Jakub Cizek

Early Stages of Precipitation Process in Al-(Mn-)Sc-Zr Alloy Characterized by Positron Annihilation

Thermal effects on the precipitation stages in as-cast Al-0.70xa0at.xa0pct Mn-0.15xa0at.xa0pct Sc-0.05xa0at.xa0pct Zr alloy were studied. The role of lattice defects was elucidated by positron annihilation spectroscopy (lifetime and coincidence Doppler broadening) enabling investigation of solutes clustering at the atomic scale. This technique has never been used in the Al-Sc- and/or Al-Zr-based alloys so far. Studies by positron annihilation were combined with resistometry, hardness measurements, and microstructure observations. Positrons trapped at defects are preferentially annihilated by Sc electrons. Lifetime of trapped positrons indicates that Sc atoms segregate at dislocations. Maximum fraction of positrons annihilated by Sc electrons occurring at 453xa0K (180xa0°C) suggests that clustering of Sc bound with vacancies takes place. It is followed by peak of this fraction at 573xa0K (300xa0°C). A rise of the contribution of trapped positrons annihilated by Zr electrons starting at 513xa0K (240xa0°C) and attaining maximum also at 573xa0K (300xa0°C) confirms that Zr participates in precipitation of the Al3Sc particles already at these temperatures. The pronounced hardening at 573xa0K (300xa0°C) has its nature in the precipitation of the Al3Sc particles with a Zr-rich shell. The contribution of trapped positrons annihilated by Mn electrons was found to be negligible.

Influence of Al2O3 Nanoparticles on the Thermal Stability of Ultra-Fine Grained Copper Prepared by High Pressure Torsion

Ultra-fine grained (UFG) Cu (grain size 80 nm) containing 0.5wt.% Al2O3 nanoparticles (size 20 nm) was prepared by high pressure torsion (HPT). Positron lifetime spectroscopy was employed to characterize the microstructure of this material, especially with respect to types and concentration of lattice defects. The evolution of microstructure with increasing temperature was studied by positron lifetime spectroscopy and X-ray diffraction measurements. The thermal stability of the Cu + 0.5 wt.% Al2O3 nanocomposite was compared with that of pure UFG Cu prepared by the same technique. The processes taking place during thermal recovery of the initial nanoscale structure in both studied materials are described.

Digital setup for Doppler broadening spectroscopy

New digital spectrometer for measurement of the Doppler shift of annihilation photons was developed and tested in this work. Digital spectrometer uses a fast 12-bit digitizer for direct sampling of signals from HPGe detectors. Analysis of sampled waveforms is performed off-line in software. Performance of the new digital setup was compared with its traditional analogue counterpart. Superior energy resolution was achieved in the digital setup. Moreover, the digital setup allows for a better control of the shape of detector signals. This enables to eliminate undesired signals damaged by pile-up effects or by ballistic deficit.

Defects in CaF2 caused by long-time irradiation and their response to annealing

Samples of CaF2 irradiated for millions of years in nature were studied by several methods, including X-Ray diffraction, positron annihilation spectroscopy (PAS), photoluminescence spectroscopy (PLS) and transmission electron microscopy (TEM). It was found that the unexpectedly high density of radiation-induced defects present in the fluorite structure (documented by TEM) causes significant micro-strains. Even annealing up to 450°C cannot completely remove these micro-strains, which are stabilised by impurities. PAS and subsequent theoretical calculations revealed the behaviour of the defects during heating. The PL spectra of irradiated fluorite were also interpreted.

Divacancy-Hydrogen Complexes in Zinc Oxide

In the present work we study Zn+O divacancies filled up with varying amount of hydrogen atoms. Besides the structure and energy-related properties of such defects, we also investigate their capability to trap positrons taking into account positron induced forces. We show that the Zn+O divacancy may trap positrons when up to two hydrogen atoms are located inside the divacancy. The calculated properties are discussed in the context of other computational and experimental studies of ZnO.

Insights into Hydrogen Gas Environment Promoted Nanostructural Changes in Stressed and Relaxed Palladium by Environmental Transmission Electron Microscopy and Variable Energy Positron Annihilation Spectroscopy

Environmental transmission electron microscopy (ETEM) and variable-energy positron annihilation spectroscopy (VEPAS) are used to observe hydrogen-induced microstructural changes in stress-free palladium (Pd) foils and stressed Pd thin films grown on rutile TiO2 substrates. The microstructural changes in Pd strongly depend on the hydrogen pressure and on the stress state. At room temperature, enhanced Pd surface atom mobility and surface reconstruction is seen by ETEM already at low hydrogen pressures ( pH < 10 Pa). The observations are consistent with molecular dynamics simulations. A strong increase of the vacancy density was found, and so-called superabundant vacancies were identified by VEPAS. At higher pressures, migration and vanishing of intrinsic defects is observed in Pd free-standing foils. The Pd thin films demonstrate an increased density of dislocations with increase of the H2 pressure. The comparison of the two studied systems demonstrates the influence of the mechanical stress on structural evolution of Pd catalysts.

Influence of dislocations on precipitation processes in hot-extruded Al–Mn–Sc–Zr alloy

Abstract Solute clustering and precipitation processes in hot-extruded Al–Mn–Sc–Zr alloy were studied. Positron annihilation studies were combined with electrical resistometry, microhardness testing and microstructural characterization. Extrusion introduced dislocations which arranged into a cell structure. Sc solutes segregated at dislocations. Moreover, fine coherent Al3(Sc,Zr) particles precipitated during extrusion and subsequent cooling. During the isochronal annealing Sc agglomerated forming Sc-rich dispersoids. Agglomeration of Sc solutes is facilitated by pipe diffusion along dislocations. Zr atoms become mobile above 240 °C and form shells surrounding Al3Sc particles. This process is not affected by dislocations. The maximum fraction of positrons annihilated by Sc and Zr electrons occurred at 300 °C, coinciding with hardening of the alloy. The resistivity changes above 300 °C are caused by precipitation of Mn-containing particles.

Investigation of interaction of hydrogen with defects in zirconia

In the present work we study theoretically hydrogen incorporated into several positions in the zirconia cubic and tetragonal lattices. These are positions in the interstitial space and in the zirconium vacancy (VZr). We examine the structure of such configurations and for VZr-related defects we also calculate selected positron characteristics in order to assess their capability of trapping positrons. It is shown that hydrogen atoms do not prefer to stay in the center of the largest interstitial space nor of VZr and they rather tend to create bonds with neighboring oxygen atoms. The positron lifetime of the VZr+1H complex is shorter than that for non-decorated VZr and positron trapping in VZr+1H complexes could, in principle, explain experimental lifetime data.