Anomalous lattice stiffening in tungsten tetraboride solid solutions with manganese under compression.

Tungsten tetraboride (WB)-based solid solutions represent one of the most promising superhard metal candidates; however, their underlying hardening mechanisms have not yet been fully understood. Here, we explore the lattice compressibility of WB binary solid solutions with different manganese (Mn) concentrations using high-pressure x-ray diffraction (XRD) up to 52 GPa. Under initial compression, the lattices of low and high Mn-doped WB alloys (i.e. WMnB and WMnB) are shown to be more and less compressible than pure WB, respectively. Then, a c-axis softening is found to occur above 39 GPa in WB, consistent with previous results. However, an anomalous sudden a-axis stiffening is revealed at ~36 GPa in WMnB, along with suppression of c-axis softening observed in WB. Furthermore, upon Mn addition, a simultaneous stiffening of a- and c-axes is demonstrated in WMnB at ~37 GPa. Speculation on the possible relationship between this anomalous stiffening and the combined effects of valence-electron concentration (VEC) and atomic size mismatch is also included to understand the origin of the nearly identical hardness enhancement in those two solid solutions compared to WB. Our findings emphasize the importance of accurate bonding and structure manipulation via solute atoms to best optimize the hardness of WB solid solutions.