Thermodynamic Coffee Lecture – Property measurement with electromagnetic levitation: density, latent heat, surface tension

Al-Ti-V alloys play an important role in technical applications. Especially, the liquid phase and its properties are of tremendous importance, as the vast majority of materials is directly made by casting. In order to understand and model those thermophysical properties, and in order to be able to optimize a material, systematic composition dependent measurements are mandatory. However, despite the importance of Al-Ti-V alloys, data is sparse for the binary and ternary systems. One reason for the lack of data is the increased chemical reactivity of liquid Ti at high temperatures. Containerless investigation methods, such as electromagnetic levitation, are therefore inevitable.

The present talk presents data on density, surface tension, and molar heat capacity for binary Al-Ti, Al-V, Ti-V and ternary Al-Ti-V alloys. For each alloy system, the composition is systematically varied and thermodynamic mixing rules based on the (sub-)regular solution model are discussed.

It is found that Al-Ti and Al-V are highly non-ideal systems with strongly negative excess volumes and positive excess surface tensions. In contrast to this, Ti-V can be approximated by the ideal solution model. The properties of the ternary alloy system are dominated by the Al-Ti subsystem. Mathematically, a large ternary parameter has to be included into the Redlich-Kister equation in order to compensate for the effect that Al-V, should have on the excess properties if binary interactions are considered alone.

Molar specific heat is discussed for liquid Ti and two Al-Ti alloys with different composition. The data as function of temperature has a positive- or negative slope, depending on the alloy system. The molar specific heat data exhibits a positive deviation from the Neumann-Kopp rule which is consistent with the negative excess Gibbs energy of Al-Ti. The finding shows that the linear approximation of the temperature dependence of the Redlich-Kister interaction parameters is oversimplified for the calculation of the molar heat capacity.          

In addition, the effect of oxygen on the surface tension is studied in detail for liquid Ti and V. Due to the highly attractive interaction between Oxygen and Titanium or Vanadium, respectively, oxygen is preferable coordinated in the bulk. The is surface virtually clean over a broad range of oxygen concentration until the latter is increased above the critical value of 1 at.-%. As a result, surface tension data is almost unaffected by the presence of oxygen in Titanium or Vanadium, as long as the quantities of oxygen remain moderate. For oxygen mole fractions larger than 1 at.-%, the surface tension decreases linearly on the logarithmic scale.