Geology Reference
In-Depth Information
In the case of an excess of point defects, recovery involves their diffusion to the
surface or to internal sinks, or there may be mutual annihilation of complementary
defects. If only vacancy diffusion is required or the sinks are closely spaced,
recovery with respect to point defects can proceed relatively rapidly. Christian
(
1975
, p.137 et seq.) gives some discussion of the kinetic considerations in the
case of vacancies in metals.
The most important recovery in deformed crystalline materials involves dis-
locations, the density of which has increased during deformation. The thermally
activated migration of dislocations to a free surface or grain boundary sink is
normally too slow to be effective in eliminating dislocations but mutual annihi-
lation may be important in reducing the dislocation density. However, even if the
latter process proceeds as far as possible, there will usually be some excess dis-
locations of one sign remaining. These dislocations, as well as possibly many that
could potentially have been annihilated, tend to arrange themselves by glide and
climb at elevated temperatures into relatively stable configurations, often repre-
sented microstructurally by ''polygonization'', which minimize the strain energy
associated with the long-range stress fields around the dislocations. Recovery
involving dislocation climb is very important in high-temperature dislocation
creep and is discussed again in
Sect. 6.5.3
where the kinetic aspects are dealt with.
3.3.3 Recrystallization
Recrystallization is a solid-state process whereby a new crystalline structure of the
same phase replaces that of the original crystal, the degree of reorganization being
such that the new crystals can no longer be regarded as structures based on the
original orientation of crystallographic axes. If the recrystallization occurs during
heat treatment subsequent to a relatively low temperature plastic deformation in
which substantial strain hardening has occurred, it is termed static recrystallization
(Poirier
1985
, Sect. 2.4.7). Alternatively, if the recrystallization occurs concur-
rently with deformation at elevated temperatures, it is termed dynamic recrystal-
lization. Dynamic recrystallization is of particular interest industrially in ''hot
working'' processes. It is also the recrystallization process that is of most interest
in geological studies.
Broadly, there are two ways in which the original crystals can be reorganized in
recrystallization, one by grain boundary migration or bulging (migration recrys-
tallization) and the other by local rotation of the crystal structure (rotation
recrystallization):
• Migration recrystallization has been widely studied in static experiments. The
term ''nucleation and growth recrystallization'' was formerly used for this
process but is inappropriate since classical nucleation theory, based on thermal
fluctuations, does not apply to recrystallization (Cahn
1983
; Christian
1975
). To
obtain static recrystallization, certain minimum values of strain and temperature
