Geology Reference
In-Depth Information
The microdynamical models of the type considered in this subsection are
limited to transient or primary creep so long as dislocation multiplication is
envisaged to continue indefinitely without counterbalancing annihilation or
immobilization. However, if a counterbalance is introduced, for example, in the
form of a recovery process, the models have wider application, especially at high
temperatures, and steady state creep can be simulated. Thus, Weertman (
1975
)
introduced recovery effects in the form of the climb of dislocations out of the pile-
ups, and Alexander and Haasen (
1968
) introduced an empirical, stress-dependent
recovery term in the dislocation density. The resulting steady-state creep rate tends
to be more highly stress dependent than a linear or Newtonian creep since, even
though the velocity term in the Orowan equation may be linearly dependent on
stress, additional stress dependence can be introduced in the mobile dislocation
density term which incorporates both the multiplication and the recovery effects
(Weertman
1975
). However, recovery processes involving climb no longer rep-
resent aspects of viscous drag control of the dislocation dynamics and the models
incorporating recovery thus take on a hybrid character, sharing their recovery
aspects with the models of the next section. In practice, there may often be a
transition from viscous drag or glide control to recovery or climb control with the
approach to steady state.
6.6.6 Thermal Models Based on Mutual Dislocation Interaction
This category of models includes, and is to a large extent comprise of, models that
are commonly referred to as models of recovery-controlled creep. These models
have been discussed mainly in connection with secondary creep or steady state
behavior. However, we shall attempt to introduce them in a slightly more general
context.
Insofar as the mutual dislocation interaction that supports the applied stress and
controls the movement of the gliding dislocations is athermal (
Sect. 6.6.3
), the rate
control in time-dependent deformation arises mainly through the thermally acti-
vated modification (recovery) of the density and configuration of the interacting
dislocation network. In this case, the role of the thermal activation is more indirect
than in the models of the previous category (
Sect. 6.6.5
), in which the primary
rate-controlling factor was the viscous drag in the thermally activated movement
of the gliding dislocations; that is, in recovery-controlled creep, the thermal
activation promotes the lowering of the barriers to dislocation motion rather than
the surmounting of given barriers.
The notion of dynamical recovery has already been invoked in discussing stage
III of the stress-strain curve in the athermal regime (
Sect. 6.6.3
). In that regime,
the recovery is envisaged as being primarily a function of strain although it may
also involve a minor degree of temperature sensitivity from thermal activation. If a
saturation hardening is attained, it is only after very large strains (
1). However,
on entering the thermal regime at higher temperatures, the dynamical recovery
