Page 248 - Materials Science and Engineering An Introduction

220 • Chapter 7 / Dislocations and Strengthening Mechanisms

7.3 CHARACTERISTICS OF DISLOCATIONS
Several characteristics of dislocations are important with regard to the mechanical prop-
erties of metals. These include strain fields that exist around dislocations, which are influ-
ential in determining the mobility of the dislocations, as well as their ability to multiply.
When metals are plastically deformed, some fraction of the deformation energy (ap-
proximately 5%) is retained internally; the remainder is dissipated as heat. The major por-
tion of this stored energy is as strain energy associated with dislocations. Consider the edge
dislocation represented in Figure 7.4. As already mentioned, some atomic lattice distortion
exists around the dislocation line because of the presence of the extra half-plane of atoms.
lattice strain As a consequence, there are regions in which compressive, tensile, and shear lattice strains
are imposed on the neighboring atoms. For example, atoms immediately above and adja-
cent to the dislocation line are squeezed together. As a result, these atoms may be thought
of as experiencing a compressive strain relative to atoms positioned in the perfect crystal
and far removed from the dislocation; this is illustrated in Figure 7.4. Directly below the
half-plane, the effect is just the opposite; lattice atoms sustain an imposed tensile strain,
which is as shown. Shear strains also exist in the vicinity of the edge dislocation. For a screw
dislocation, lattice strains are pure shear only. These lattice distortions may be considered
to be strain fields that radiate from the dislocation line. The strains extend into the sur-
rounding atoms, and their magnitude decreases with radial distance from the dislocation.
The strain fields surrounding dislocations in close proximity to one another may
interact such that forces are imposed on each dislocation by the combined interactions
of all its neighboring dislocations. For example, consider two edge dislocations that have
the same sign and the identical slip plane, as represented in Figure 7.5a. The compres-
sive and tensile strain fields for both lie on the same side of the slip plane; the strain field
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Defects in Metals interaction is such that there exists between these two isolated dislocations a mutual re-
Why do Defects pulsive force that tends to move them apart. However, two dislocations of opposite sign
Strengthen Metals? and having the same slip plane are attracted to one another, as indicated in Figure 7.5b,
and dislocation annihilation occurs when they meet. That is, the two extra half-planes of
atoms align and become a complete plane. Dislocation interactions are possible among
edge, screw, and/or mixed dislocations, and for a variety of orientations. These strain
fields and associated forces are important in the strengthening mechanisms for metals.
During plastic deformation, the number of dislocations increases dramatically.
The dislocation density in a metal that has been highly deformed may be as high as
2
10 10 mm . One important source of these new dislocations is existing dislocations,
which multiply; furthermore, grain boundaries, as well as internal defects and surface
irregularities such as scratches and nicks, which act as stress concentrations, may serve
as dislocation formation sites during deformation.

Figure 7.4 Regions of compression
(green) and tension (yellow) located
around an edge dislocation.
(Adapted from W. G. Moffatt, G. W. Pearsall,
and J. Wulff, The Structure and Properties of
Materials, Vol. I, Structure, p. 85. Copyright Compression
© 1964 by John Wiley & Sons, New York.) Tension