Thermophysical properties of liquid Al-Ti-V alloys

Kurzfassung

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.