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Introduction
The goal of experimental deformation studies is to
activate the same processes that occur in nature, but under
known and controlled conditions. In order to activate
crystal plastic deformation processes at relatively fast
laboratory strain rates (10-4/sec to 10-7/sec), experiments
are done at higher temperatures and confining pressures (and
thus water fugacities) than those in nature. Most
experiments on quartz aggregates have been done 'as-is',
with the naturally occurring water content of ~0.1-0.2 wt%,
but for some a small amount (~0.1-0.2 wt%) of water was
added, to investigate the effect of this important variable.
Over the range of experimental strain rates, addition of
~0.15 wt% water has approximately the same effect on
dislocation creep strengths as increasing the temperature by
100°C.
Most of the experimentally deformed samples illustrated
in this chapter have been subjected to axial compression at
a constant strain rate, and the compression direction in the
photos is vertical. The samples start out as cylinders 6.3 mm
(0.25") in diameter and ~15 mm (0.6") long; they are
shortened by up to 65%. The photos have been taken from the
center portions of the samples. One sample was subjected to
a combination of compression and shear, using pistons cut at
45° to the apparatus compression direction. At the end
of most deformation experiments, the temperature is rapidly
quenched (down to 300°C in < 2 minutes) while the
sample remains under differential stress, in order to
preserve the deformation microstructures. However at the end
of a few experiments, the differential stress was removed
and the sample was allowed to remain at P and T, to study
the effects of static annealing.
Many photos of experimentally deformed samples show
horizontal extension cracks; these result from decompression
at the end of the experiments, after deformation and
quenching. The thin sections are extra thin and doubly
polished, in order to more clearly show details of small
recrystallized grains.
The photos in this chapter illustrate the transition from
semi-brittle flow (distributed microcracks and dislocations)
to crack-free dislocation creep, which occurs with
increasing temperature or decreasing strain rate (and thus
with decreasing flow stress). As explained briefly in the
Preface to this contribution, three microstructurally
distinct regimes of dislocation creep, associated with
distinct processes of dynamic recrystallization, have been
identified in experimentally quartz aggregates, and these
same regimes have been recognized in quartz aggregates
naturally deformed at lower temperatures and slower strain
rates. The distinctions between these regimes are explained
more fully in the individual photo captions. One image
illustrates the asymmetry in microstructures that results
from a component of simple shear, and another pair of images
illustrates the effect of static annealing after
deformation.
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