Deformation Mechanisms: Crystal Plasticity - Materials Science for Structural Geology - page 149

temperature and strain-rate regimes. Only a bare outline will be attempted of these
models here, not only for reasons of space but also because, until there is an
adequate observational basis for the structural and mechanistic assumptions
required for particular theories, there is little practical point in their elaborate
development beyond what is useful for guiding further experimental work. This
observational basis for theoretical development is still very inadequate, on the
whole, for most minerals.
In attempting to classify the theoretical models in a broad view, two types of
distinction in behavior can be made. The first concerns temperature sensitivity,
distinguishing between:
1. athermal regimes—weak temperature dependence
2. thermal regimes—strong temperature dependence.
The second type of distinction concerns the predominant sort of dislocation
interaction determining the flow stress, distinguishing between:
1. models in which the mutual interaction between dislocations plays the main
controlling part
2. models in which the interaction of the dislocation with other structural entities
is dominant.
The second distinction can also be expressed as one between population control
and glide control in the sense that, in the former case, the control depends in an
essential way on there being a multiplicity of dislocations while, in the latter case,
factors of a different kind are exerting control on the glide behavior of individual
dislocations.
Before considering particular groups of models, it is interesting to look, in a
general way, at the implications of the existence of the athermal and thermal
regimes of behavior. As pointed out in Chap. 4 , it is often observed that there are
both low-temperature and high-temperature regimes, separated by an athermal
regime, although among particular materials the actual temperature ranges of the
regimes may vary widely relative to some characteristic temperature such as the
melting point. In the light of the present chapters on deformation mechanisms, this
tripartite tendency (Fig. 6.18 ) is seen to apply particularly where dislocation
mechanisms are involved. In the thermal regimes, the temperature sensitivity of
the flow stress can here be presumed to reflect the existence of thermally sur-
mountable barriers controlling dislocation movement, of which there is potentially
a wide range, as we have seen in earlier sections. However, the existence of two
separate thermal regimes would seem to imply that there are two distinct classes of
such barriers, reflected in two ranges of values of the activation energy E,as
follows:
1. Values of E small relative to the binding or cohesive energy per atom, say, less
than 1 eV (100 kJ mol -1 ). Such values may be typical of the Peierls energy,
the kink nucleation energy or the solute interaction energy in many materials,
especially metals or simple ionic crystals. Since the rate at which barriers are