Thermophysical properties of liquid Ti-Al-V alloys under oxygen influence

Kurzfassung

In this work, reliable systematic thermophysical property data on density and surface tension under the influence of oxygen is presented for pure liquid aluminum, titanium and vanadium, for the liquid binary alloys Al-Ti, Al-V and Ti-V as well as for one section through the ternary Al-Ti-V system. Compiling this data serves three distinct purposes. Firstly, the foundation of a thermophysical data base for the ternary Al-Ti-V is laid. Secondly, this thermophysical data helps gaining a fundamental insight into the general mixing behavior in this widely used alloy system. Lastly, the obtained data is investigated under the influence of oxygen both to reassess the validity of the previously developed data foundation and to understand the fundamental interaction of oxygen with the liquid Al-Ti-V system and liquid metals in general. To provide appropriate experimental techniques for both density and surface tension measurements, a new, specialized electromagnetic levitation furnace (EML) is constructed. Electromagnetic levitation is a container-less measurement technique which circumvents many experimental challenges associated with traditional container-based measurement methods when investigating highly reactive metal melts such as the liquid Al-Ti-V system. With the EML, the Al-TiV samples can be processed without the risk of contamination, allowing for a higher undercooling and higher measurement precision. Combining the well-established EML setup with a heating laser as well as a novel oxygen control system (OCS) allows for highly precise measurements of the thermophysical properties under the influence of oxygen by changing the oxygen partial pressure present in the process atmosphere during the experiment. Density and surface tension are measured as a function of temperature for the pure components, the AlV and the Ti-V system as well as for one ternary AlxTi(1-x)0.5V(1-x)0.5 section. From the temperature dependent measurements, the compositional dependence of the density and surface tension can be determined for all different sub-systems. Using this approach, a comprehensive dataset was generated for the Al-Ti-V system, considering previous investigations. Subsequently, different established thermodynamic models for the binary systems were compared against the measured data. The results were then used to gain a more fundamental insight into the general mixing mechanisms, including segregation, of the three elements. Evaluating different thermodynamic models against the obtained data allows for an optimized prediction of the thermophysical properties of the binary, liquid sub-systems. The groundwork for an equally comprehensive prediction capability for the complete Al-Ti-V system, was set by exemplary expanding the established models by the ternary AlxTi(1-x)0.5V(1-x)0.5 section. The OCS is the key instrument to evaluate the influence of oxygen on the thermophysical properties of the Al-Ti-V system. To fully understand the operation principles of the OCS, for which an instrument similar in construction is also intended to expand the EML facility onboard the international space station (ISS), several system investigations were carried out. These lay the foundation for future research involving the OCS on earth and on board the ISS. After successfully integrating the OCS to the EML furnace on ground and establishing a general understanding of its operation, first oxygen dependent measurements were carried out for pure liquid vanadium. The oxygen influence was exemplary investigated on pure liquid vanadium in two different ways. First, the oxygen partial pressure of the process atmosphere was adjusted while simultaneously measuring the surface tension. Subsequently, the oxygen mole fraction of the measured sample was changed by adding oxide powder to the pure vanadium during sample preparation. The results were used to reevaluate the previous thermophysical property measurements against the background of oxygen influence and to gain first insights into the interaction mechanisms between liquid vanadium and oxygen. The first oxygen dependent measurements confirm the reliability of the developed thermophysical database and the prediction quality of the employed models. For liquid vanadium oxygen has only a minor influence on the surface tension for comparably high oxygen concentrations of up to 1 at.-%. Strong similarities can be found in this regard when comparing liquid vanadium with liquid titanium. The vast differences in the interaction with oxygen of liquid vanadium (liquid titanium respectively) and liquid aluminum opens up many exciting research possibilities.