Evidence for mechanical softening-hardening dual anomaly in transition metals from shock-compressed vanadium

Solids usually become harder and tougher under compression and turn softer at elevated temperature. Recently, the compression-induced softening and heating-induced hardening dual anomaly was predicted in group VB elements such as vanadium. Here, the evidence for this counterintuitive phenomenon is reported. By using accurate high-temperature, high-pressure (HP) sound velocities measured at Hugoniot states generated by shockwaves, together with first-principles calculations, we observe not only the prominent compression-induced sound velocity reduction but also strong heating-induced sound velocity enhancement in shocked vanadium. The former corresponds to the softening in the shear modulus by compression, whereas the latter reflects the reverse hardening by heat. These experiments also unveil another anomaly in Young's modulus. Based on the experimental and theoretical data, we infer that vanadium might transition from body-centered cubic into two different rhombohedral phases at \ensuremath{\sim}79 and 116 GPa along the Hugoniot, respectively, which implies a dramatic difference in static and dynamic loading, as well as the significance of deviatoric stress and rate-relevant effects in HP phase transition dynamics.