Messung von Diffusionskoeffizienten in hochschmelzenden, metallischen Flüssigkeiten mit Scherzellentechnik8

摘要

The determination of inter- and self-diffusion coefficients of refractory sample systems requires the usage of a new type of crucible materials, measuring installations and -methods. For the measurements within the scope of this work, the shear-cell method was combined with an isothermal furnace, which was also developed for the use in weightlessness. The shear-cell furnace built by the company Astrium (Airbus) and the Technical University of Freiberg was used in order to establish its characteristics regarding function and temperature distribution as well as for the measurement of the diffusion coefficient. In order to achieve this, a series of changes regarding its overall structure had to be implemented. During the calibration measurements, temperatures of 1500°C within the shear-cell could repeatedly be achieved. The isothermal area along the sample was of 90 mm; the temperature gradient along the heating zone was a maximum of 2 K. The furnace itself can be heated up to temperatures of 1650°C. The heating parameters for 5 different diffusion temperatures were determined in order to guarantee an isothermal area of ±1K along the capillaries. This was necessary in order to compensate heat loss along the shear cell and in preparation for each diffusion test series with the graphite shear cell. This experiment differs from other already existing shear-cells due to the high amount of measuring points, the length and the amount of samples as well as the extreme temperature stability of the experiment buildup. Because the construction can be tilted, the melting column can be aligned with gravitational acceleration, which favours the creation of a stable density layer within the melting pillar. The fusing of the samples in a horizontal position avoids segregation effects. The isothermal area minimizes the transport of material within the melt due to convection. Pre-stressed repositories are balancing volume expansion or shrinkage of the samples. This ensures a complete filling of the capillaries and prevents free surfaces on the capillary walls or the formation of bubbles within the melt. The shear processes at the beginning and at the end of the diffusion process allow a precise determination of the soak time and prevent the influence of solidification phenomena on the concentration profile. In this work, the inter-diffusion and self-diffusion coefficients were measured on Al-Cu. To achieve this, several shear cell experiments were conducted under identical process conditions and with varying concentration gradients around the average concentration of Al$_{87,5}$Cu$_{12,5}$ in the graphite shear-cell. CT analysis and the determination of concentration profiles with AAS were used for the evaluation of the results. The analysis with different measurement techniques reduces the amount of errors. The newly determined inter-diffusion coefficients differ from previously published results. The analysis of the concentration profiles were performed with x-ray tomography and chemical AAS analysis. The calculated self-diffusion and inter-diffusion coefficients match the assumed 10% measurement error. The minor deviations of the measured results when using two different measuring processes over a high amount of experiments prove the reliability and the precision of the shear cell method that was used in this work. In the context of another dissertation at the Institute of Materials Physics in Space, shear- cell experiments to measure the inter-diffusion on the system AgCu and the ternary AlCuAg alloy were carried out parallel to this work. Because of the high isothermal area and the increased amount of sample segments that could be analysed, the shear cell method presented in this work represents an adequate measurement method for sample systems with limited x-ray contrast. To date, the mutual influence of the alloying components as well as the correct choice of evaluation methods render the collection of precise measurement results on ternary sample systems rather difficult. Four shear-cell measurements around the area of the AlCuAg-eutectic were carried out to complete and verify the results obtained. Despite the lack of process observations, it is clear that the shear-cell, in combination with an isothermal furnace, provides reproducible results on ternary alloys, which also correlate with the results of neutron radiography, within the limit of tolerance regarding measurement and analysis/evaluation errors. Alongside the measurements described on the Al-Cu, Ag-Cu and AlCuAg systems, shear-cell experiments to determine the inter-diffusion coefficient on varying Al-Ni systems were also carried within the scope of this work. Part of this work focuses on the analyses of chemical compatibility between potential crucible materials for the production of a high temperature shear cell and high-melting Al-Ni melts. The analyses resulted in the production of a shear-cell made of BN, with 35% ZrO$_2$, which combines the characteristics of the graphite cell with the chemical stability of BN. The first shear cell experiments include measurements around Al$_{71,02}$Ni$_{28,98}$. In an additional experiment, measurements to determine the inter-diffusion coefficient were also carried out on Al$_{25}$Ni$_{75}$. With this boron nitride cell, it was possible for the very first time to perform inter-diffusion shear cell experiments on high-melting Al-Ni at process temperatures above 1400°C for the very first time.