Ivo Marek

The Structure and Mechanical Properties of High-Strength Bulk Ultrafine-Grained Cobalt Prepared Using High-Energy Ball Milling in Combination with Spark Plasma Sintering

In this study, bulk ultrafine-grained and micro-crystalline cobalt was prepared using a combination of high-energy ball milling and subsequent spark plasma sintering. The average grain sizes of the ultrafine-grained and micro-crystalline materials were 200 nm and 1 μm, respectively. Mechanical properties such as the compressive yield strength, the ultimate compressive strength, the maximum compressive deformation and the Vickers hardness were studied and compared with those of a coarse-grained as-cast cobalt reference sample. The bulk ultrafine-grained sample showed an ultra-high compressive yield strength that was greater than 1 GPa, which is discussed with respect to the preparation technique and a structural investigation.

A Lifetime of Metallic Nanoparticles in Heat Exchange Liquids

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.

Phase Transformation Induced Self-Healing Behavior of Al-Ag Alloy

Self-healing alloys are promising materials that can decrease the consequences of accidents. To detect crack formation in a material is simple task that can be performed by e.g., sonic or ultrasound detection, but it is not always possible to immediately replace the damaged parts. In this situation, it is very advantageous to have the chance to heal the crack during operation, which can be done e.g., by annealing. In this paper, self-healing behavior was proven by TEM (Transmission electron microscope) observation of crack healing after annealing. The crack was observed in the rapidly solidified Al-30Ag alloy with non-equilibrium phase composition formed by a minor amount of Ag2Al and a supersaturated solid solution of Ag in an fcc-Al matrix (fcc = face centered cubic). After annealing at 450 °C, equilibrium phase composition was obtained by forming a higher amount of Ag2Al. This phase transformation did not allow the crack to be healed. Subsequent annealing at 550 °C caused recrystallization to a supersaturated solid solution of Ag in fcc-Al, followed by a return to the mixture of fcc-Al and Ag2Al by cooling, and this process was accompanied by the closing of the crack. This observation proved the self-healing possibilities of the Ag2Al phase. Practical application of this self-healing behavior could be achieved through the dispersion of fine Ag2Al particles in a structural material, which will enrich the material with self-healing properties.

Structure and properties of nanocrystalline nickel prepared by selective leaching at different temperatures

Abstract Nanocrystalline nickel is an interesting material for catalysis, and also, like all nanocrystalline metals, it has potential for structural application. Our aim was to develop a method for preparation of precursor materials for powder metallurgy. Because of this, selective leaching of binary alloy was chosen as it leads to production of nanocrystalline clustered (sub)micro particles. In this work, the preparation of Ni particles by selective leaching of Al matrix from Al-50 wt.% Ni in NaOH water solution is described. It was found that structure and magnetic properties were strongly dependent on leaching temperature, which was proven by characterization of Ni particles leached at −20, 0, 20, 40, 60 and 80°C. The microstructure of as-prepared particles was observed by HRTEM and also by small angle neutron scattering. Magnetic properties were characterized by measuring of saturation magnetization. It was proven that with increased leaching temperature the grain size of prepared nanocrystalline particles increased. Also the value of saturated magnetization follows the same trend. The amount of hydrogen stored in nickel particles is independent on leaching temperature.

Structure of Nanocrystalline Nickel Prepared by Powder Metallurgy

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.

Microstructure and Mechanical Properties of Rapidly Solidified Al-Fe-X Alloys

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.

Preparing of Silver Nanoparticles by Selective Dissolving of Aluminium Matrix

Metallic nanoparticles and nanocrystalline bulk materials are recently extremely popular. It is because of their properties and wide field of possible applications.In this work, silver nanoparticles were prepared by selective dissolving of the aluminium matrix from rapidly solidified (RS) AlAg30 (wt. %) alloy in the sodium hydroxide solution. This alloy was prepared by melt-spinning technique. The influence of production parameters such as sodium hydroxide concentration, dissolving temperature and using of ultrasound bath on the phase composition, morphology and size of the nanoparticles was studied.Phase compositions of the initial alloy and prepared nanoparticles were investigated by X-ray phase analysis. Furthermore, the nanoparticles were studied by means of transmission (TEM) electron microscope. Nanoparticle size was determined by image analysis of TEM micrographs and these results were also compared to the size of nanoparticles determined from diffraction patterns using Scherrer calculator.It was found that concentration of sodium hydroxide has significant effect on phase composition of the product and the dissolving temperature influences the particle size.