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

TL;DR

This study provides experimental and theoretical evidence of a dual anomaly in vanadium, showing compression-induced softening and heating-induced hardening, along with new phase transition insights under shock compression.

Contribution

It reports the first combined experimental and first-principles evidence of the dual softening-hardening anomaly in vanadium and reveals new high-pressure phase transitions.

Findings

Compression reduces shear sound velocity in vanadium.

Heating increases sound velocity, indicating hardening.

Vanadium transitions from BCC to two rhombohedral phases at high pressures.

Abstract

Solid usually becomes harder and tougher under compression, and turns softer at elevated temperature. Recently, compression-induced softening and heating-induced hardening (CISHIH) 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 sound velocities measured at Hugoniot states generated by shock-waves, 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 shear modulus by compression, whereas the latter reflects the reverse hardening by heat. These experiments also unveil another anomaly in Young's modulus that wasn't reported before. Based on the experimental and theoretical data, we infer that vanadium might transition from BCC into two different rhombohedral (RH1 and RH2) phases at about 79GPa and 116GPa 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 high-pressure phase transition dynamics.