  To examine the microstructural evolution that occurs during transient creep,
we deformed olivine aggregates to different strains that spanned the initial
transient deformation. Two sets of samples with different initial grain sizes
of 5 $\mu$m and 20 $\mu$m were deformed in torsion at T = 1523 K, P = 300 MPa,
and a constant shear strain rate of 1.5 $\times$ 10$^{-4}$ s$^{-1}$. Both sets
of samples experienced strain hardening during deformation. We characterized
the microstructures at the end of each experiment using high-angular resolution
electron backscatter diffraction (HR-EBSD) and dislocation decoration. In the
coarse-grained samples, dislocation density increased from 1.5 $\times$
10$^{11}$ m$^{-2}$ to 3.6 $\times$ 10$^{12}$ m$^{-2}$ with strain. Although the
same final dislocation density was reached in the fine-grained samples, it did
not vary significantly at small strains, potentially due to concurrent grain
growth during deformation. In both sets of samples, HR-EBSD analysis revealed
that intragranular stress heterogeneity increased in magnitude with strain and
that elevated stresses are associated with regions of high geometrically
necessary dislocation density. Further analysis of the stresses and their
probability distributions indicate that the stresses are imparted by long-range
elastic interactions among dislocations. These characteristics indicate that
dislocation interactions were the primary cause of strain hardening during
transient creep. A comparison of the results to predictions from three recent
models reveals that the models do not correctly predict the evolution in stress
and dislocation density with strain for our experiments due to a lack of
previous such data in their calibrations.
