Geology Reference
In-Depth Information
are required and the process is driven primarily by the ''stored'' energy asso-
ciated with the structural defects resulting from the prior deformation, especially
the dislocations. The recrystallization process is thought to involve the bulging
out of high-angle grain boundaries to generate strain-free crystal (Bailey and
Hirsch
1962
). It is also an important process in dynamic recrystallization
(Poirier
1985
, Sect. 6.3.1).
• Rotation recrystallization results from the progressive relative rotation of
subgrains until they can be viewed as distinct grains (Poirier
1985
, Sect. 6.3.1).
It is only observed to occur during deformation as a dynamic recrystallization,
occurring concurrently with deformation at elevated temperatures. It seems to
have been first reported in quartz (Hobbs
1968
) but has now also been recog-
nized in the hot working of metals (Sakai
1989
). A model for the process has
been proposed by Shimizu (
1998
,
1999
).
The overall extent of recrystallization, measured by the volume fraction of the
specimen recrystallized at constant temperature, can often be described by the
Avrami relation (Christian
1975
,p.19)
n
¼
1
exp
kt
n
ð
Þ
ð
3
:
13
Þ
where t is elapsed time, k, n are parameters, and n commonly being *3-4.
The new grain size upon completion of recrystallization can be greater or smaller
than the original grain size. Subsequent to recrystallization, further growth of some
grains may occur at the expense of others, resulting in a more of less uniform
coarsening of the grain structure. The driving force for this coarsening, termed grain
growth, is the reduction in grain boundary energy, generally a much smaller quantity
than the stored energy from plastic deformation. Grain growth is usually much
slower than recrystallization owing to this smaller driving force. Sometimes, how-
ever, continued heating after recrystallization of previously deformed materials
gives rise to high rates of selective growth of certain grains, to which the terms
''exaggerated grain growth'' or ''secondary recrystallization'' are applied. Such an
effect seems to occur in marble (Schmid et al.
1980
), although in the case of metals it
is said to be only observed in sheet material (Christian
1965
, p. 737). The kinetics of
grain growth can also be importantly influenced by the behavior of impurities
segregated in the grain boundaries since they affect the grain boundary energy. Thus,
there may be different rates of grain boundary migration depending on whether the
impurities migrate with the grain boundary or are left behind (Guillopé and Poirier
1979
; Lücke and Stüwe
1971
).
3.3.4 Solid-State Transformations and Reactions
Polymorphic phase transitions are relatively simple in that no long-range material
transport is necessary. In general, such transformations are heterogeneous and can
be classified into two types, distinguished mainly by whether the atom movements
