High-pressure studies of size dependent yield strength in rhenium diboride nanocrystals.

The superhard ReB system is the hardest pure phase diboride synthesized to date. Previously, we have demonstrated the synthesis of nano-ReB and the use of this nanostructured material for texture analysis using high-pressure radial diffraction. Here, we investigate the size dependence of hardness in the nano-ReB system using nanocrystalline ReB with a range of grain sizes (20-60 nm). Using high-pressure X-ray diffraction, we characterize the mechanical properties of these materials, including bulk modulus, lattice strain, yield strength, and texture. In agreement with the Hall-Petch effect, the yield strength increases with decreasing size, with the 20 nm ReB exhibiting a significantly higher yield strength than any of the larger grained materials or bulk ReB. Texture analysis on the high pressure diffraction data shows a maximum along the [0001] direction, which indicates that plastic deformation is primarily controlled by the basal slip system. At the highest pressure (55 GPa), the 20 nm ReB shows suppression of other slip systems observed in larger ReB samples, in agreement with its high yield strength. This behavior, likely arises from an increased grain boundary concentration in the smaller nanoparticles. Overall, these results highlight that even superhard materials can be made more mechanically robust using nanoscale grain size effects.