Martin Vlach

Defect studies of ZnO single crystals electrochemically doped with hydrogen

Various defect studies of hydrothermally grown (0001) oriented ZnO crystals electrochemically doped with hydrogen are presented. The hydrogen content in the crystals is determined by nuclear reaction analysis and it is found that already 0.3at.% H exists in chemically bound form in the virgin ZnO crystals. A single positron lifetime of 182ps is detected in the virgin crystals and attributed to saturated positron trapping at Zn vacancies surrounded by hydrogen atoms. It is demonstrated that a very high amount of hydrogen (up to ∼30at.%) can be introduced into the crystals by electrochemical doping. More than half of this amount is chemically bound, i.e., incorporated into the ZnO crystal lattice. This drastic increase of the hydrogen concentration is of marginal impact on the measured positron lifetime, whereas a contribution of positrons annihilated by electrons belonging to O–H bonds formed in the hydrogen doped crystal is found in coincidence Doppler broadening spectra. The formation of hexagonal shape ...

Microstructure and mechanical properties of the AA6082 aluminium alloy with small additions of Sc and Zr

Abstract The effect of Sc and Zr addition on microstructure and hardness development in a commercial as-cast and solution-treated (530 °C/45 min) AA6082 alloy was studied. The electrical resistivity decrease and hardness increase in the temperature range 180 – 300 °C are caused by precipitation of β′′ and/or β′ needles of the Mg – Si system. Precipitation and morphology changes of Al – Mn – Fe system phases in AA6082 and AA6082-ScZr alloys influence the resistivity significantly, but have a negligible effect on hardness. The initial hardness of Sc and Zr containing AA6082 alloys is higher mainly due to the presence of Al3(Sc, Zr) particles. This difference is less pronounced in the peak-hardening state. Natural ageing of solution-treated alloys influences microstructure development in both alloys when annealed subsequently up to 240 °C.

Effect of Sc and Zr Additions on Microstructure and Mechanical Properties of Conventional Cast and P/M Aluminium

The influence of plastic deformation and heat-treatment on the precipitation of Al3(Sc, Zr) particles and the effect of these precipitates on hardening and softening processes of dilute ternary Al-0.2wt.%Sc-0.1wt.%Zr alloy was investigated. Behaviour of two differently prepared alloys (mold cast and prepared by powder metallurgy – PM) was investigated in as-prepared and in cold rolled state. Both alloys exhibit the same peak age hardening, PM one reaches it already during extrusion at 350°C. Both cold rolled alloys are highly resistant against recovery, which proceeds without rapid hardness decrease at high temperatures. Evolution of hardness agrees well with that of resistivity and with TEM observation.

Phase transformations in non-isothermally annealed as-cast and cold-rolled AlMnScZr alloys

Abstract The effect of Mn addition on the microstructure, thermal and mechanical properties in as-cast and cold-rolled Al–Sc–Zr alloys was studied. Electrical resistometry, differential scanning calorimetry and microhardness measurements were used. Transmission electron microscopy, electron backscatter diffraction and X-ray diffraction of specimens quenched from temperatures of pronounced changes in resistivity helped to identify the microstructural processes responsible for resistivity changes. The distinct microhardness increase observed after annealing above ∼320°C is caused by precipitation of the Al3Sc particles. The cold-rolling prior to a heat treatment has no substantial effect on temperature position of the Al3Sc-phase precipitation. The formation of Al6Mn and/or Al6(Mn,Fe) particles is responsible for the intensive resistivity decrease of the cold-rolled materials compared to the as-cast materials. Precipitation of these particles has an insignificant effect on microhardness. The apparent activation energy for the precipitation of the Al3Sc particles was determined.

Phase transformations in creep resistant MgYNdScMn alloy

Abstract Mg-4 wt.% Y-2 wt.% Nd-1 wt.% Sc-1 wt.% Mn alloy was squeeze-cast. It benefited from the addition of Nd which decreased the solubility of Y in Mg, as is known from WE alloys. Isochronal annealing of the alloy showed relative resistivity changes similar to those obtained in other Mg–R.E.–Sc–Mn alloys (R.E. – rare-earth element). The resistivity decrease observed is due to the complex precipitation process, namely a combination of decomposition sequences in the Mg–Y–Nd, Mg–R.E.–Mn, and Mn–Sc systems. The increase in the resistivity on isochronal annealing up to temperatures over 500 °C is due to the re-solution of the phases in the first two systems. The Mn2Sc phase coarsens only during this annealing treatment. The creep resistance of the alloy at high temperatures is better than that of low Sc content Mg alloys with Yand Gd in the T5 condition and also than that of WE alloys in the T6 condition.

Hydrogen Interaction with Defects in Nanocrystalline, Polycrystalline and Epitaxial Pd Films

Hydrogen interaction with defects and structural development of Pd films with various microstructures were investigated. Nanocrystalline, polycrystalline and epitaxial Pd films were prepared and electrochemically loaded with hydrogen. Structural changes of Pd films caused by absorbed hydrogen were studied by in-situ X-ray diffraction combined with acoustic emission and measurement of electromotorical force. Development of defects during hydrogen loading was investigated by positron annihilation spectroscopy. It was found that hydrogen firstly fills open volume defects existing already in the films and subsequently it occupies also interstitial sites in Pd lattice. Absorbed hydrogen causes volume expansion, which is strongly anisotropic in thin films. This introduces high stress into the films loaded with hydrogen. Acoustic emission measurements revealed that when hydrogen-induced stress achieves a certain critical level rearrangement of misfit dislocations takes place. The stress which grows with increasing hydrogen concentration can be further released by plastic deformation and also by detachment of the film from the substrate.

Hydrogen Interaction with Defects in ZnO

In the present work hydrothermally grown ZnO single crystals were electrochemically charged with hydrogen. The influence of hydrogen on ZnO microstructure was investigated by positron annihilation spectroscopy (PAS) combined with X-ray diffraction (XRD) using synchrotron radiation. Hydrogen concentration in the samples was determined by nuclear reaction analysis (NRA). It was found that a high concentration of hydrogen can be introduced into ZnO by electrochemical loading. At low concentrations, absorbed hydrogen causes elastic volume expansion of ZnO crystal. At higher concentration, hydrogen-induced stresses exceed the yield stress in ZnO and plastic deformation of the crystal takes place leading to formation of a defected subsurface layer in the crystals.

Influence of Deformation on Precipitation Kinetics in Mg-Tb Alloy

Precipitation effects in age-hardenable Mg-13wt.%Tb alloy were investigated in this work. The solution treated alloy was subjected to isochronal annealing and decomposition of the supersaturated solid solution was investigated by positron annihilation spectroscopy combined with transmission electron microscopy, electrical resistometry, differential scanning calorimetry and microhardness measurements. Peak hardening was observed at 200°C due to precipitation of finely dispersed particles of β phase with the D019 structure. Vacancy-like defects associated with β phase particles were detected by positron annihilation. At higher temperatures precipitation of β and subsequently β phase takes place. Formation of these phases lead to some additional hardening and introduces open volume defects at precipitate/matrix interfaces. To elucidate the effect of plastic deformation on the precipitation sequence we studied also a Mg-13wt.%Tb alloy with ultra fine grained structure prepared by high pressure torsion. In the ultra fine grained alloy precipitation of the β phase occurs at lower temperature compared to the coarse grained material and the peak hardening is shifted to a lower temperature as well. This effect can be explained by enhanced diffusivity of Mg and Tb atoms due to a dense network of grain boundaries and high density of dislocations introduced by severe plastic deformation. Moreover, dislocations and grain boundaries serve also as nucleation sites for precipitates. Hence, precipitation effects are accelerated in the alloy subjected to severe plastic deformation.

Influence of Natural Ageing on Precipitation Processes during Isochronal Annealing in MgGd Alloys

The Mg-10 wt. % Gd and Mg-15 wt. % Gd alloys produced by squeeze casting were solution treated at 500 °C for 8 hours and subsequently naturally aged for more than 2 months. Electrical resistivity of both materials measured at 77 K decreases, if the alloys are kept at room temperature after quenching from the solution temperature. This change accompanied by a microhardness increase almost saturates after 2 months and is caused most probably by solute atoms clustering. Phase transformations and microhardness changes were investigated during isochronal annealing in both naturally aged alloys in comparison to just solution treated ones. Electrical resistivity changes measured at 77 K were used to characterize microstructure development. Transmission electron microscopy was performed at selected states heat treated in the identical way. The Mg15Gd supersaturated solid solution isochronally annealed up to 500 °C immediately after the solution treatment decomposes into following successive phases: β ́ ́ (D019) metastable → β ́ (cbco) metastable → β (Mg5Gd) stable. All three possible orientation relationship modes of the metastable β ́ (cbco) phase existed at lower temperatures (up to 280 °C) but only one mode persists up to 330 °C. Precipitation of the β ́ (cbco) phase has not been observed in the Mg10Gd alloy annealed isochronally immediately after the solution treatment. The natural ageing does not change the precipitation sequence but concentration of Gd atoms involved in individual precipitation processes is influenced in both alloys. Peak hardening increases after natural ageing in the Mg15Gd alloy, shifts to higher temperatures and the temperature region of peak hardening extends.

Precipitation Processes in Mg-Y-Nd Alloys

Successive precipitation processes in solution treated Mg3Y3Nd alloy were studied by electrical resistometry, by differential scanning calorimetry and by microhardness. The results were compared to those in the commercial WE43 alloy. Five various phases known from the Mg-Gd-and Mg-Nd-type decomposition sequences form, dissolute or transform in the Mg3Y3Nd alloy. The main difference in the WE43 precipitation sequence is the absence of the β1 phase particles. Electron microscopy confirmed that not the phase type of hardening particles but their morphology, size and orientation to the matrix determine the hardening effectiveness. Differential scanning calorimetry revealed exothermic effects connected to precipitation processes. Apparent activation energies of individual precipitation processes correspond to those in the MgTbNd and in MgNd alloys.