Synthesis and High-Pressure Mechanical Properties of Superhard Rhenium/Tungsten Diboride Nanocrystals

Rhenium diboride is an established superhard compound that can scratch diamond and can be readily synthesized under ambient pressure. Here, we demonstrate two synergistic ways to further enhance the already high yield strength of ReB₂. The first approach builds on previous reports where tungsten is doped into ReB₂ at concentrations up to 48 at. %, forming a rhenium/tungsten diboride solid solution (Re_0.52W_0.48B₂). In the second approach, the composition of both materials is maintained, but the particle size is reduced to the nanoscale (40-150 nm). Bulk samples were synthesized by arc melting above 2500 °C, and salt flux growth at ∼850 °C was used to create nanoscale materials. In situ radial X-ray diffraction was then performed under high pressures up to ∼60 GPa in a diamond anvil cell to study mechanical properties including bulk modulus, lattice strain, and strength anisotropy. The differential stress for both Re_0.52W_0.48B₂ and nano ReB₂ (n-ReB₂) was increased compared to bulk ReB₂. In addition, the lattice-preferred orientation of n-ReB₂ was experimentally measured. Under non-hydrostatic compression, n-ReB₂ exhibits texture characterized by a maximum along the [001] direction, confirming that plastic deformation is primarily controlled by the basal slip system. At higher pressures, a range of other slip systems become active. Finally, both size and solid-solution effects were combined in nanoscale Re_0.52W_0.48B₂. This material showed the highest differential stress and bulk modulus, combined with suppression of the new slip planes that opened at high pressure in n-ReB₂.