Microstructural and Micromechanical Evolution of Olivine Aggregates During Transient Creep

Abstract: 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.