Geology Reference
In-Depth Information
involves thermally activated cross-slip or climb, or in alloys, escape from solute
pinning. Also the mobile dislocations may eventually traverse many subgrains, the
spacing of the dislocation sources being greater than the subgrain size, and the
subboundaries themselves may have some mobility, contributing a minor compo-
nent to the strain. Martin and coworkers also show that there is an internal stress
field extending some way into the subgrain interior from the subboundary dislo-
cations, which are therefore not fully mutually compensating in respect of their
long-range elastic stresses (not in ''equilibrium'' in the sense of Frank
1955
). In
minerals, insofar as mutual interaction of dislocations on multiple slip systems is
important, there may well be similar behavior.
The parameter most frequently used to characterize the cellularity of the dis-
location substructure is the cell or subgrain diameter. However, depending on
ideas of what most influences the flow strength, other parameters have also been
used, especially the dislocation link length within sub boundaries or sub boundary
mesh size (Ardell and Przystupa
1984
; Lin et al.
1985
; Morris and Martin
1984a
;
Öström and Ahlblom
1980
; Öström and Lagnerborg
1980
).
Coarser-scale structure reflecting heterogeneity in deformation behavior is seen
at the microscopic or grain scale. Microscopically visible slip bands (
Sect. 6.1
;
Fig.
6.2
), themselves representing a heterogeneity in dislocation activity, are often
distributed heterogeneously, especially in grains in polycrystals, giving rise to
various sorts of deformation bands. These bands may be defined by differences in
the amount of activity of a primary slip plane, by the localized activity of a
secondary slip system, by the alternation of active slip systems, and so on, and they
form regions of corresponding heterogeneity in distribution of dislocations. At
higher temperatures and lower strain rates, subgrain formation may also become
evident,
the
optical
observations
detecting
coarser-scale
subgrains
than
the
submicroscopic.
Optical or scanning electron microscope (SEM) observations on slip band
structures generally require careful polishing of the specimen prior to deforma-
tion. The distribution of the dislocations themselves, such as those defining sub-
grain boundaries, can also be revealed by suitable etching after the deformation
provided the dislocation density is relatively low; for techniques see Wegner and
Christie (
1985a
,
b
). The optical transparence of minerals permits further tech-
niques of observation in transmission optical microscopy, not available for metals,
which exploit birefringence and stress-optical effects. For example, small orien-
tation changes revealed by observation of thin sections between crossed polarisers
permit subgrain boundaries to be located (Fig.
6.14
), and other optical features
such as ''lamellae'' (Fig.
6.15
) reveal the presence of localized concentrations of
dislocations or other local heterogeneities (Christie and Ardell
1974
; McLaren
et al.
1970
). At low dislocation densities, the dislocation configuration itself can
also be revealed by decoration techniques (Kohlstedt et al.
1976
). For further
information on observation techniques, see Nicolas and Poirier (
1976
), and Hobbs
et al. (
1976
).
