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ABSTRACT In this work, failure of a magnesium aluminate spinel (MgAl₂O₄) is investigated at the microscale by concurrent in situ imaging and loading within a transmission electron microscope. The goal of the effort is to quantitively measure the grain boundary fracture toughness of a spinel bi‐crystal and study the toughness property disparity between the grain boundary and lattice (measured in an earlier effort). Additionally, the mode mixity dependence of the grain boundary fracture properties is measured as the applied loading configuration is varied. By placing a notch aligned with the grain boundary at the top or bottom edge of a bi‐crystal beam sample, bending experiments can generate grain boundary failure with different mode mixites. Critical energy release rates and mode mixity indicators for each sample were extracted through three‐dimensional finite element analysis (FEA), validated by comparison of particle tracking measurements with the FEA results. For opening‐dominated fracture, the grain boundary exhibited a lower fracture energy when compared to the single crystal lattice. Alternatively, shear‐dominated modes exhibit much larger toughness. 

Recent work in ultra-high temperature in situ electron microscopy has presented the need for accurate, contact-free temperature determination at the microscale. Optical measurement based on thermal radiation (pyrometry) is an attractive solution but can be difficult to perform correctly due to effects, such as emissivity and optical transmission, that must be accounted for. Here, we present a practical guide to calibrating and using a spectral pyrometry system, including example code, using a Czerny-Turner spectrometer attached to a transmission electron microscope. Calibration can be accomplished using a thermocouple or commercial heated sample holder, after which arbitrary samples can be reliably measured for temperatures above ∼600∘C. An accuracy of 2% can be expected with the possibility of sub-second temporal resolution and sub-Kelvin temperature resolution. We then demonstrate this capability in conjunction with traditional microscopic techniques, such as diffraction-based strain measurement for thermal expansion coefficient, or live-video sintering evolution. 

We demonstrate using theoretical, computational, and experimental studies a morphological instability, in which a polycrystalline nanorod breaks up at grain boundaries (GBs) into an array of isolated domains. Our theoretical model is used to establish a neutral stability surface demarcating stable and unstable perturbations. It is shown that GBs play a destabilizing role in which the critical wavelength for the instability decreases with the increase in the GB energy. We carry out phase field simulations, which reveal accelerated pinch-off kinetics with the increase in the GB energy and predict temporal evolution of interfacial profiles in quantitative agreement with experimental observations.