Geology Reference
In-Depth Information
by an increase in Peierls stress. If the dislocation is dissociated, segregation to the
fault strip between the partial dislocations (Suzuki atmosphere) will have a similar
effect. However, unless the Peierls stress is sufficiently high to dominate the dis-
location behavior, the role of the binding interaction with the point defect in the
core will be less important than the elastic interaction since most of the dislocation
energy tends to be in the elastic field.
The interaction of point defects with the long-range elastic stress field is dis-
cussed in terms of the elastic distortion associated with the point defect. In the
more refined treatments, the point defect is not regarded as a rigid inclusion to be
accommodated volumetrically in an elastic matrix but it is viewed as also
undergoing elastic distortion itself, with elastic constants different from those of
the matrix. The size misfit aspect is then sometimes referred to as a paraelastic
interaction and the modulus difference aspect as a dielastic interaction (Haasen
1983
). The elastic interaction energy (negative for attractive interaction) can be
shown to be finite for both edge and screw dislocations. In the case of screw
dislocations, in which there is no dilatancy to first order in the elastic field, the
interaction arises through the dielastic effect involving the shear modulus, as well
as to some extent through second-order elastic effects. For the cases in which the
temperature is high enough for redistribution of point defects to occur, their
equilibrium concentration in the neighborhood of the dislocation can be calculated
taking into account the elastic interaction energy per defect and any necessary
formation energy for the defect. Thus, an atmosphere of point defects can be
established around the dislocation, giving a pinning effect. This atmosphere is
known as a Cottrell atmosphere in the case of an enhanced concentration of solute
atoms. If the total energy of interaction of the atmosphere with the dislocation can
be calculated as a function of the displacement of the dislocation, then the inter-
action force between the dislocation and its atmosphere can be obtained.
Finally, electrostatic interaction will occur between a charged dislocation and
charged point defects, a situation that might be expected to be particularly relevant
in nonmetallic compounds. However, it is not known, in general, how important
this type of interaction is in practice. In the case of alkali halides, at least, it is
thought to be of negligible importance mechanically compared with the elastic
interactions (
Sect. 6.2.7
).
The mechanical implications of the interactions between point defects and
dislocations depend in general, on the extent to which the distribution of point
defects is at equilibrium with the dislocation configuration initially present and on
the rate at which redistribution can occur during dislocation motion. The effects
are mainly of significance where the defects are solute atoms. Three particular
situations may be selected for comment:
1. If, following introduction of the dislocations, the temperature has been suffi-
ciently low that redistribution of point defects in the neighborhood of the
dislocations has not occurred, then during deformation the dislocations move
through a more or less random but fixed assemblage of centers of interaction
which represent local potential barriers to be overcome by the applied stress in
