SURFACE TENSION OF LIQUID TI, V AND THEIR BINARY ALLOYS MEASURED BY ELECTROMAGNETIC LEVITATION

Abstract

Ti-based alloys are prime candidates for construction materials used for applications in extreme conditions and environments, due to the combination of light weight with high strength, high temperature stability, large corrosion resistance as well as their bio-compatibility. Alongside this rising industrial demand comes an increasing need for precise knowledge of the thermophysical properties of Ti-based alloys, as input for process optimization, phase calculation and atomic modelling. The Ti-Al-V system is currently the most relevant alloy system when it comes to industrial application. While, now, there are some data regarding the thermophysical properties of the Ti-Al [1] side of the system, reliable and systematic data for the Ti-V system are still sparse. One of the major reasons for this is the difficult processability of liquid Ti-V at elevated temperatures, due to the high chemical reactivity of the system. Electromagnetic levitation offers a container-less measurement method for density as well as surface tension without the risk of contamination of the sample by container walls. Density is hereby measured in a shadow graph technique where an expanded laser is directed onto the levitating sample. The molar volume is subsequently obtained from integration over the profile edge curve of the sample shadow captured by a camera on the other side of the sample. Surface tension is measured by means of the oscillating drop technique. Here a high-speed camera records the surface oscillations of the molten droplet and the frequency spectra are evaluated according to the sum formula of Cummings and Blackburn. The obtained density and surface tension data are analyzed by means of different thermodynamic models. In case of the density, it is found that Ti-V obeys the ideal solution model, so no excess volume needs to be considered [2]. The surface tension data can be evaluated using Butler’s thermodynamic model. Again, near perfect agreement is found only with the ideal solution model. Even while utilizing the levitation method, contact between the melt and the surrounding atmosphere cannot completely be prevented. Oxygen, plays a crucial role for metallic melts in most applications, since already small oxygen contents can greatly influence the thermophysical properties of liquid alloys [3]. Therefore, in a second step, the influence of oxygen on the before studied thermophysical properties of the Ti-V system will be reviewed