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Positron Lifetime Spectroscopy of Nanocrystalline Copper

100%
C̆ížek J., Procházka I., Vostrý P., Chmelík F., Islamgaliev R.
Acta Physica Polonica A
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1999
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vol. 95
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issue 4
487-495
EN
Positron lifetime spectroscopy was applied to investigate the thermal stability of nanocrystalline copper prepared by severe plastic torsion deformation. Positrons annihilating in as prepared specimens exhibited free positron component τ_{1} and two defect components τ_{2}=164 ps and τ_{3}=255 ps. Evolution of the lifetimes and relative intensities of all the three components with increasing annealing temperature during step-by-step isochronal annealing up to 630°C was studied. Behaviour of positrons in nanocrystalline copper could not be interpreted in the frame of conventional 3-state trapping model due to highly inhomogeneous defect distribution. Therefore a modified trapping model was developed and applied to explain the experimental results.
2
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Electrical Resistivity and Positron Lifetime Studies of Precipitation Effects in Al-Cu-Based Alloys

86%
Melichova O., Vostrý P., Procházka I., Cieslar M., Stulíková I., Faltus J.
Acta Physica Polonica A
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1999
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vol. 95
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issue 4
627-631
EN
The improved workability of the commercial automatic machine designed alloy Al-Cu-Bi-Pb is guaranteed by the presence of Pb. Nevertheless, the toxic element Pb reduces some of the alloy properties. Therefore new Pb-free machinable Al-based alloys are developed. The Al-Cu-Bi-Sn alloy belongs to these non-traditional materials. The contribution deals with the investigation of precipitation effects in Al-Cu-Bi-Sn alloy during step-by-step isochronal annealing up to 500°C after previous solution heat treatment by means of positron annihilation spectroscopy completed with electrical resistivity measurements and results of independent transmission electron microscopy studies. The used combination of experimental methods gives the possibility to detect separately the redistribution of Sn and Cu atoms in the matrix and to study the influence of vacancies on this process.
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Electrical Resistivity and Positron Lifetime Studies of Precipitation Effects in Al-Cu Alloy

86%
Αda P., Cieslar M., Vostrý P., Bečvář F., Νovotný I., Procházka I.
Acta Physica Polonica A
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1995
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vol. 88
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issue 1
111-117
EN
Decomposition of the supersaturated solid solution of 2 at.% Cu in Al was investigated by means of electrical resistivity and high-resolution positron lifetime measurements. The phase composition of this alloy was determined by transmission electron microscopy. Electrical resistivity measurements were performed by a classical four-point method. Positron lifetime spectra were measured by means of a spectrometer consisting of two BaF_{2} detectors and a standard fast-slow coincidence system. The specimens were first exposed to the solution heat treatment at 783 K for 19 hours with a subsequent quenching. Then the specimens were isochronally annealed in the temperature range 293 ÷ 573 K. Annealing responses of electrical resistivity and positron annihilation were studied. Resistivity measurements as well as transmission electron microscopy observations confirmed the well-known decomposition sequence of the Al-Cu alloy. The decomposition of the alloy was manifested by the decrease in the intensity of positron lifetime component τ_{2} = (207 ±2 ) ps correlated with simultaneous appearance and increase in the intensity of τ_{3} = (180 ± 5) ps component. Component τ_{2} originates from positron annihilation in vacancies trapped at the Guinier-Preston zones while τ_{3} comes from annihilation of the positrons localized in the misfit dislocations at coherent precipitates of the Al_{2}Cu phase. The shortest observed component τ_{1} apparently belongs to annihilation of untrapped positrons.
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