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Vickers Hardness and Positron Annihilation Study of Eurofer97 and ODS Eurofer

100%
Bartošová I., Čížek J., Lukáč F., Slugeň V.
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vol. 125
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issue 3
702-705
EN
This paper describes investigations of Eurofer97 and oxide dispersion strengthened version of this steel, in as received state and after annealing. Vickers hardness test was performed on the as received specimens and after annealing at various temperatures to obtain information about development of the strength of studied materials. After identifying the temperature region where significant changes of the strength take place we applied positron annihilation spectroscopy to probe the microstructural changes in ferritic/martensitic steels. An observation of phase transformation was observed in both steels at various temperatures indicating different behaviour of the base material and its strengthened version. All applied methods proved the importance of strengthening by oxides to improve properties of steels.
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Characterization of Defects in Titanium Created by Hydrogen Charging

76%
Hruška P., Čížek J., Knapp J., Melikhova O., Havela L., Mašková S., Lukáč F.
Acta Physica Polonica A
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2017
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vol. 132
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issue 5
1606-1610
EN
Hydrogen interaction with vacancies in α-Ti was investigated employing positron lifetime spectroscopy combined with ab initio theoretical modeling of vacancy-hydrogen complexes. Ab initio modeling revealed that multiple hydrogen atoms up to 7 can be trapped at vacancies in the α-Ti lattice. Trapped H atoms are located close to the nearest neighbor tetrahedral sites around the centre of vacancy. Lifetimes of positrons trapped at vacancies associated with various numbers of hydrogen atoms were calculated. Positron lifetime measurement of H-loaded α-Ti samples revealed that phase transition into the hydride phase introduced dislocations. Vacancies were created by H loading as well and agglomerated into small vacancy clusters.
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Defects Studies of ZnO Single Crystals Prepared by Various Techniques

76%
Lukáč F., Čížek J., Procházka I., Melikhova O., Anwand W., Brauer G.
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EN
The aim of the present work was a comparison of defects in ZnO crystals grown by various techniques available nowadays, namely hydrothermal growth, pressurized melt, Bridgman method growth and vapor phase growth. Positron annihilation spectroscopy was employed as a principal tool for characterization of defects in ZnO crystals grown by above mentioned various techniques. ZnO crystals can be divided into two groups: (i) hydrothermal grown crystals, which exhibit positron lifetime of 179-182 ps and (ii) ZnO crystals grown by the other techniques (pressurized melt, Bridgman method, vapor phase growth) which are characterized by the lower lifetimes falling in the range of 160-173 ps. Comparison of experimental data with ab initio theoretical calculations revealed that HT grown ZnO crystals contains Zn vacancies associated with hydrogen atom in a bond-centered site. On the other hand, ZnO crystals prepared by other techniques contain most probably stacking faults created by stresses induced by temperature gradients in the melt.
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