Papers by Author: Maria Sozańska

Abstract: Heat resistance and microstructure stability at elevated temperature in HR6W alloy is the result of strong appreciation of the solution by adding tungsten and strengthening precipitation. Structural studies were made after the annealing process at a temperature of 750°C. The heat treatment was carried out for up to 5000 hours. Microstructure stability of HR6W alloy was evaluated by the action of elevated temperature. Identification of precipitates produced using the method of selective electron diffraction (SAED) and chemical composition analysis using a transmission electron microscope by technique STEM.

Abstract: The Mg-Al alloys are the best-known and most commonly used magnesium alloys (especially AZ91 alloy). However, the AZ91 alloy offers insufficient corrosion resistance. Many investigations show that hydrogen is the main corrosive factor appearing during chemical reactions between magnesium and water in electrolyte solution. The main intermetallic phase in the AZ91alloy is the Mg₁₇Al₁₂ (β phase), which is a hydrogen trap. During corrosion, magnesium hydride forms inside the β phase, and this phase is brittle fractured when the inner stress caused by hydrogen pressure and expansion stress due to the formation of magnesium hydride is higher thanthe fracture strength. We examined the corrosion behaviour of AZ91 and AE44 magnesium alloysin 0.1M Na₂SO₄ solution and 3.5% NaCl solution. We analysed two Mg-Al alloys in order todetermine the various effects of hydrogen on these materials.

Abstract: Modern magnesium alloys containing rare earth (RE) elements from the Mg-Y-RE-Zr and Mg-Al-RE systems are characterized by low density and good mechanical properties. Therefore, these alloys are used in the automotive and aerospace industries. However, magnesium alloys offer insufficient corrosion resistance in environments containing electrolyte solutions. Hydrogen is themain corrosive factor appearing during chemical reactions between magnesium and water in anelectrolyte solution. The results showed that when samples were immersed in 0.1M sodium sulfate solution, some cracks were observed inside the Al11RE3 and Al8CeMn4 intermetallic phases. Phase identification was performed by electron backscatter diffraction (EBSD) analysis. The microstructure of the alloys before and after corrosion was observed using a scanning electron microscope (SEM).

Abstract: The turbine blades made of directionally solidified nickel-based superalloys are exposed to combination of high temperature and aircraft environment, in which appear corrosive elements like sulphur, sodium and vanadium (hot corrosion). Corrosion resistance of superalloys is mainly dependent on their structure and chemical composition. Therefore, it is important to be aware of the correlation between the hot corrosion and changes in chemical composition and morphology of a surface of the material. The following paper presents the influence of sulphur on the microstructure of directionally solidified nickel-based superalloy. The research was carried out in Na₂SO₄ environment at two temperatures of 850^oC and 900^oC (below and above the melting point of salt, 884°C). The results show scale morphology on material surface and changes in chemical composition of surface of nickel superalloy.

Abstract: The presented paper is devoted to the study of hydrogen diffusion characteristics in the C-Mn-Si TRIP 800 steel. The steel was tested in three different states: in as-received state after hot and cold rolling and subsequent heat treatment; and furthermore after 5% and 10% tensile deformation. Hydrogen diffusion characteristics were studied by means of electrochemical permeation method. Two build up transients corresponding to lower and higher charging current densities as well as a decay transient were recorded during experiment. The lowest values of hydrogen diffusion coefficient (from 1 to 3.5.10^-7 cm².s^-1) were observed during the first build up transient; the value of 3.5.10^-7 cm².s^-1 corresponded to 10% tensile deformation. During the 2^nd build up transient corresponding to the higher charging current density, hydrogen diffusion coefficients increased markedly reflecting thus the fact that hydrogen trapping was less pronounced. For decay transients hydrogen diffusion coefficients were situated between values obtained for the 1^st and 2^nd build up transients. In all studied states, a rather high sub-surface hydrogen concentration was observed during the 1^st build up transient rising to 12.6 ppm of hydrogen in as-received state. The obtained results are explained taking into account steel microstructure and hydrogen trapping.

Abstract: Magnesium alloys have low density densities and high specific strengths that are comparable to steels and titanium alloys. Therefore, they are widely used as structural materials in the automotive and aerospace industries. However, the use of magnesium alloys is hindered by the fact that they offer insufficient resistance against corrosion, even in diluted electrolyte solutions. We examined alloys from the Mg-Y-RE-Zr and Mg-Al-RE systems (WE43 and AE44) that are used in the domestic and international automotive and aerospace industries. In these applications, the alloys are exposed to corrosion in environments containing electrolytes. It is commonly known that hydrogen is the main corrosive factor, appearing during chemical reactions between magnesium and water in an electrolyte solution. Selecting rare earth-containing magnesium alloys allows us to analyse the various effects of hydrogen on these materials. Hydrogen interacts with the selected alloys in a manner that depends strongly on alloy structure and chemical composition—these factors cause variations in the concentration, solubility, and diffusion rate of hydrogen in the host material. After hydrogen uptake, the cracking velocity of each alloy phase is different and is related to cracking micromechanisms. Our results show that when samples were immersed in 0.1M sodium sulfate solution, hydrogen atoms diffused into the material and enriched the intermetallic phases. With increased immersion time, magnesium hydride fractures in a brittle manner when the inner stress caused by hydrogen pressure and the expansion stress due to the formation of magnesium hydride are higher than the fracture strength.

Abstract: We considered the degradation of hollow turbine blades made of ZhS6K nickel-based superalloy after service in an aircraft engine. The blades were coated with a diffusive aluminide coating (Al-Si) to improve resistance to oxidation and hot corrosion. Turbine blades work under extreme conditions and a complex state of stress. During service, creep and fatigue occur. The interaction among hot combustion gases causes oxidation of the surface layer, hot corrosion, and micro-cracking of the coating. Moreover, changes occur in the morphology of the γ’ phase just under the coating, and transformations of the primary carbides take place. The factors limiting the lifetime of a turbine blade are the quality of the aluminide coating and the microstructure of the superalloy, depending on the service parameters (the temperature and the duration service). We found that exposure to high temperatures above the critical value for several seconds substantially decreased the engine power and its durability. We analysed the microstructure, chemical composition, and phase composition of turbine blades after service. An evaluation of the extent of degradation was performed using scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDS), and electron backscatter diffraction (EBSD). The EBSD technique was used to analyse the phase composition in micro-areas, especially to identify carbides before and after transformations.

Abstract: The trials conducted in sulfur– and chlorine–rich environment were aimed at simulating the conditions prevalent in operating power boilers. The tests conducted in the temperatures of 700°C and 750°C have been selected in such a way as to enable reference between them and the supercritical parameters of boiler steam. The analysis of the rate of high–temperature corrosion kinetics in the Sanicro 25 steel was carried out for up to 3000 hours. The aim of the tests was to establish the oxidation kinetics and evaluate the destruction of the new generation of Sanicro 25 steel in atmosphere containing Cl and S in temperature of 700°C and 750°C during up to 3000 hours of operation.

Abstract: The trials conducted in sulfur– and chlorine–rich environment were aimed at simulating the conditions prevalent in operating power boilers. The tests conducted in the temperatures of 700°C and 750°C have been selected in such a way as to enable reference between them and the supercritical parameters of boiler steam. The analysis of the rate of high–temperature corrosion kinetics in the HR6W alloy was carried out for up to 3000 hours.

Abstract: We examine the mechanical and physical properties of Sanicro 25 steel that are relevant to the performance requirements of supercritical boilers (e.g., resistance to high-temperature corrosion). Sanicro 25 is mainly used in the construction of power plant components. Materials research has demonstrated the stability of properties at elevated temperatures (700 to 750°C) during long-term use. We conducted tests of corrosion resistance for Sanicro 25 steel, and confirmed its resistance to oxidation in steam and exhaust gas heat .