Grain Boundary Fracture of a Magnesium Aluminate Spinel Bi‐Crystal: Microscale Experiments With Varying Mode Mixity

ABSTRACT In this work, failure of a magnesium aluminate spinel (MgAl 2 O 4 ) 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.