The role of grain boundaries in plastic deformation
Keywords: grain boundaries
forsterite
micropillar
in-situ deformation
nanoindentation

Grain boundaries in polycrystalline materials underpin deformation processes in both low- and high- temperature regimes. The study of grain boundary types relies on the investigation of small volumes of material containing one interface. This thesis presents small-scale mechanical tests on forsterite grain boundaries. Forsterite is the Mg-rich end member of the solid solution of the mineral olivine.
Olivine is a key geological material influencing large-scale, long-term planetary 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 the yield stress. Laboratory observations have demonstrated that aggregates with smaller grains are stronger than aggregates with coarser grains. However, the interactions between intracrystalline defects and grain boundaries leading to this effect remain unresolved in olivine. Moreover, 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. In this study, we use high-precision mechanical testing of synthetic forsterite bicrystals with well characterised interfaces and directly observe and quantify the mechanical properties of olivine grain boundaries. We perform room-temperature spherical and Berkovich nanoindentation tests on low- and high- angle grain boundaries. We observe that in the high-angle grain boundary sample, the interface acts as a site of microplasticity and as a barrier to incoming dislocations. The low-angle grain boundary does not interact with other crystalline defects in a measurable manner. Additionally, we conduct in-situ micropillar compression tests at high-temperature (700C) on low- and high-angle grain boundaries. In these in-situ experiments, the boundary is contained within the micropillar and oriented at 45 degrees to the loading direction to promote shear along the boundary. We observe differences in deformation style between the pillars containing the grain boundary and the pillars in the crystal interior. The 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 advanced microstructural characterization confirms that under these experimental conditions, sliding did not occur along the grain boundary. These observations support the hypothesis that at our experimental conditions grain boundaries are relatively stronger compared to the crystal interior, and some grain boundaries can act as potent sources of dislocations in olivine.
Studying grain boundaries and their interaction with crystalline defect relies on small-scale, high-precision mechanical testing and microstructural characterization. One such technique is instrumented nanoindentation with a continuous stiffness measurement, which has gained increased popularity in material sciences. Studies implementing spherical nanoindentation 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 machine stiffness 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) and validate our proposed workflow against key benchmarks. We apply the resulting calibrations to data collected in materials with varying ductility (olivine, titanium, tungsten) to extract indentation stress-strain curves. We synthesize these analysis routines in a single workflow for use in future studies aiming to extract and process data from spherical nanoindentation.
Another strategy for investigating grain boundaries and microstructure evolution represents in-situ mechanical testing. In order to test materials under simple shear studies implement different strategies to transform a far-field compressive or tensile applied stress into shear stress in both mesocale and microscale specimens. In this study, we describe an apparatus developed to investigate in-situ ductile materials by directly applying a simple shear to a thin sheet of material. We propose a novel sample geometry compatible with established techniques for strain characterization and specimens with a wide range of starting microstructures. We demonstrate the capabilities of our apparatus by deforming solder alloys and aluminium, and use optical digital image correlation and electron microscopy for the in-situ characterization of the deforming sample.