Riaboshtan V. Thermal stability of nano- and microcrystalline copper-based pseudo-alloys

This thesis presents the results of experimental and theoretical studies of structure formation and patterns of structural changes under the influence of temperature in copper pseudoalloys with a low concentration of alloying elements such as molybdenum and tantalum, which were obtained by the method of vacuum condensation from the vapor phase (PVD method). The aim of the dissertation is to study the processes of formation of vacuum copper condensates with a low concentration of alloying elements (molybdenum/tantalum), structural changes that occur in them under the influence of temperature, and mechanisms of thermal stabilization of the structure by alloying elements in the matrix metal in different structural and phase states. The object of the study is the thermal stability of micro- and nanostructured materials, which is an urgent problem for all dispersed alloys and composite materials. The subject of the study is the processes and phenomena that lead to a significant increase in thermal stability and mechanical properties in Cu-Mo and Cu-Ta systems, with the content of the alloying component in the range from 0.3 to 0.4 at%. The methodological and theoretical basis of the study is the fundamental principles of the theory of physical adsorption, methodological developments and scientific works of leading scientists on the study of adsorption and recrystallization processes. In solving the problems of the dissertation research, modern general scientific and special methods were used: methods for obtaining samples in the form of pseudo-alloy foils; special methods for studying the state of the structure (microscopy, electron microscopy, X-ray diffractometry, X-ray fluorescence method (XRF) and energy dispersive X-ray spectroscopy (EDS)); methods for selecting physical properties (e.g, measurement of microhardness using the "ПMT-3" hardness tester); method of temperature treatment in vacuum; special methods for obtaining, processing and calculating information provided by structure images obtained in various ways; mathematical and computer modeling; abstract and logical methods for theoretical generalizations of research results. The relevance of the research topic is related to the problem of increasing the stability of nano- and submicrocrystalline metals, which have unique functional properties, but their practical use is limited by the low thermal stability of the initial state. Theoretical and experimental results available in the world literature suggest that this problem can be solved by targeted doping of matrix metals with elements that have exclusive physical and chemical properties in relation to them. The atoms of these substances under certain technological conditions of metal production or heat treatment are able to form segregations at the grain boundaries of matrix metals and, as a result, stabilize the initial structural state and physical and mechanical properties under temperature or other energy effects. The scientific novelty of the study lies in the phenomena that were first observed during the experiments, as well as the theories put forward regarding the mechanisms by which these phenomena occur. The dissertation work provides new knowledge about the mechanisms of formation of the structure of copper pseudo-alloys during their deposition from the vapor phase in vacuum and the conditions of formation and decay of anomalous solutions of alloying elements that are not normally soluble in the copper matrix; about the mechanisms and factors that increase the thermal stability of micro- and nanostructured materials based on copper. The applicant recorded a number of new phenomena during the experiment and put forward a number of theses explaining these phenomena. Based on the experimental results, the physical interaction between the atoms of the alloying component and the grain surface of the matrix metal was modeled. The results of experimental and theoretical studies obtained in this work can be used as a scientific basis for new technologies for the production of nano- and submicrocrystalline metals in the form of films, foils, coatings, and massive products with an ultra-high combination of strength and conductive properties. The peculiarity of these metals in comparison with existing analogues produced in Ukraine and abroad is a specific structural state stable up to 0.8-0.9 of the melting point of the matrix metal, which is 200-300°C higher than the similar indicators currently used for copper-based alloys. The developed copper-based metals can be used as coatings and independent elements of electrical contact materials, tips, electrodes for chiseled welding, highly conductive and thermally stable elements of electronic equipment, etc. The results obtained in this thesis can be used in research centers and universities in Ukraine and abroad, where similar research topics are being developed.