Geology Reference
In-Depth Information
3.3 Changes in Crystalline Organization
3.3.1 General
We now consider some particular rate processes that may be viewed as simple
reactions, or transformations, involving only one reactant and one product. These
are the organizational processes affecting the atomic structure of the crystalline
grains and the granular structure of the polycrystalline body. Here it is useful to
distinguish, in general, between homogeneous and heterogeneous processes.
A homogeneous process is one that proceeds simultaneously in all parts of the
body concerned. A heterogeneous process is one that is, at any instant, localized at
certain discrete sites or along a discrete front, the location of the activity migrating
through the body with time in order to affect all parts. However, the distinction
may be to some extent a matter of scale; a process that is heterogeneous when
viewed at one scale may appear statistically homogeneous when viewed on
another scale.
In rock deformation studies, interest in organizational processes arises for
several reasons. First, there is a tendency for the structural disruption caused by the
deformation processes themselves to be restored, as in recovery and recrystalli-
zation processes. Second, there may be a tendency for the re-equilibration of a
body to changed environmental conditions involved with the deformation, leading
to phase transitions, precipitation, or other solid-state reactions that are coupled
with the deformation of the body. Third, the primary growth or dissolution of
crystals may be concerned, especially if a fluid phase is present in pores.
3.3.2 Recovery
The term recovery, in its original metallurgical usage, refers to the restoration or
change, without recrystallization, of physical properties such as indentation,
hardness, or electrical resistivity during the annealing of a body that has been
previously plastically deformed. It is now used more generally to refer to any more
or less homogeneous intracrystalline response to the presence of structural defects
in excess of an equilibrium concentration that have been introduced by plastic
deformation or other cause such as irradiation. Thus, the term is now used in a
mechanistic sense for a reorganization within the crystal with respect to structural
defects based on the original crystal structure, which may proceed either con-
currently with or subsequent to the deformation or other disrupting process.
The structural defects involved in recovery may be point defects or extended
defects. In the first case, recovery to an equilibrium state may be achieved in
available time if the temperature is sufficiently high. However, in the second case,
involving dislocations or stacking faults that would need to be completely elimi-
nated to achieve thermodynamic equilibrium, recovery is usually only partial.
