Papers by Author: Alena Michalcová

Abstract: Nanocrystaline nickel was prepared by selective leaching technology. Consequently, the powder was compacted by spark plasma sintering method. This process is suitable due to its high heating rates, which leads to relatively low thermal exposition of compacted material. The dependence of structure of compacted material on preparation conditions is described in this paper.

Abstract: This work is focused on lifetime prediction of metallic nanoparticles in heat exchange nanofluids. Copper, nickel and iron nanoparticles were studied in 40 wt.% aqueous solution of potassium formate and propylene glycol. Materials were observed by means of mass loss exposure tests, linear polarization resistance and potentiodynamic measurements. Potassium formate solution is not suitable bearing liquid for metallic nanoparticles. Propylyne glycol seems promising, however additional corrosion prevention needs to be applied.

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: Nearly equi-atomic Ni-Ti alloys (nitinol) show shape memory behavior, superelasticity, high strength, excellent corrosion resistance and biocompatibility, making them of interest for various biomedical applications. In processing, they experience various heat treating steps. The present work illustrates influence of a short-time heat treatments (several minutes) at moderate temperatures (around 500°C) on transformation temperature Af and on mechanical properties.

Abstract: Pressureless reactive sintering production of iron aluminides is always connected with high porosity of the product. Previous research showed that silicon reduces the porosity significantly. In this work, the effect of alloying elements (Cu, Ni) on the reactive sintering behaviour and on the porosity of Fe-Al and Fe-Al-Si alloys was studied. Microstructure, phase composition, mechanical and tribological properties were studied as functions of alloy composition.

Abstract: An alloy containing Al – 3wt.% Cr – 3wt.% Fe – 0.8wt. % Ce, was prepared by melt spinning. Structure of obtained ribbons was observed by light, scanning and transmission electron microscopy. It was found out that the structure is very fine. Microhardness of cross sectioned ribbons was also measured. Defects in structure were determined by positron annihilation spectroscopy. The thermal stability of the alloy was observed by comparing rapidly solidified ribbons and ribbons annealed at 400°C and at 500°C for 100 h

Abstract: Aluminium-chromium based alloys are promising candidates for manufacture of light components exposed to elevated temperatures. The work describes properties of Al-6.0wt.%Cr- 2.1wt.%Fe-0.5wt.%Ti alloy. The rapidly solidified powder was prepared by the pressure nitrogen melt atomization. The powder was then subject to heat treatment in order to investigate solid state phase transformations. Compaction of the powder was carried out by hot extrusion after preheating at 450 °C. Microstructure, phase composition and structural transformations on heat treatment were investigated in the as-atomized powder, as well as in the as-extruded alloy. It is found that metastable state of the rapidly solidified powder is characterized by presence of quasi-crystalline phases and supersaturated solid solution. Heating before and during the hot extrusion induces decomposition of the supersaturated solid solution and quasicrystalline to crystalline phase transformations. The hot extruded alloy has a refined recrystallized structure that remains very stable aven after long-term annealing at 400 °C. Mechanical properties of the extruded alloy are discussed in terms of strengthening mechanisms.

Abstract: In the presented paper, properties of Al-Cr-Fe-Ti alloy produced by powder metallurgy (PM) are described. Rapidly solidified powder alloy was prepared by the pressure nitrogen melt atomization. The granulometric powder fraction of less than 45 μm was then hot-extruded. Microstructure of the as-extruded material comprised recrystallized α-Al grains and spheroids of intermetallic phases. Tensile strength of the investigated material was similar to that of a conventional casting Al-Si alloy commonly used in elevated temperature applications. Excellent thermal stability of the PM Al-Cr based material, which much exceeded the elevated temperature casting alloy, was proved by room temperature tensile tests after long-term annealing at elevated temperature. Reasons for the observed thermal stability of the investigated PM alloy are discussed.