Geology Reference
In-Depth Information
Fig. 6.7 Depicting a a single
kink and b a double kink. The
heavy line is the configuration
of the dislocation; the light
line represents the Peierls
potential peaks and the
dashed lines represent the
Peierls troughs
(a)
(b)
potential barrier and the stress for motion of a kink, respectively. The secondary
Peierls barrier is only likely to be substantial in the case of narrow dislocations, as
in covalent solids, but, as long as the crystal is at absolute zero temperature, it will,
in any case, be of little importance since, once the kink has traversed the dislo-
cation line, the line will then tend to move forward as a whole at the Peierls stress
itself, in the absence of the thermally activated nucleation of kinks at a lower
stress.
Raising the temperature above absolute zero will first have the effect of flat-
tening somewhat the profile of the Peierls potential, and hence reducing the Peierls
stress. However, this effect is unlikely to be a very marked one (Friedel
1964
,
p. 67) and it will eventually be overshadowed by the much more important effect
of the thermally activated nucleation of kinks on the dislocation, rendering the
Peierls considerations for straight segments of dislocations no longer relevant. We
therefore now have to consider kinks more seriously, especially in relation to their
thermally activated behavior.
6.2.4 Kinks and Jogs
As mentioned in the previous section, if a dislocation line in a given slip plane lies
partly in one Peierls valley and partly in the next, the crossover is called a kink.If
there is a second crossover nearby bringing the dislocation back into the first
valley, the combination is called a double kink or, better, a kink pair (Seeger
1984
)
(Fig.
6.7
). Kinks are only sharp or narrow, and usefully distinguished as entities,
when the Peierls potential is relatively high.
Although propagating a kink along a dislocation gives the possibility of
advancing the dislocation under lower stress than is needed in the absence of
kinks, the continuing motion of the dislocation in this way requires the continuing
