Microscopic evidence of a flat melting curve of tantalum

Abstract

New data on the high-pressure melting curve of Ta up to 48 GPa are reported.Evidence of melting from changes in sample texture was found in five different experiments using scanning electron microscopy. The obtained melting temperatures are in excellent agreement with earlier measurements using x-ray diffraction or the laser-speckled method but are in contrast with several theoretical calculations. The results are also compared with shock-wave data.These findings are of geophysical relevance because they confirm the validity of earlier experimental techniques that resulted in low melting slopes of the transition metals measured in the diamond-anvil cell, including iron.

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

“…The existence of a phase transition (such as the postulated bcc → hex-ω one) at high pressures and temperatures has also been proposed as a way to explain marked differences in the melt line between DAC and shock-wave experiments. [4][5][6][7] However, so far no direct evidence of a high-pressure phase transition in Ta has been reported in DAC experiments. A non-equilibrium phase transformation in the shock experiments has been proposed as a plausible explanation for the experimentally observed transition at low pressures.…”

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

“…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. The large defect densities thought to be present under shock compression may also be pertinent to apparent contradictions in measured melting temperatures at high pressure [11][12][13]. Common methods of interrogating sample response, such as velocimetry techniques using VISAR (Velocity Interferometer System for Any Reflector) [14], rely on measurements of the velocity of the free surface in order to deduce the sample's response [15].…”

Flash melting of tantalum in a diamond cell to 85 GPa

“…Problems associated with these techniques are the short time scale and chemical contamination of Ta, respectively. Previous LHDAC studies [1,18] that attributed melting to optical observations of textural changes on the sample surface lacked chemical analysis, and from our present observations, chemical contaminations of Ta during prolonged heating were highly likely in these studies. The in-situ XRD measurements in the LHDAC [20] that reported the only experimental steep melting curve relied on the appearance of a diffuse scattering ring and/or diffraction spots arising from fast recrystallization [20,32].…”

“…Most measurements on Ta melting in the laser-heated diamond cell (LHDAC) using optical and textural observations [1,18] and synchrotron X-ray diffraction (XRD) [19] have produced flat melting curves (dT/dP → 0) with one exception [20]. Both shock data and theory have predicted steep but strongly varying melting slopes.…”