Nanoindentation experiments on a high-angle grain boundary (60 * misorientation) in a pure forsterite bicrystal reveal that the interface acts as a source of dislocations. * Nanoindentation experiments on a high-angle grain boundary (60 * misorientation) in a pure forsterite bicrystal reveal that the interface acts as an obstacle to incoming dislocations, leading to pileups of dislocations.* Nanoindentation experiments on a subgrain boundary (13 * misorientation) in a pure forsterite bicrystal do not detect the impact of the interface on dislocations.

Rheological properties of olivine influence large‐scale, long‐term deformation processes on rocky planets. Studies on the deformation of olivine at low temperatures and high stresses have emphasized the importance of a grain‐size effect impacting yield stress. Laboratory studies indicate that aggregates with finer grains are stronger than those with coarser grains. However, the specific interactions between intracrystalline defects and grain boundaries leading to this effect in olivine remain unresolved. In this study, to directly observe and quantify the mechanical properties of olivine grain boundaries, we conduct nanoindentation tests on well characterized bicrystals. Specifically, we perform room‐temperature spherical and Berkovich nanoindentation tests on a subgrain boundary (13°, [100]/(016)) and a high‐angle grain boundary (60°, [100]/(011)). These tests reveal that plasticity is easier to initiate if the high‐angle grain boundary is within the deformation volume, whereas the subgrain boundary does not impact the initiation of plasticity. Additionally, the high‐angle grain boundary acts as a barrier to slip transmission, whereas the subgrain boundary does not interact with dislocations in a measurable manner. We suggest that the distribution of grain‐boundary types in olivine‐rich rocks might play a role in generating local differences during deformation.

Instrumented spherical nanoindentation with a continuous stiffness measurement has gained increased popularity in material science studies in brittle and ductile materials alike. These investigations span hypotheses related to a wide range of microphysics involving grain boundaries, twins, dislocation densities, ion-induced damage and more. These studies rely on the implementation of different methodologies for instrument calibration and for circumventing tip shape imperfections. In this study, we test, integrate, and re-adapt published strategies for tip and machinestiffness calibration for spherical tips. We propose a routine for independently calibrating the effective tip radius and the machine stiffness using three reference materials (fused silica, sapphire, glassy carbon), which requires the parametrization of the effective radius as a function of load. We validate our proposed workflow against key benchmarks, such as variation of Young's modulus with depth. We apply the resulting calibrations to data collected in materials with varying ductility (olivine, titanium, and tungsten) to extract indentation stress-strain curves. We also test the impact of the machine stiffness on recently proposed methods for identification of yield stress, and compare the influence of different conventions on assessing the indentation size effect. Finally, we synthesize these analysis routines in a single workflow for use in future studies aiming to extract and process data from spherical nanoindentation.

<p>The mechanics of olivine deformation play a key role in large-scale, long-term planetary processes, such as the response of the lithosphere to tectonic loading or the response of the solid Earth to tidal forces, and in short-term processes, such as the evolution of roughness on oceanic fault surfaces or postseismic creep within the upper mantle. Many previous studies have emphasized the importance of grain-size effects in the deformation of olivine. However, most of our understanding of the role of grain boundaries in deformation of olivine is inferred from comparison of experiments on single crystals to experiments on polycrystalline samples.</p><p>To directly observe and quantify the mechanical properties of olivine grain boundaries, we use high-precision mechanical testing of synthetic forsterite bicrystals with well characterised interfaces. We conduct nanoindentation tests at room temperature on low-angle (13<sup>o</sup> tilt about [100] on (015)) and high-angle (60<sup>o</sup> tilt about [100] on (011)) grain boundaries. We observe that plasticity is easier to initiate if the grain boundary is within the volume tested. This observation agrees with the interpretation that certain grain-boundary configurations can act as sites for initiating microplasticity.</p><p>As part of continuing efforts, we are also conducting in-situ micropillar compression tests at high-temperature (above 600<sup>o</sup> C) within similar bicrystals. In these experiments, the boundary is contained within the micropillar and oriented at 45<sup>o</sup> to the loading direction to promote shear along the boundary. In these in-situ tests, our hypothesis is that the low-angle grain boundary displays a higher viscosity relative to the high-angle interface. Key advantages of performing in-situ experiments are the direct observation of grain-boundary migration or sliding, simplified kinematics of a single boundary segment, and  potentially changes in style of deformation with different grain-boundary character.</p><p>These small deformation volume experiments allow us to qualitatively explore the differences between the crystal interior and regions containing grain boundaries. Overall, the variation in strain and temperature in our small scale experiments allows the fundamental investigation of the response of well characterised forsterite grain boundaries to deformation. </p>

The mechanics of olivine deformation play a key role in long-term planetary processes, including  the response of the lithosphere to tectonic loading or the response of the solid Earth to tidal forces,  and in short-term processes, such as post-seismic creep within the upper mantle. Previous studies  have emphasized the importance of grain-size effects in the deformation of olivine. Most of our  understanding of the role of grain boundaries in the deformation of olivine is inferred from comparison  of experiments on single crystals to experiments on polycrystalline samples, as there are no direct  studies of the mechanical properties of individual grain boundaries in olivine. In this study, we use  high-precision mechanical testing of synthetic forsterite bicrystals with well characterized interfaces  to directly observe and quantify the mechanical properties of olivine grain boundaries. We conduct  in-situ micropillar compression tests at high-temperature (700• C) on bicrystals containing low-angle (4• tilt about [100] on (014)) and high-angle (60• tilt about [100] on (011)) boundaries. During  the in-situ tests, we observe differences in deformation style between the pillars containing the  grain boundary and the pillars in the crystal interior. In the pillars containing the grain boundary,  the interface is oriented at ∼ 45• to the loading direction to promote shear. In-situ observations  and analysis of the mechanical data indicate that pillars containing the grain boundary consistently  support elastic loading to higher stresses than the pillars without a grain boundary. Moreover, the  pillars without the grain boundary sustain larger plastic strain. Post-deformation microstructural  characterization confirms that under the conditions of these deformation experiments, sliding did not occur along the grain boundary. These observations support the hypothesis that grain boundaries are stronger relative to the crystal interior at these conditions.