Geology Reference
In-Depth Information
1. At sufficiently high temperatures, climb of dislocations may be important. Its
role in compensating for a lack of slip systems is discussed by Groves and
Kelly (
1969
). They show that, acting alone, six independent climb systems are
required for a general, nondilatational strain, and that six different Burgers
vectors, not necessarily crystallographically nonequivalent, are required to
provide this set of climb systems. They also point out that, for dislocations of a
given Burgers vector, climb motion produces two independent strain compo-
nents additional to those due to glide in a specific plane, although the new strain
components are not necessarily independent of those contributed by other
active glide systems, and so further examination is needed to establish to what
extent the climb increases the number of independent strain components
overall. If dislocations can glide on any plane as well as climb, then a general
strain is possible if there are three noncoplanar Burgers vectors (as in olivine).
2. Also at sufficiently high temperatures, sliding on grain boundaries introduces
another mode of relative displacement in the aggregate. The extent to which
this sliding can be counted as contributing to the available independent modes
of deformation is not easy to see and would seem to depend on the nature of the
mechanisms accommodating the sliding (see
Sects. 5.2.2
and
7.1.3
on the
interdependence of the sliding and accommodation processes). In the case of
accommodation by diffusion, there can be an independent contribution to the
overall deformation if the accommodation involves components of strain that
cannot be produced by available climb or glide systems. However, in the case
of accommodation by plastic deformation within the grains, the requirements
on availability of slip systems for the accommodation of the sliding will be
identical to the requirements for compatible deformation of grains in the
absence of sliding and so the occurrence of slip-accommodated sliding at grain
boundaries does not introduce any additional independent modes of deforma-
tion. Thus, on this view, sliding at grain boundaries only introduces additional
independent modes of deformation insofar as it involves atom transfer mech-
anisms for accommodation which in themselves are independent mechanisms.
Grain boundary migration is not a mechanism of deformation and so does not
enter into the present kinematical considerations although it may have impor-
tant dynamical consequences through its effect on the stresses needed to operate
the various mechanisms.
3. Dilatancy of the aggregate permits accommodation by the development or
elimination of voids. Void formation may consist of cavitation, as occurs in
creep of metals at high temperature at atmospheric pressure, of microcracking,
as in the deformation of rocks under confining pressure at any temperature
(Paterson
1969
), or of variation in pore space in an already porous rock
(cf. deformation of sand). Whenever dilatancy is involved, a significant
pressure dependence of the flow stress can be expected, as was observed, for
example, in magnesium oxide by Paterson and Weaver (
1970
).
4. Twinning mechanisms may contribute in appropriate cases, although the single-
sense property of the twinning shear makes twinning a less effective accom-
modation mechanism than slip. Kinking can also contribute in a similar way to
