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

The central objective of this work was to study thermophysical properties of liquid Al, Ti, as well as the creation and completing of materials databases with systematically varied compositions in the binary Al-Ti system. Also, it has been found that thermophysical property data on the binary Al-Ti system are also a good starting point for investigations on multicomponent alloys and industrially used compounds, as well as the need for thermophysical data for materials processing. The overall method used for the measurements of this work was electromagnetic levitation (EML). As Al-Ti alloys are highly reactive and generally show high evaporation at elevated temperatures, conventional container-based techniques render vast difficulties. Therefore, contact-free electromagnetic levitation methods offer many advantages, enable to minimize error sources and measure with high accuracy. The measurements were performed at the German Aerospace Center (DLR), Institute of Materials Physics in Space in Cologne, Germany and at the Fukuyama lab at Tohoku University in Sendai, Japan. High-temperature density, ρ, measurements as a function of temperature were performed over the whole Al-Ti composition range. The data were analyzed concerning the density at liquidus temperature, the molar volume, the density temperature coefficients, the thermal volume expansion coefficient, and the excess molar volume. For all Al-Ti compositions, the density decreases linearly with increasing temperature. Additionally, the density at liquidus temperature decreases gradually with increasing Al mole fraction from 4.12 g/cm^3 to 2.30 g/cm^3 for pure Ti and Al, respectively. It was found that Al-Ti exhibits significantly large negative values for the excess molar volume. It was suggested that shrunk Al radii upon mixing contribute to that mechanism. The normal spectral emissivity, ε, of four compositions in the Al-Ti system was measured in dependence of the wavelength and temperature. It was found that all compositions show negligible temperature dependence. The emissivity results at a wavelength of 940 nm, amount to 0.30, 0.32, 0.40 and 0.37 for Al70Ti30, Al50Ti50, Al20Ti80 and Ti, respectively. The composition dependence of the emissivity results of the Al-Ti melts is in very good agreement with literature data of binary and multi-component Al-Ti-based alloys. The obtained values were used as input data for the isobaric heat capacity measurements of this work, analogously conducted of the same four compositions of liquid Al-Ti melts as a function of temperature. For all compositions, the isobaric heat capacity shows negligible temperature dependence with average values of 57.59, 46.20, 54.61 and 48.50 J/K/mol for Al70Ti30, Al50Ti50, Al20Ti80 and Ti, respectively. The composition dependence of the isobaric heat capacity results at constant pressure was compared to results by the Neumann-Kopp rule and literature. The deviation between the measured values and the Neumann-Kopp rule, the excess heat capacity was evaluated, exhibiting positive values for all compositions. High-temperature viscosity, η, of liquid Al50Ti50 was measured as a function of temperature during parabolic flight measured in the DLR μg-EML facility, TEMPUS. The results were fitted by an Arrhenius law with 2.23 mPas and 1.27∙104 J/mol corresponding to the Arrhenius fit parameters, η_∞ and E_A, respectively. The isothermal viscosity at 1750 K shows a positive deviation from the linear composition dependence of the pure elements. The Hirai model, the Kaptay model and the Kozlov model were calculated to predict the composition dependence of the viscosity in the Al-Ti system with the closest agreement of the experimental results found with the Kozlov model. The surface tension, γ, of ten compositions in the Al-Ti system was measured as a function of temperature, including Al50Ti50 surface tension data obtained under microgravity conditions. For all compositions the surface tension decreases linearly with rising temperature yielding linear fit parameters; the surface tension at liquidus temperature, and the surface tension temperature coefficient. In addition, within the measured composition range, the surface tension increases gradually with decreasing Al mole fraction at the respective liquidus temperatures. The isothermal surface tension at T=1950 K is in good agreement with predictions according to the Butler and the Chatain model for subregular nonideal solutions. A positive excess surface tension was evaluated over the whole composition range with a suppressed surface segregation of the surface-active component Al due to interatomic attractions. Additionally, the influence of oxygen on Al and Ti melts was investigated with regards to the surface tension. Surface tension measurements were conducted as a function of time, the oxygen partial pressure in the surrounding gas and chemical addition of oxygen to Ti in the form of TiO2. For all composition, the surface tension results showed a negligible dependence on the oxygen partial pressure over the measured range. However, with increasing chemical addition of oxygen to Ti in the form of TiO2, the surface tension decreased gradually. All results for Ti-O were fitted by the Belton/Szyszkowski adsorption equation. Overall it was suggested in the case of liquid Ti, that the reduction of the surface tension of occurs at higher oxygen concentrations than assumed and covered (> 10 Pa). Concerning liquid Al, the maximum oxygen partial pressure boundaries in the chamber and the sample’s vicinity for a “clean” surface were calculated. The Al surface tension results for ‘clean conditions’ are in very good agreement with corresponding literature results. It was suggested that these measurements relate to oxide free surfaces with either zero or minor adsorbed oxygen particles. Finally, all thermophysical properties investigated in this work were compared with special regards to excess properties and the relations to the enthalpy of mixing and the Gibbs free energy of formation. Overall it was shown that liquid Al-Ti alloys generally exhibit highly nonideal behaviour for all investigated thermophysical and thermodynamic properties. Most of the properties are directly or indirectly interacting, however, no unified trend over all properties could be observed as the range of processes contributing are too wide-ranging