Sound velocities, shear modulus, and yield strength of tungsten to 141 GPa

Abstract

Tungsten is an elemental metal known for its high melting point and hardness, but experimental information about its mechanical properties at high pressures is limited. Here, longitudinal sound velocity, VL, of tungsten compressed in a diamond anvil cell up to P = 141 GPa was measured using picosecond laser ultrasonics. Thus, the range of static pressures for which its VL is known was extended by more than one order of magnitude, corresponding to a 31% increase in density compared with that at atmospheric pressure. We have found that the VL grows monotonically with pressure and reaches 7.75 ± 0.18 km/s at P = 141 GPa. Applying an earlier published equation of state, ρ(P), we derived the pressure-dependent shear modulus, G(P), and transversal sound velocity, VT(P), of tungsten. Both parameters continuously increase with pressure, but their growth rates diminish upon compression. Based on these data, the evolution of tungsten ductility/brittleness with pressure was assessed using Pugh's ratio, B/G, and a transition from intermediate- to weakly-brittle state was recognized. Combining our G(P) with previously measured yield strength of tungsten at high pressures, σy(P), the ratio ∂σy/∂G = 0.017 ± 0.002 was established, and σy ∼ 7.5 GPa at P = 141 GPa was estimated from linear extrapolation. Finally, we have found that the linear dependence of VL on density, known as Birch's law, holds for tungsten up to the maximal pressure of our work.