Papers by Author: Ivan Procházka

Abstract: The commercial Al–Zn–Mg–Cu-based alloys (7xxx series) are widely used in metalworking, automotive and aircraft industries as well as in aeronautical applications. The positive effect of the Sc,Zr-addition on mechanical properties of laboratory Al-based alloys is generally known. The microstructure, mechanical and thermal properties of the conventionally cast, heat-treated and cold-rolled Al–Zn–Mg–Cu (–Sc–Zr) alloys during isochronal annealing and natural ageing were studied. Microstructure observation by scanning electron microscopy and transmission electron microscopy proved the Zn,Mg,Cu-containing eutectic phase at grain boundaries. The distinct changes in microhardness curves as well as in a heat flow of the alloys studied are mainly caused by dissolution of the clusters/Guinier-Preston (GP) zones and precipitation of particles from the Al–Zn–Mg–Cu system. An easier diffusion of Zn, Mg and Cu atoms along dislocations in the cold-rolled alloys is responsible for the precipitation of the Zn,Mg,Cu-containing particles at lower temperatures compared to the cast alloys. Microhardness values of the heat-treated alloys increase immediately from the beginning of natural ageing due to the formation of the clusters/GP zones. Addition of Sc and Zr elements results in a higher hardness above ~ 270 °C due to a strengthening by coherent secondary Al₃(Sc,Zr) particles with a good thermal stability. Sc,Zr-addition has probably no influence on the evolution of the solute clusters/GP zones.

Abstract: The effect of cold-rolling on mechanical, thermal, and electrical properties as well as microstructure behaviour of the Al-2.93wt.%Mg-0.34wt.%Mn-0.33wt.%Si-0.22wt.%Fe-0.19wt.%Cr-0.24wt.%Sc-0.06wt.%Zr was studied. The material was investigated during step-by-step isochronal annealing in a temperature range from room temperature up to 540 °C and during isothermal annealing at 200, 450 and 550 °C. Precipitation reactions were studied by electrical resistometry, conductivity, (micro) hardness measurements and differential scanning calorimetry. The hardening effect appears due to the additional precipitation of the Al₃Sc and/or Al₃(Sc,Zr) particles. The distinct changes in residual resistivity ratio above ~ 330 °C are probably caused by precipitation of the Mn (,Fe,Cr)-containing particles. This precipitation process is highly influenced by cold rolling but it has a negligible effect on hardness. The apparent activation energy values for additional formation of the Al₃Sc and/or Al₃(Sc,Zr) particles were determined. The kinetics of the Al₃(Sc,Zr)-phase precipitation seems to be independent of Mn-and Mg-addition in the studied alloys. A partial recrystallization of the cold-rolled alloy was registered by electron backscatter diffraction after annealing at 550 °C. The initial difference in microhardness introduced by cold rolling is almost removed after annealing at 550 °C/30 min.

Abstract: The aim of this work was investigation of hydrogen interaction with defects in Ti. Well-annealed Ti samples were loaded with hydrogen either electrochemically or using H₂ gas phase. The hydrogen content and the phase composition of hydrogen loaded samples was determined by thermogravimetric analysis and by X-ray diffraction (XRD) respectively. Positron lifetime (LT) spectroscopy was employed for investigation of defects created by hydrogen loading. High-temperature H₂ gas loading led to complete transformation of the whole sample into δ-TiH₂ while low-temperature H₂ gas loading and the electrochemical loading resulted in a non-uniform structure with most of the hydrogen absorbed in a sub-surface layer. All hydrogen-loaded samples exhibit positron lifetime component of ≈ 170 ps corresponding to positrons trapped at dislocations. Vacancy clusters were observed in the electrochemically loaded sample and the H₂ gas phase loaded sample at low temperature.

Abstract: Porosity in several zirconia-based pressure compacted nanopowders was studied using the positron lifetime technique combined with the mass-density measurements. Two kinds of pores were identified: (i) the larger pores of ≈ 10 to 19 nm diameter arising likely from a formation of secondary particle aggregates, and (ii) the smaller ones (≈ 1 nm) which are obviously of a more complex origin.

Abstract: Positron annihilation spectroscopy (PAS) was employed for characterization of defects in the hydrothermally (HT) grown zinc oxide single crystals irradiated by high energy ions. Defects created in ZnO crystals by 2.5 MeV protons, 7.5 MeV N³⁺ and 167 MeV Xe²⁶⁺ ions were compared. The virgin ZnO crystals contain Zn-vacancies associated with hydrogen. Ion implantation introduced additional defects, namely Zn+O di-vacancies in crystals irradiated by protons and small vacancy clusters in samples implanted by N and Xe ions.

Abstract: In the present work, positron annihilation spectroscopy was employed for investigation of hydrogen-induced defects in Pd. Well annealed Pd samples were electrochemically charged with hydrogen and development of defects during hydrogen loading was investigated. At low concentrations (α-phase, x_H < 0.017 H/Pd) hydrogen loading introduced vacancies. When the hydrogen concentration exceeds 0.017 H/Pd, particles of hydrogen rich α’-phase are formed. This generates dislocations in the sample in order to accommodate the volume mismatch between the α and the α’-phase. Moreover, additional vacancies are introduced into the sample by crossing dislocations. Vickers hardness testing revealed that absorbed hydrogen causes hardening of the sample. In the α-phase region the hardness increases due to solid solution hardening caused by dissolved hydrogen. At higher hydrogen concentrations when dislocations were created hardness increases due to strain hardening caused by dislocations.

Abstract: A Pd-H system is a model system suitable for studying interactions of hydrogen with metals. In the present work, we studied hydrogen-induced buckling of thin Pd films deposited on various substrates with different bonding strengths (sapphire, glimmer) and also the effect of deposition temperature. Lattice expansion and phase transitions were investigated by X-ray diffraction of synchrotron radiation. The influence of the substrate and microstructure of the film on the buckling process and phase transformation to palladium hydride are discussed.

Abstract: In the present work, defects created by implantation of hydrothermally grown ZnO single crystals of high quality with H⁺ ions were investigated by positron annihilation lifetime (LT) spectroscopy combined with measurements of optical transmittance (OT) and photoluminescence (PL). First, zinc vacancies attached with one hydrogen impurity (V_Zn – 1H) atom were identified in the virgin ZnO single crystal. The ZnO single crystals were then bombarded by H⁺ ions with the energy of 2.5 MeV to the fluence of 10¹⁶ cm^-2. It was found that V_Zn – V_O divacancies were introduced into ZnO by H⁺-implantation. Effects of H⁺-implantation on the optical activity of defects in ZnO lattice are characterised in the light of the present OT and PL data.

Abstract: The Mg-Zn-Y system attracts interest of researchers due to the formation of quasicrystalline particles which are believed to improve mechanical properties of Mg-based alloys. In the Mg-Zn-Y-Nd-Zr alloy (WE43 modified by addition of Zn) studied here the formation of icosahedral phase (I-phase) with quasicrystalline structure competes with cubic W-phase. Grain boundary phases in Mg-Zn-Y-Nd-Zr alloys subjected to various heat treatments were characterized. It was found that the portion of the I-phase in Mg-Zn-Y-Nd-Zr alloy can be increased by a suitable heat treatment. Moreover, the solidification process is influenced by the cooling rate resulting in a striking difference in the morphology of grain boundary phases.

Abstract: Hydrogen interaction with vacancies and dislocations in Pd were investigated in the present work. Well annealed and plastically deformed Pd samples were electrochemically doped with hydrogen up to various hydrogen concentrations. Subsequently the samples were subjected to linear annealing (10 K/min) and hydrogen desorption was studied by differential scanning calorimetry (DSC). An endothermic peak caused by hydrogen desorption was observed in the DSC curve of well annealed sample at ∼ 178 °C. In plastically deformed samples this peak is shifted to higher temperatures since hydrogen is trapped at dislocations and its diffusivity is suppressed. Moreover, it was found that if the atmosphere surrounding the heated sample contains oxygen the endothermic hydrogen desorption peak is followed by a strong exothermal peak caused by fusion of desorbed hydrogen with oxygen into water vapour molecules. To avoid this undesired effect DSC measurements have to be done in a protective atmosphere which does not contain oxygen.