Page 49 - Mechanical Behavior of Materials

50 Chapter 2 Structure and Deformation in Materials

Figure 2.14 The two basic types of dislocations: (a) edge dislocation, and (b) screw
dislocation. (From [Hayden 65] p. 63; used with permission.)

Line defects are called dislocations and are the edges of surfaces where there is a relative
displacement of lattice planes. One type is an edge dislocation, and the other is a screw dislocation,
both of which are illustrated in Fig. 2.14. The edge dislocation can be thought of as the border of
an extra plane of atoms, as shown in (a). The dislocation line shown identifies the edge of the extra
plane, and the special symbol indicated is sometimes used.
The screw dislocation can be explained by assuming that a perfect crystal is cut as shown
in Fig. 2.14(b). The crystal is then displaced parallel to the cut and finally reconnected into the
configuration shown. The dislocation line is the edge of the cut and hence also the border of
the displaced region. Dislocations in solids generally have a combined edge and screw character
and form curves and loops. Where many are present, complex tangles of dislocation lines
may form.
Grain boundaries can be thought of as a class of surface defect where the lattice planes change
orientation by a large angle. Within a grain, there may also be low-angle boundaries.Anarrayof
edge dislocations can form such a boundary, as shown in Fig. 2.15. Several low-angle boundaries
may exist within a grain, separating regions of slightly different lattice orientation, which are called
subgrains.
There are additional types of surface defects. A twin boundary separates two regions of a crystal
where the lattice planes are a mirror image of one another. If the lattice planes are not in the proper
sequence for a perfect crystal, a stacking fault is said to exist.

2.4 ELASTIC DEFORMATION AND THEORETICAL STRENGTH

The discussion of bonding and structure in solids can be extended to a consideration of the
physical mechanisms of deformation, as viewed at the size scales of atoms, dislocations, and
grains. Recall from Chapter 1 that there are three basic types of deformation: elastic, plastic, and