Page 144 - Materials Science and Engineering An Introduction

116 • Chapter 4 / Imperfections in Solids

upper front region of the crystal is shifted one atomic distance to the right relative to the
bottom portion. The atomic distortion associated with a screw dislocation is also linear
and along a dislocation line, line AB in Figure 4.5b. The screw dislocation derives its name
: VMSE from the spiral or helical path or ramp that is traced around the dislocation line by the
Screw
atomic planes of atoms. Sometimes the symbol is used to designate a screw dislocation.

Figure 4.5 (a) A screw
dislocation within a crystal.
(b) The screw dislocation in
(a) as viewed from above. The
dislocation line extends along
line AB. Atom positions above
the slip plane are designated
by open circles, those below by
solid circles.
[Figure (b) from W. T. Read, Jr.,
Dislocations in Crystals, McGraw-
Hill Book Company, New York,
NY, 1953.]
C

A

D

Dislocation
line
Burgers vector b

(a)

A B

b

D C
(b)