Papers by Author: Pavel Novák

Abstract: Iron aluminides have been considered as materials resistant against high temperature oxidation in air and sulphur-containing environments. Previous research of our team proved that the oxidation resistance in the air can be significantly improved by the addition of silicon. Fe-Al-Si alloys have also very good mechanical properties at high temperatures. However, the resistance in the environments simulating combustion gasses have not been studied yet. This work focuses on the oxidation resistance in carbon dioxide, which is the main component of the combustion gasses. It was found that the Fe-Al-Si, Fe-Al-Si-Ni and Fe-Al-Si-Ti alloys have lower oxidation resistance in carbon dioxide containing atmosphere than in the air due to carbon diffusion to the material and even to the formation of carbides below the oxide layer. It leads to the spallation of the oxide layer, especially in FeAl20Si20Ni20 alloy.

Abstract: Ti-Al-Si alloys are materials for high-temperature applications. They are characterized by low density, good mechanical properties and excellent resistance against oxidation in comparison with other commonly used alloys, for example nickel alloys or stainless steels. The preparation of Ti-Al-Si is very problematic due to high melting points of the intermediary phases, the high reactivity of melt with the melting crucibles and with the atmosphere in the furnace or formation of the cracks and pores during the process. Powder metallurgy seems to be a promising method for preparation of Ti-Al-Si alloys but there are still many complications. In this work, Ti-Al-Si alloys were prepared by unconventional powder metallurgy techniques and the aim of this work was to describe the problems during the sintering of these materials and their solution.

Abstract: In this paper, recently developed ternary FeAl20Si20 (wt.%) alloy with promising high-temperature oxidation and wear resistance was prepared by mechanical alloying in a high-energy ball mill. The possibility to speed-up the mechanical alloying process by replacing aluminium (and partly silicon) elemental powder by the pre-alloyed powder (AlSi30) with relatively fine dispersion of Si in the Al-Si eutectic was examined. The microstructure, phase composition and mechanical properties after various time of mechanical alloying were characterized. The effect of using the pre-alloyed powders on kinetics of mechanical alloying is compared with the results obtained on batches prepared from elemental powders.

Abstract: The effect of processing conditions on microstructure and mechanical properties of Fe-Al-Si powders was studied by means of scanning electron microscopy, X-ray diffraction and nanoindentation. Fe-Al-Si alloy powder was prepared from pure elemental powders by mechanical alloying. Microstructure and mechanical properties of powders were characterized after various durations of mechanical alloying. Special sample preparation technique was developed allowing to characterize the properties of individual powder particles after each step of processing in a planetary ball mill. This step-by-step characterization allowed to find the optimum conditions for subsequent spark plasma sintering.

Abstract: NiTi alloy is usually prepared by casting and forming. As an innovative process, reactive sintering powder metallurgy is tested worldwide, aiming to prepare pure NiTi alloy easily from nickel and titanium powders. This process enables to prepare both porous and low-porosity alloy, depnding on the process conditions. However, the formation of NiTi phase in this process is always accompanied by the Ti₂Ni phase, which is hard, brittle, less corrosion resistant and does not have the shape memory. In this work, various alloying elements (Al, Si, Mg, Fe, Nb, V) were added to Ni-Ti alloy in order to lower the amount of Ti₂Ni phase or at least to minimize its undesirable effect on the alloy properties. The reactive sintering behaviour, phase composition and mechanical properties of Ni-Ti-X alloys were described.

Abstract: The aim of this work was to describe the dependence of microstructure of NiTi shape memory alloy on the conditions of powder metallurgy processing route. The technology consisted of blending of elemental Ni and Ti powders, uniaxial cold pressing and reactive sintering. The effects of reactive sintering temperature, heating rate, holding duration and particle size were determined. The proposed technology can be used as the alternative production route of NiTi to minimize the contamination of the alloy.

Abstract: Germanium is an element which is used in metallurgy in a very small extent. Much more significant is its use as a semiconductor material. Most of magnesium alloys are usually used for applications at ambient temperature. The significant decrease in mechanical properties is observed already at the temperature higher than 150°C. This is the reason for the effort to prepare a new low-priced magnesium based alloys with improved mechanical properties at elevated temperatures, e.g. for components of combustion engines. Therefore, new unconventional alloying elements are studied for increase the thermal stability of magnesium alloys. The effect of germanium on the microstructure and mechanical properties of Mg-Ge alloys at ambient and elevated temperatures was studied in this paper.

Abstract: Alloys based on TiAl and FeAl aluminides are modern materials for high-temperature applications in automotive or aerospace industry due to low density combined with good high-temperature mechanical properties and oxidation resistance. Previous works proved that the addition of silicon to these alloys improves the oxidation resistance as well as the thermal stability. In this work, the mechanism of the silicon effect was investigated by observing the microstructure of the oxide layer and the near-surface area of the Ti-Al-Si and Fe-Al-Si alloys prepared by reactive sintering powder metallurgy. It was found that silicon improves the compactness of the oxide layers. The oxide layers on Fe-Al-Si alloys are formed by Al₂O₃ and small amount of iron oxide (Fe₂O₃) while Ti-Al-Si alloys cover by TiO₂ and Al₂O₃ during the oxidation. Due to aluminium depletion of the alloy, a layer of silicides is formed under the oxide layer, thus acting as the additional protection against high-temperature oxidation.

Abstract: Rapidly solidified aluminium alloys have many interesting properties such as higher thermal stability and strength, when compared with conventional cast alloys. Due to these properties, RS alloys seem to be prospective for using in automotive or aircraft industry. Aim of this work was to compare the differences in microstructure of alloys containing Fe, Ni and Cr which were prepared by different solidification rate. Alloys were prepared by melt spinning, melting with follow-up quenching into the water and by conventional casting with pouring into brass mould. Microstructure of prepared alloys was investigated by scanning electron microscope; phase composition was determined by x-ray diffraction. In this experiment, microhardness was measured in the initial state of all types of alloys; rapidly solidified alloys were also annealed to determine thermal stability by microhardness measurement. Results indicate that higher solidification rate refines the microstructure which is composed of supersaturated solid solution of alloying elements in aluminium and stable and meta-stable intermetallic phases. Hardness of the alloys increases and microstructure refines with solidification rate.

Abstract: Mg-based alloys are prospective materials for reversible hydrogen storage in the form of metallic hydrides. Usually, hydrogen saturation is carried out at high temperatures and high hydrogen pressures. This is the reason for the high cost of metallic hydrides in comparison with other hydrogen storage methods. Electrochemical hydriding, on the other hand, can be realized at room temperature. Moreover, this process does not need any hydrogen atmosphere. In the presented work, electrochemical hydriding of several Mg-Ni-Mm-based alloys (Mm = mishmetal) is performed. Hydriding efficiency, mechanism and kinetics are described. It is shown that the additions of Ni, Mm and the formation of eutectic structures support hydriding of alloys.