Untitled Document
Laboratory studies of Dislocation Damping
Robert Farla1, Harri Kokkonen1, John Fitz Gerald1, Auke
Barnhoorn1,2, Ulrich Faul3 and Ian Jackson1
1 Research School of Earth Sciences, Australian National University,
Canberra, ACT 0200, Australia
2 Now at Department of Earth Sciences, Utrecht University, 3508 TA Utrecht,
The Netherlands.
3 Department of Earth Sciences, Boston University, Boston, MA 02215, USA
Last year we presented some preliminary results on dislocation annealing
in fine-grained synthetic olivine [1]. Since analyses of these results
showed that dislocations can be preserved over laboratory timescales,
the first experiments to search for dislocation damping in polycrystals
have now been conducted.
Calculations showing the resolved
shear stress (contours) for uniaxial compression (left) and torsion
(right).
Two similar deformed synthetic olivine specimens
have been investigated so far. The first specimen, 6618, has been deformed
in compression up to 2.3% strain, the second, 6646, to 22%. The resulting
dislocation densities are 2.3 and 3.6 µm-2 respectively. The
specimens were exposed to a maximum temperature of 1100°C, and
to oscillating torques of 1 - 1000s periods at each temperature decrement,
generating maximum shear strains of around 10-5. The strains are sensed
from displacements measured with parallel-plate capacitance transducers.
The first attenuation experiment in torsion on specimen 6616 yielded
a surprise null result. This led to some calculations for resolved shear
stresses in relation to deformation in compression or torsion and assuming
that, at high temperatures, the dominant slip system for olivine involves
slip in the [100] direction on the (010) glide plane. The main results
are shown in figure 1 for [100](010) slip in a single crystal olivine.
The two panels (a) and (b) represent deformation in compression and torsion
respectively. The parameters α and β (and γ) are the direction cosines
relative to the crystal axes [α, β, γ] and some major crystal directions
are labelled. In uniaxial compression, the resolved shear stress as indicated
by the contours in (a) has its maximum value of 0.5 for compression parallel
to [110]c. The contours in panel (b) describe the azimuth-dependent resolved
shear stress for different torsion axes. Note that torsional deformation
around the [110]c is the least favoured orientation for stressing [100](010).
In contrast, a cylinder axis parallel to [111]c (ie making equal angles
with all 3 principal unit-cell axes) is a good trade-off if a single
crystal were to be deformed in compression then transferred to attenuation
measurements in torsion. For the case of polycrystalline olivine, preliminary
calculations show that prior torsional deformation should increase the
anelastic strain by 6-fold relative to prior compressional deformation.
Q-1 data for a deformed (6646) and undeformed specimen (6585).
Figure 2 shows measurements of Q-1 for both deformed specimens with different
dislocation densities in comparison to a similar synthetic but undeformed
olivine specimen. Only specimen 6646 with the higher dislocation density
shows enhanced attenuation and only at the highest temperatures (> 1000°C).
In nature, dislocation density should be in equilibrium with the prevailing
tectonic stresses in the upper mantle. Dislocation damping will become
progressively more important as the grain-boundary dissipation decreases
with increasing grain size. Accordingly, dislocation damping could dominate
in melt-free material under upper-mantle conditions.
Since the resolved shear stress calculations indicated the need, prior
torsional deformation experiments will be done through a collaboration
with the University of Minnesota in 2009. Also for single crystals we
plan to investigate the possibility of deforming along [111]c in compression.
For both materials, subsequent measurement of the forced torsional behaviour
will proceed at ANU.
[1] Farla RJM
, Kokkonen H, Fitz Gerald JD, Barnhoorn A, Faul UH and
Jackson I (2008) Dislocation recovery in fine-grained synthetic olivine.
In preparation for submission to Physics and Chemistry of Minerals.