Geology Reference
In-Depth Information
where Ds is a small drop in applied stress from s to s
Ds and Dt is the time
interval before straining is observed to recommence after the drop. This quantity
tends to be markedly higher than similar measures of static recovery, indicating
that the stress itself is assisting the recovery processes (for example, through the
action of the climb force,
Sect. 6.2.1
). There is thus the possibility that the tem-
perature dependence of r, expressed as an activation energy, may include a stress
term as well as the expected activation energy for self-diffusion.
Dynamic recrystallization (
Sect. 3.3.3
) is a further process that may modify the
build-up and structure of the population at elevated temperature. In the case of
rotation recrystallization, its occurrence may not have an obvious mechanical
effect because the new grains are formed in an apparently continuous transition
from the earlier formed subgrain boundaries. For example, in the experimental
deformation of marble at 1,000-1,300 K and constant strain rate, no abrupt
changes in flow stress are observed to coincide with the appearance of new grains
evidently formed by rotation recrystallization (Griggs et al.
1960
; Schmid et al.
1980
).
In contrast, the onset of migration recrystallization, often observed in hot
working of metals, can have a marked mechanical effect, usually a softening,
sometimes cyclic (McQueen
1977
; Mecking and Gottstein
1978
; Roberts
1984
;
Sellars
1978
). For this type of recrystallization to occur, a sufficient build-up in
stored energy associated with the dislocation substructure is necessary and high
angle boundaries must be sufficiently mobile. Thus, factors that inhibit recovery
(such as low stacking fault energy in f.c.c. metals) are favorable to its occurrence.
Where both types of recrystallization are observed in the same material, two
distinct domains of behavior are found, with rotation recrystallization tending to
occur at lower temperatures and strain rates (or stresses) than migration recrys-
tallization, presumably due to inhibition of grain boundary mobility by impurities
in this domain (Guillopé and Poirier
1979
; Tungatt and Humphries
1981
,
1984
);
Fig.
6.17
. A succinct review of dynamic recrystallization is given by Poirier (
1985
,
pp. 179-190).
6.6 Dislocation Theories of Flow in Single Crystals
6.6.1 Introduction: Athermal and Thermal Models
In preceding sections, we have discussed the factors governing both the motion of
the individual dislocation and the collective properties of the population of dis-
locations in a deforming crystal. We now combine these considerations in dis-
cussing how the macroscopic flow stress is determined during a deformation that
occurs primarily by dislocation glide. This procedure leads to the establishment of
theoretical flow laws for crystal plasticity.
