Fracture strength of grains and grain boundaries in a dual–phase high-entropy ultra-high temperature ceramics

The fracture behaviour of grains and grain boundaries plays an important role in the design of novel ceramics with improved macromechanical deformability. This was investigated first time in a dual-phase high-entropy carbide/boride ceramic, using microcantilever bending, linear beam theory and micro/nanofractography. Microcantilevers were focused ion beam milled from the carefully polished sample surface with random locations containing both carbide and boride grains and grain boundaries in the beam volume. Scanning electron microscopy-based fractography analysis revealed that the bending strength of grain/phase boundaries varies in the range of 0.9-6.7GPa, depending on the orientation and location of the boundaries in the beam. The fracture of carbide grains was found to be initiated mainly on submicron size volume defects with strength values of 4.5-9.5GPa, while the fracture origin in most boride grains was identified as surface defects introduced by specimen preparation, which resulted in higher fracture strengths of 9.0-12.3GPa.