Tibor Berecz

Spinodal decomposition of the ferritic phase, a thermally induced phase transformation process is studied in an LDX 2101 type lean-duplex stainless steel by different examination methods. This phase transformation in duplex stainless steels has special importance because it is responsible for the so called "475 °C embrittlement". This deterioration process causes a decrease in toughness and corrosion resistance. Fe-Cr alloys, thus several stainless steels, are susceptible to the spinodal decomposition at intermediate and low temperatures (<600 °C). Therefore, it is important to study this phenomenon in novel lean-duplex stainless steels as well. Our results revealed that spinodal decomposition occurs to a lesser extent in the studied type of lean-duplex stainless steel but its effect is not negligible in applications.

Thermally induced deterioration processes were studied in cold worked (up to 60% deformation) SAF 2507 type super-duplex stainless steel (SDSS) by differential thermal analysis (DTA). DTA results revealed two transformations. Parent and inherited phases of these transformations were examined by other methods too, such as micro-hardness tests, optical metallography and X-ray diffraction (XRD). Finally, these transformations were identified as the formation of α’- and σ-phases. Formation of strain-induced martensite (SIM) and recrystallization were not experienced until 1000 °C, despite high degree of cold working. Activation energies of the σ-phase precipitation and α’-phase formation were determined from the Kissinger plot, through DTA measurements—they are 275 and 220 kJ/mol, respectively—in good agreement with the values found in the literature.

The paper presents the microstructure and corrosion behavior of an AlTiNiCuAgSn new equiatomic multicomponent alloy. The alloy was obtained using the vacuum arc remelting (VAR) technique in MRF-ABJ900 equipment. The microstructural analysis was performed by optical and scanning electron microscopy (SEM microscope, SEM-EDS) and the phase transformations were highlighted by dilatometric analysis and differential thermal analysis (DTA). The results show that the as-cast alloy microstructure is three-phase, with an average microhardness of 487 HV_0.1/15. The obtained alloy could be included in the group of compositionally complex alloys (CCA). The corrosion resistance was studied using the potentiodynamic method in saline solution with 3.5% NaCl. Considering the high corrosion resistance, the obtained alloy can be used for surface coating applications.

Electron backscatter diffraction (EBSD) and X-ray diffraction line profile analysis (XLPA) methods are suitable to determine dislocation densities within crystalline structures. A software was developed to obtain dislocation density from misorientations measured between the adjacent points by EBSD. However, this EBSD-based method can estimate only the density of geometrically necessary dislocations (GND), while XLPA method can give the total dislocation (TD) density. In this paper, GND densities are determined and compared them to the TD densities in as-received and severe plastic deformed copper samples. Based on these results, estimations are tested to provide reliable GND densities and to appreciate the applicability of this calculation method. Furthermore, correlations are found between TD densities and yield strengths estimated by hardness tests.

There are two widely used examination methods to study microstructure of polycrystalline materials, namely the automated electron backscatter diffraction (EBSD) and the X-ray diffraction line profile analysis (XLPA). Both methods are suitable to determine quality and quantity of crystallographic defects or irregularities within crystalline structures. However, the EBSD method can be used to estimate the density of only geometrically necessary dislocations (GND), while the XLPA method can give the total dislocation density. A novel software was developed for the determination of GND density from misorientations measured between the neighbouring pixels in EBSD images. Therefore EBSD method is a local, while XLPA method is a non-local procedure to determine dislocation densities. In fact, the calculated GND density depends on the applied step size of EBSD measurements, namely GND densities increase with decreasing the step size. So this leads to a question of how large applied step size of EBSD measurements gives a good approximation for the real GND density in a given polycrystalline material.

The aim of this work was to analyze the performance of joints made by TIG (Tungsten Inert Gas) welding process in austenitic and duplex stainless steels with special regards to their corrosion resistance. Three different types of stainless steel were butt welded with TIG method. Ferric-chloride test and electrochemical treatments revealed how does the TIG process affects the corrosion resistance depending upon the alloy used for welding the joint. This work focuses on the weldability of the 2304, 2404 and 304 type stainless steel heterogeneous welds.

Lath martensitic microstructures were produced in steels containing 0.15% of carbon and various amounts of molybdenum and nickel. The behavior of the lath martensitic microstructure was studied during cold rolling in steels with six different chemical compositions. All materials have endured 75% deformation without damage. In addition, the effects of heat treatments on microstructures and hardness were investigated as well.

Surface preparation for electron backscatter diffraction (EBSD) measurements requires a lot of time and experience. We chose a lath martensitic iron based alloy to demonstrate the efficiency of ion polishing techniques. The average image quality (IQ) values from the EBSD measurements were assigned to be the characteristic parameter for surface goodness. The ideal ion sputtering time and the angle of incidence were determined, and the corresponding inverse pole figure (IPF) and IQ maps were compared to the mechanical polishing treatment.