sometimes the edge dislocation in Figure 4.4 is represented by
the symbol #, which also indicates the position of the dislocation line. An edge disloca-
tion may also be formed by an extra half-plane of atoms that is included in the bottom
portion of the crystal; its designation is a . #
screw dislocation Another type of dislocation, called a screw dislocation, may be thought of as being
formed by a shear stress that is applied to produce the distortion shown in Figure 4.5a: the

Burgers vector Figure 4.4 The atom positions around an
b edge dislocation; extra half-plane of atoms
shown in perspective.
(Adapted from A. G. Guy, Essentials of Materials
Science, McGraw-Hill Book Company, New York,
NY, 1976, p. 153.)
Edge
dislocation
line