Superhard Rhenium/Tungsten Diboride Solid Solutions.

Rhenium diboride (ReB2), containing corrugated layers of covalently bonded boron, is a superhard metallic compound with a microhardness reaching as high as 40.5 GPa (under an applied load of 0.49 N). Tungsten diboride (WB2), which takes a structural hybrid between that of ReB2 and AlB2, where half of the boron layers are planar (as in AlB2) and half are corrugated (as in ReB2), has been shown not to be superhard. Here, we demonstrate that the ReB2-type structure can be maintained for solid solutions of tungsten in ReB2 with tungsten content up to a surprisingly large limit of nearly 50 atom %. The lattice parameters for the solid solutions linearly increase along both the a- and c-axes with increasing tungsten content, as evaluated by powder X-ray and neutron diffraction. From micro- and nanoindentation hardness testing, all of the compositions within the range of 0-48 atom % W are superhard, and the bulk modulus of the 48 atom % solid solution is nearly identical to that of pure ReB2. These results further indicate that ReB2-structured compounds are superhard, as has been predicted from first-principles calculations, and may warrant further studies into additional solid solutions or ternary compounds taking this structure type.