Shock-induced phase transformation in tantalum

Abstract

-A TEM study of pure tantalum and tantalum-tungsten alloys explosively shocked at a peak pressure of 30 GPa is presented. While no omega phase was found in shock-recovered pure Ta and Ta-5W that contain mainly a cellular dislocation structure, shock-induced omega phase was found in Ta-10W that contains evenly-distributed dislocations with a density higher than 1 x 10 12 cm -2 . The shock-induced α (bcc) → ω (hexagonal) transition occurs when dynamic recovery of dislocations become largely suppressed in Ta-10W shocked under dynamic pressure conditions. A dislocation-based mechanism is proposed for the shock-induced phase transformation.

Femtosecond X-Ray Diffraction Studies of the Reversal of the Microstructural Effects of Plastic Deformation during Shock Release of Tantalum

“…Whilst defect densities consistent with observed plastic strain rates are not found in recovered samples [6,7], such high densities are observed in many MD simulations [8,9], and long prior to those simulations the generation of high densities of homogeneously nucleated dislocations at the shock front had been proposed [3]. A resolution to this discrepancy is suggested by further MD simulations that show that upon the shock unloading at a free surface, and subsequent rarefaction, most of the dislocations annihilate [10] implying that post facto analysis of recovered samples may at best not provide a full picture of the conditions present during the passage of the shock itself.…”

Shock-induced plasticity in tantalum single crystals: Interatomic potentials and large-scale molecular-dynamics simulations

“…A non-equilibrium phase transformation in the shock experiments has been proposed as a plausible explanation for the experimentally observed transition at low pressures. 8 These issues underscore some of the current controversies regarding the high-temperature/highpressure properties of this system. Surprisingly, while there have been many computational studies of the thermal and mechanical properties of Ta at high pressures, [9][10][11] there has not been, to date, a systematic atomistic simulation study of the dynamical response of Ta to shock-wave and high strain-rate loading.…”

Predicted alternative structure for tantalum metal under high pressure and high temperature

“…However, what Hsiung observed in the recent experiment is the pseudo-hex-ω with fractional coordinates: (0,0,0), (2.056/3,0.944/3,1/2), and (0.944/3,2.056/3,1/2). 24 We started from these initial coordinates in our optimiza-tions, but obtained the ideal hex-ω with coordinates: (0,0,0), (2/3,1/3,1/2), and (1/3,2/3,1/2). And the hightemperature phonon dispersion curves of hex-ω Ta also show considerable imaginary frequencies, suggesting it is not dynamically stable.…”