Messung von Diffusionskoeffizienten in ternären AlCuAg-Legierungen und deren Subsystemen

摘要

Within the scope of this thesis inter- as well as self-diffusion coefficients have been determined by experimental series of measurements on a ternary Al-Cu-Ag system at the eutectic as well as on the binary constituent systems. Through the design of a state of the art ex-situ long-capillary facility and combined with the application of new and complementary methods of measurement with strong enhanced precision this investigation offers a methodical approach to these systems and identifies reproducible and accurate diffusion coefficients. This was achieved by the utilization and development of different methods for the sample preparation and the chemical post-mortem sample analysis by atomic absorption spectrometry. In the course of this study ex-situ series of measurements have been conducted on a binary AlCu sub-system. The determined results were compared with literature data. These diffusion measurement were compared to those collected by the use of long-capillary X-ray radiography or shear cell measurement in an isothermal oven. It is shown that even with a high number of similar experiments, neither the statistical mean nor the diffusion coefficient with the lowest value allows a conclusion to be made regarding to the actual diffusion coefficient. A significant result of the discussion of the different diffusion techniques in this thesis is that diffusion experiments require an in-situ process control. Moreover, Neutron radiography should be applied for liquid alloys which do not show sufficient contrast between the sample pairs for radiography methods or, in the case of multi-component systems, which are not respectively only conditionally suitable for this purpose. In the binary AlAg system seven different sample pairs were tested within a concentration level of 0-45 at % silver. By using radiography long-capillary experiments, in-situ interdiffusion coefficients within the entire concentration level were identified. Additionally the self-diffusion coefficient of pure silver as well as the self-diffusion coefficient of a mean silver concentration of 20.4 at % was determined at the Time of Flight spectrometer (TOFTOF) at FRM II in Munich. Thus, the validity of the Darken equation for this concentration within the measurement error was confirmed. A relationship between the atomic dynamic and the packing density with the detected interdiffusion coefficient could have been established through the calculation of the thermodynamic factor and the packaging density, within the considered concentration level. A detailed error analysis proves the influence of free surfaces, temperature variation and diffusion blocking on the interdiffusion coefficient, which enforces together with the previous mentioned AlCu measurement the necessity of an in-situ process control. In the course of this study the measured diffusion coefficient of ternary Al-Cu-Ag alloys with the composition surrounding the eutectic are presented. Furthermore, the different measurement methods, which led to the submitted results, are discussed and their application under different conditions compared, additionally, a selection of a qualified evaluation process for the diffusion profile is discussed. More specifically, a series of experiments with the support of the classic ex-situ long-capillary, the shear cell method, the long-capillary at the neutron source of the FRM II in Munich and ex-situ long-capillary experiments on the research rocket Mapheus were conducted. One substantial finding does confirm the achieved results in the binary system: without an in-situ process control it is not possible to reach a conclusion with regard to the diffusion coefficient with an accuracy better than 50 %. Also, by increasing the number of experiments, there are no improvements in the experimental error range shown within this system, which would satisfactory value of the diffusion coefficient. Only after having applied other more recent techniques, is the quality and error level of the determined ex-situ results able to be assessed. In general, for complex ternary systems the quasi-binary diffusion analysis of with alloys with a constant concentration of a certain component cannot be implemented as an a priori application. In those systems, the mass diffusion of a component can be influenced by the chemical concentration gradient of a considered alloy component through interaction with other alloy elements of the system. In order to capture this behaviour, a diffusion matrix was developed within the scope of this study, which allows the entire description of the measured data. For the first time the ternary Al-Cu-Ag system was investigated systematically and with different diffusion measurement methods. The universal validity of the results are to be verified beyond this study considering other multi-component systems with different thermodynamic driving forces.