Deformation Mechanisms: Granular Flow - Materials Science for Structural Geology - page 226

(1) The granular flow model readily allows very large strain to be achieved
without concomitant change in grain shape, as is observed, and it gives ready
explanation of observed effects such as grain-neighbour switching and cavi-
tation. In the case of atom transfer models, an extraordinary amount of grain
boundary migration would have to be invoked and rationalized in order to
explain retention of equant grain shape while increasing or decreasing the
number of grains in a cross-section.
(2) The stress-strain rate relations are often non-Newtonian (Chokshi and Lang-
don 1985 ), in contrast to the Newtonian Nabarro-Herring or Coble models, an
observation more readily accommodated in granular flow models.
(3) When the shear strength of grain interfaces is low, for reason of high tem-
perature, partial melting, or the presence of a wetting phase, sliding on the
interfaces can be expected to be the primary response to applied shear stress,
although the accommodation processes brought into play by compatibility
requirements are likely to be rate controlling.
As for atom transfer models ( Chap. 5 ) , the accommodation processes that are
necessary for maintaining strain compatibility must, however, be seen as an
inseparable aspect of the deformation process. The distinction emphasized here is
that the relative grain movements play the primary role, in the sense of defining the
overall deformation, and that the accommodation processes are brought in only to
the extent necessary to maintain compatibility, whereas the reverse of this hier-
archy of roles was envisaged in the atom transfer models. The accommodation
processes can be either of an atom transfer type (any of the processes in Chap. 5 )
or of a crystal-plastic type ( Chap. 6 ).
In the spirit of this approach, we shall attempt first to develop a general
deformation model that is analogous to the temperature insensitive granular flow
models of Sect. 7.2 but with thermally-activated processes playing the essential
roles of accommodation and rate control. In this context, we shall then review a
number of the more specific models for superplastic flow that have been proposed
from time to time.
As a preliminary, certain aspects of the physics of superplastic deformation
need closer examination. In particular, the concept of sliding on a migrating grain
boundary and the constraints on accommodation processes present conceptual
difficulties. However, there is a paucity of microstructural evidence reflecting these
physical processes. This lack may be due, in part, to the inadequacy of the con-
ventional observational methods, which are more appropriate to intracrystalline
plasticity, and, in part, to there being actually fewer microstructural features
remaining because of an increased tendency for them to be annealed out at the
higher temperatures and smaller grain sizes generally involved. Dislocation den-
sities tend to be low and substructures absent, while grain boundaries tend to be
straight or only gently curved (for example, Ball and Hutchinson 1969 ; Kaibyshev
et al. 1978 ; Padmanabhan and Davies 1980 , pp. 91-95; Parayil et al. 1990 ).
However, much more microstructural study in association with experimental work
is clearly needed.