Papers by Author: Natalia Sobczak

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Authors: Natalia Sobczak, Zbigniew Gorny, Marzanna Ksiazek, Waldemar Radziwill, P.K. Rohatgi
Authors: Fabrizio Valenza, Rafal Nowak, Natalia Sobczak, Alberto Passerone, Michele Di Foggia, Maria Luigia Muolo
Abstract: The need of increased efficiency of industrial gas turbines comes also through the improvement of the composition of superalloys (addition of new solutes) and of the manufacturing technologies involved in the investment casting process of the turbine blades. Thus, the knowledge of the interactions between the ceramic materials used for casting and the molten superalloys must be deepened in order to minimize the formation of internal defects, to improve the casting surface and to optimize finishing and casting operations. In this work, a study of the wetting behaviour of some Ni- or Co -based superalloys, used for the fabrication of turbine blades, has been performed with reference to the interactions of these alloys in the molten state with the silica-aluminate based ceramic materials forming the shell or the core in the casting process. Wettability tests have been performed by means of the sessile drop method at 1500°C; the characterization of the interfaces between the molten drop and the substrates has been made on solidified sessile drop samples by SEM/EDS analysis to check the final characteristics of the interfaces. The results are discussed in terms of chemical interactions in relation to the processing parameters and as a function of the surface and interfacial energetic properties of the systems.
Authors: Joanna Wojewoda-Budka, Natalia Sobczak, Jerzy Morgiel, Rafal Nowak
Abstract: Interface reactions between liquid aluminium and ZnO single crystal substrates of <1-100> orientation (at 1273 K under vacuum) were examined using scanning and transmission electron microscopy techniques. The substrates were subjected to the “pushing drop” tests when liquid is deposited from the capillary on the substrate surface and then, after appropriate contact time, it is pushed away. After short time of interaction with <1-100>ZnO substrate, three phases were detected: α-Al2O3, the alumina of unknown type and ZnAl2O4 spinel formed due to the solid state reaction between Al2O3 and ZnO.
Authors: Jerzy J. Sobczak, Ludmil B. Drenchev, Natalia Sobczak, Rajiv Asthana
Abstract: This paper discusses some theoretical aspects of design of ultralight metallic materials using analytical and heuristic arguments. Potential application of syntactic foams to obtain metal-matrix composites lighter than air is also analyzed. Carbon allotropes (fullerenes, colossal carbon tubes) and some non-carbon materials are considered as components of ultralight metal-matrix composites. Calculations for the size of fullerenes, number of atoms in their structure, and coating thickness required to produce ultralight composites are presented. It is concluded that 3D carbon molecules (fullerenes) and colossal carbon tubes are the most promising components to design ultralight metallic materials which can be lighter than air.
Authors: Maria Luigia Muolo, Fabrizio Valenza, Natalia Sobczak, Alberto Passerone
Abstract: The ultra high temperature performance of ceramic-based complex structures may require the development of liquid-assisted joining techniques; this in turn requires the definition of the wettability of these materials by various metals over a wide range of compositions and temperatures. After a short description of the relevant experimental aspects of wettability studies at high temperatures, a discussion is presented on how these results can be used to derive chemical and structural information on the solid-liquid interactions. Reference is made mainly to metal-ceramic systems; a summary of the results of sessile drop tests under carefully controlled conditions is given in relation to the wettability and the interfacial characteristics of systems based on transition metals (Zr, Hf) diboride ceramics in contact with liquid Ag, Cu, Au and Ni and of some of their alloys with Ti, Zr, Hf and B to promote/control wettability. In particular, the utilization of phase diagrams is discussed, as one of the most powerful tool to design the filler alloy compositions for the optimization of joining (brazing) processes.
Authors: Rafal Nowak, Natalia Sobczak, Edmund Sienicki, Jerzy Morgiel
Abstract: The reaction product region, formed between molten aluminium and MgO and MgAl2O4 single crystals of three different crystallographic orientations, was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) coupled with X-ray energy dispersive spectrometry (EDS). The Al/MgO and Al/MgAl2O4 couples were produced under ultra high vacuum at 800, 900 and 1000°C. The observations proved the redox reactions of Al with both MgO and MgAl2O4. Independently of crystallographic orientation of initial oxide single crystals, the reaction product region (RPR) was formed and it was built of oxide particles surrounded by continuous metallic phase. For Al/MgO couples, the RPR was composed of two layers, where in the first layer, the oxide phase was Al2O3 while in the second layer, the MgAl2O4 was identified. In the case of Al/MgAl2O4 couples, a single layer was distinguished and only the Al2O3 phase was recognized.
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