Density and excess volume of the liquid Ti–V system measured in electromagnetic levitation

The density of the liquid Ti–V system was systematically measured using the optical dilatometry method in electromagnetic levitation. Possible error sources have been discussed and minimized. A linear temperature dependency with negative slope of the density was found for all investigated alloys. Pure vanadium shows the highest density, pure titanium the lowest with every measured alloy ranging between these two extrema. The molar volume was utilized in order to interpret the compositional density dependency. No significant excess volume was evident. It was therefore shown that the Ti–V system acts like an ideal solution regarding density, molar volume and temperature coefficient. This result allows to reliably calculate the density for the complete Ti–V system at any given temperature

Surface tension and excess volume of the liquid Ti-V system measured in electromagnetic levitation

Due to their light weight, high strength, increased ductility, large corrosion resistance, and biocompatibility, Ti-based alloys have raised significant interest in recent years. They are ideal candidates for operation under extreme conditions, such as high temperature or aggressive chemical environment. alpha + beta titanium alloys in particular are of high interest for aerospace applications as well as for medical applications, with vanadium being one of the most prominent beta stabilisers. The addition of vanadium can elevate the thermal as well as the corrosive stability of Ti-Al alloys, especially those of lower aluminum content. The fast-growing interest in these alloys requires precise knowledge of thermophysical properties of the liquid phase as input for process optimization, phase calculation and atomic modelling. Density and the molar volume are two of the most fundamental thermophysical properties. The rather high melting temperatures of titanium and vanadium of 1941K (1668°C) and 2183K (1910°C), respectively greatly complicate their measurement using conventional container-based methods. Due to the highly reactive nature of the liquid Ti-V system, a container-less measurement needs to be implemented in order to avoid any reactions of the investigated liquid with existing container walls. In this work the already established optical dilatometry method is used for the density and molar volume determination of the liquid Ti-V system in electromagnetic levitation [1]. So far, there is not yet any model or rule of thumb in order to predict the molar volume of any liquid alloy, its density, its excess volume, or even the sign of the latter. Titanium alloys generally show a strongly non-ideal behavior with regard to their mixing properties, depending on the alloying element [2]. However, it has been shown that liquid alloys consisting of elements with similar electronic configuration, which is the case for titanium and vanadium, seem to exhibit almost ideal behavior with respect to the molar volume [3]. It is therefore especially interesting to investigate, how Ti-V behaves with respect to the molar volume and density. The present work uses electromagnetic levitation in order to containerlessly measure density and thermal expansion of Ti-V as function of both, temperature and composition. Thereupon, the molar volume of the Ti-V system is discussed in relation to existing trends predicting the excess volume of metallic alloys. First data is presented

Impact of convection on the damping of an oscillating droplet during viscosity measurement using the ISS-EML facility

Oscillating droplet experiments are conducted using the Electromagnetic Levitation (EML) facility under microgravity conditions. The droplet of molten metal is internally stirred concurrently with the pulse excitation initiating shape oscillations, allowing viscosity measurement of the liquid melts based on the damping rate of the oscillating droplet. We experimentally investigate the impact of convection on the droplet’s damping behavior. The effective viscosity arises and increases as the internal convective flow becomes transitional or turbulent, up to 2–8 times higher than the intrinsic molecular viscosity. The enhanced effective viscosity decays when the stirring has stopped, and an overshoot decay pattern is identified at higher Reynolds numbers, which presents a faster decay rate as the constraint of flow domain size becomes influential. By discriminating the impact of convection on the viscosity results, the intrinsic viscosity can be evaluated with improved measurement accuracy

Oxygen partial pressure control for microgravity experiments

A system consisting of a high-temperature yttrium-stabilized zirconia (YSZ) based oxygen ion pump and potentiometric sensor enables precise measurement and control of oxygen partial pressure, pO2, at elevated temperatures within 0.2 to 10^-20 bar. <the principle of operation as well as the influence of temperature and gas buffers like H2/H2O on the pO2 is discussed. The ion pump is controlled by a microcontroller system and adjusts the oxygen partial pressure with an uncertainty of Delta-log(pO2)<0.02 and response times between 5 and 90s over the entire pO2 range. The oxygen ion pump is tested in combination with the electromagnetic levitation. Here, the surface tension of molten Ni at 1720 °C as a function of oxygen partial pressure is determined. A good agreement of this measurement with calculated value confirms the applicability of the system for high-temperature measurement and control of pO2. The developed hardware is suitable for the electromagnetic levitation facility onboard the international space station (ISS)

Surface tension of liquid Al-Cu binary alloys.

Surface tension data of liquid Al–Cu binary alloys have been measured contactlessly using the technique of electromagnetic levitation. A digital CMOS-camera (400 fps) recorded image sequences of the oscillating liquid sample and surface tensions were determined from analysis of the frequency spectra. Measurements were performed for samples covering the entire range of composition and precise data were obtained in a broad temperature range. It was found that the surface tensions can be described as linear functions of temperature with a negative slope. Moreover, they monotonically decrease with an increase in the aluminium concentration. The observed behaviour with respect to both temperature and concentration is in agreement with thermodynamic model calculations using the subregular solution approximation

Ground-Based Electromagnetic Levitation (EML) for the Measurement of Thermophysical Properties

This section gives a short description of ground based electromagnetic levitation and its technical foundations. In addition, the various diagnostic means for the measurement of the following thermophysical properties of liquid metals and alloys will be described briefly: density, surface tension, self-diffusion coefficient, electrical conductivity, spectral normal emissivity, heat capacity, and thermal conductivity

Thermophysical properties of multicomponent liquid alloys and their measurement at elevated temperature

Economic-, cultural- and social- evolution goes hand in hand with the development and continuous improvement of new materials. The casting of metals, their optimization and the advancement of their production processes is especially crucial in this context. The liquid phase plays a key role in this context, as the vast majority of materials is made directly from the melt or under its direct involvement. For any process of materials development, the properties of the liquid metal need to be known precisely. As liquid metals are generally highly reactive at elevated temperature, the use of levitation methods is inevitable. In addition, technical alloys are generally multicomponent materials. This makes the investigation of the concentration dependence of their properties especially important. The present talk will describe electromagnetic levitation and corresponding diagnostic methods in order to contactlessly determine thermophysical properties such as density, surface tension, specific heat and several others. The concentration dependence of these properties will be discussed systematically. Three distinct classes of materials can be identified where the members of each class exhibit qualitatively the same behavior with respect to the mixing rules and applicable solution models. Furthermore, the effect of oxygen on the surface tension of liquid metals will be discussed using the Butler equation