Thermophysical properties of liquid vanadium under oxygen influence

Abstract

Vanadium is one of the most important alloying elements in the recently upcoming Ti-alloy systems. Despite its high industrial relevance, reliable data on the thermophysical properties, such as surface tension, which is a key property when designing metal manufacturing processes, is still rare. Like titanium, vanadium has a high solubility of oxygen which can significantly impact the surface tension of liquid metals [1]. In this work, it is our goal to investigate how oxygen influences the surface tension of liquid vanadium.

In order to process such a highly reactive liquid metal, the non-contact method of electromagnetic levitation was applied. The surface tension is measured by means of the oscillating drop technique. A novel Oxygen Control System (OCS), based on Y2O3-stabilized ZrO2 tubes, was implemented to adjust and measure the oxygen content of the surrounding noble gas atmosphere during the measurement. Accessible oxygen partial pressures range between 10-24 an 10-3 bar. Vanadium samples were alloyed with different amounts of vanadium(V)-oxide powder, to create samples with oxygen mole fractions xo between 0.25 at. % and 18 at. %. Afterwards, the surface tension of the samples was measured over a broad temperature range. In a second series of measurements the surface tension of samples of identical composition was measured at a constant temperature while simultaneously increasing the oxygen mole fraction in the process atmosphere. Thereby, the surface tension for liquid vanadium was obtained as a function of oxygen xO in the bulk as well as oxygen partial pressure in the process atmosphere.

All samples show a linear decline in surface tension with increasing temperature. Simultaneously the surface tension amongst the samples decreases with increasing oxygen mole fraction. Up to a threshold of 1.0 at. % bulk oxygen, only a marginal decrease in surface tension at a constant reference temperature can be observed. Beyond that threshold, a stronger, nearly linear decay in surface tension with log(xo) becomes apparent. When repeating the measurements while changing the oxygen partial pressure in the process atmosphere from 10-11 to 10-3 bar, almost no change in surface tension can be observed for samples with a bulk oxygen mole fraction of up to 3.0 at. %. For samples with bulk oxygen mole fractions larger than 10 at. %, the surface tension slightly decreases when increasing the oxygen partial pressure up to 10-4 bar. When further increasing the oxygen partial pressure the surface tension starts to rapidly decrease. It was attempted to link both findings and compare the results with simple, already existing model calculations.