Štefan Nagy

Ni-NiO Porous Preform with Controlled Porosity Using Al 2 O 3 Powder

The Ni-NiO skeleton seems to be a good candidate for various applications in industry such as corrosion-proof filters or components in refrigerating systems and as preforms for reactive infiltration with molten metals.The present work was focused on preparation of Ni-NiO composite with higher, controlled porosity. Sintering of pure Ni powder always leads to a substantial closed porosity in almost whole sample volume [1,2]. To eliminate this, we added Al2O3 particles with diameter of-32 +20 μm into the Ni powder (-75 +45 μm diameters) and sintered this loose powder mixture (Ni + 25 vol. % Al2O3) in air by progressive heating up to 800 °C followed by 2 hours isothermal exposure. As a control, pure Ni powder was sintered under the same conditions. Thermal oxidation of loose powder samples performed in alumina crucible indicates that the strongest oxidation occurred in the top part of sample, while the bottom part was the least oxidized. Therefore, it was necessary to run the thermal oxidation once more, but out of the crucible, to ensure the sufficient diffusion of oxygen to the whole volume of sample.

Microstructure and Erosion Resistance of Zirconium Diboride Ceramics Infiltrated by Pure Copper

Cu/ZrB2 composite was prepared by gas pressure infiltration of molten metal into ceramic preform. Microstructure and erosion resistance of composite was investigated. The microstructure was analysed by light microscopy and scanning electron microscopy. The chemical compositions were analysed using energy dispersive X-ray spectroscopy. Good penetration of copper along the grain boundaries of the 60% porosity sintered ceramics was analysed in the whole volume of composite. The interfacial morphology shows the regular interfaces without any macroscopic reactions [1]. Cu/ZrB2 composite was subjected to 60 spark discharges to investigate the ablation resistance. Linear dependence of the amount of loss material on the number of electrical discharge analytical cycles for Cu/ZrB2 composite was determined.

Fracture Description of AZ61 Mg-Al2O3 Materials Studied by “In Situ Tensile Test in SEM“

In situ observation of AZ61 Mg alloy with 5 wt. % of Al2O3 in the SEM was performed to study influence of the weight fraction of Al2O3 particles on the deformation and fracture description during the tensile test. Structure of the experimental materials was also analysed; microstructures were heterogeneous, with randomly distributed globular Al2O3 particles (average diameter of 25 nm) and Mg17Al12 intermetallic phase (average diameter of 0.4 mm). It was shown that during the tensile deformation the failure of Mg17Al12 particles and decohesion of the matrix-Al2O3 particles interphase boundary started simultaneously. Decohesion resulted from the different physical properties of matrix and Al2O3 particles. The influence of the Al2O3 weight fraction on the final fracture was evident; for the material with 5 wt. % of Al2O3, the fracture surface was approximately perpendicular to the loading direction. The fracture surface had transcrystalline ductile character.

Microstructure and Properties of Composites Prepared by Reactive Pressure Infiltration of Aluminium into Metal and Ceramic Powder Preforms

Nickel aluminides exhibit very attractive high temperature properties. However, due to high melting temperatures they are difficult to prepare. Gas pressure reactive infiltration is a relatively cheap technology that provides composites where nickel aluminides are formed due to mutual reaction between Ni powder and molten aluminium forced to penetrate into powder preform. The feasibility of this concept is demonstrated in this work. Ni powder and/or Ni+25 vol. % Al2O3 powder mixture, respectively, were mechanically pressed and then infiltrated with aluminium using 5 MPa argon gas pressure at the temperature of 750 °C for 120 s. Al/Al2O3 composite using loose alumina powder was prepared in similar manner for comparison. The microstructure of composites was observed by scanning electron microscopy and newly formed intermetallic phases were analysed by energy-dispersive X-ray spectroscopy. Relative elongations during additional thermal cycling up to 800 °C had been recorded. Composites were additionally characterized by hardness measurements.