Geology Reference
In-Depth Information
Although direct experimental testing of (
6.23a
) would be difficult, it is widely
accepted for application to problems such as climb creep and the growth of
prismatic loops when it is thought that pipe diffusion is negligible.
6.5 Dislocation Populations and Their Evolution
In a real crystal undergoing plastic deformation, the dislocations tend to be neither
uniformly distributed nor equally mobile, and the density of the dislocation
assemblage and its configuration tend to evolve during straining. These structural
and behavioral factors have to be taken into account in any microdynamical theory
of flow. Microstructural observation therefore plays a vital part in guiding the
proper development of an adequate theory of macroscopic flow. Observations are
required both at the optical microscope scale, to reveal deformation banding,
subgrain formation, etc., at this scale, and at the submicroscopic scale, to reveal the
actual dislocation configurations, the formation of pile-ups, dipoles, loops, tangles,
cellular arrangements, etc., often described as ''substructure''.
Even a crude description of the dislocation population must, in general, take
account of two aspects, which may be designated as the mean density and the
''cellularity'' (Kocks
1985a
). These two aspects will be dealt with in the first two
subsections to follow, and the more profound microstructural reorganizations
involved in recovery and recrystallization will be touched upon in the third
subsection.
6.5.1 The Configuration of the Dislocation Assemblage
The configuration of individual dislocation segments can reflect the nature of the
barriers to dislocation motion and of the interactions between dislocations. This
configuration can be revealed by transmission electron microscopy (TEM). When
the crystal is oriented in the electron beam so that a Bragg diffraction condition is
near to being satisfied, the slight variations in orientation due to the variation in
distortion in the long-range elastic stress field give rise to local increase or
decrease in the intensity of diffraction near the dislocation core compared with
elsewhere, and hence to the dislocation being imaged by the diffraction contrast
effect Figs.
6.10
,
6.11
,
6.12
,
6.13
,
6.14
,
6.15
,
6.16
).
The observation of long straight dislocation lines of simple crystallographic
orientation points to a strong interaction between the dislocation and the structure
itself. This interaction may consist of a large amplitude of the Peierls potential in
the case of a nonextended dislocation, or it may involve an effective deepening of
the Peierls valley by extension of the dislocation core into planes other than the
slip plane, as in the case of climb dissociation (
Sect. 6.2.5
). Examples of straight
dislocations suggesting structural control are shown in Figs.
6.10
a and
6.11
a,
