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In the present work point defects in ZnO crystals were characterized by positron lifetime spectroscopy combined with back-diffusion measurement of slow positrons. Defects in ZnO crystals grown by various techniques were compared. Hydrothermally grown ZnO crystals contain defects characterized by lifetime of ≈181 ps. These defects were attributed to Zn vacancies associated with hydrogen. ZnO crystals prepared by other techniques (Bridgman, pressurized melt growth, and seeded chemical vapour transport) exhibit shorter lifetime of ≈165 ps. Positron back-diffusion studies revealed that hydrothermally grown ZnO crystals contain higher density of defects than the crystals grown by other techniques. The lowest concentration of defects was detected in the crystal grown by seeded chemical vapor transport.

Positron annihilation spectroscopy (PAS) combined with optical methods was employed for characterisation of defects in the hydrothermally grown ZnO single crystals irradiated by 167 MeV Xe26+ ions to fluences ranged from 3×10^12 to 1×10^14 cm−2. The positron lifetime (LT), Doppler broadening as well as slow-positron implantation spectroscopy (SPIS) techniques were involved. The ab-initio theoretical calculations were utilised for interpretation of LT results. The optical transmission and photoluminescence measurements were conducted, too. The virgin ZnO crystal exhibited a single component LT spectrum with a lifetime of 182 ps which is attributed to saturated positron trapping in Zn vacancies associated with hydrogen atoms unintentionally introduced into the crystal during the crystal growth. The Xe ion irradiated ZnO crystals have shown an additional component with a longer lifetime of ≈ 360 ps which comes from irradiation-induced larger defects equivalent in size to clusters of ≈ 10 to 12 vacancies. The concentrations of these clusters were estimated on the basis of combined LT and SPIS data. The PAS data were correlated with irradiation induced changes seen in the optical spectroscopy experiments.

The positron annihilation spectroscopy was employed for characterisation of defects in pure Ti with ultra fine grained (UFG) structure. UFG Ti samples were prepared by two techniques based on severe plastic deformation (SPD): (i) high pressure torsion (HPT) and (ii) equal channel angular pressing (ECAP). Although HPT is the most efficient technique for grain refinement, the size of HPT-deformed specimens is limited. On the other hand, ECAP is less efficient in grain refinement but enables to produce larger samples more suitable for industrial applications. Characterisation of defects by positron annihilation spectroscopy was accompanied by hardness testing in order to monitor the development of mechanical properties of UFG Ti.

A novel digital coincidence Doppler broadening (D-CDB) spectrometer was employed for energy resolved investigations of two-quantum annihilation-in-flight (TQAF). The TQAF phenomenon was studied using monoenergetic positrons produced in a slow positron beam and also using fast positrons. Because of a low background the measurements on the slow positron beam could be performed in a close geometry and the TQAF contribution in the two-dimensional gamma ray energy spectra fills a 'bowl-like' area delimited by a hyperbolic curve and a kinematical cut-off determined by the kinetic energy of positrons. With decreasing positron energy the area of TQAF contribution becomes smaller and disappears completely for slow positrons with energies below ~ 100 eV. The measurements with fast positrons were restricted to a limited range of angles between the annihilation gamma rays and the TQAF events contribute to a hyperbolic band in gamma ray energy spectrum.

In the present work hydrogen interaction with vacancies was investigated in Fe-Al alloys with various concentration of vacancies. The Fe-Al samples were implanted with low energy H+ ions (100 keV). This procedure introduced high hydrogen concentration into relatively narrow sub-surface region in the depth of ~500 nm. Variable energy positron annihilation spectroscopy (VEPAS) was employed for investigation of hydrogen interaction with vacancies in the sub-surface region. This study revealed formation of vacancy hydrogen complexes in the sub-surface region. Thermal stability of vacancy-hydrogen complexes was investigated as well.

Iron aluminides are perspective materials for high temperature structural applications due to a high mechanical strength and excellent corrosion resistance. Hardness of Fe-Al alloys shows non-trivial dependence on chemical composition and thermal treatment of samples and cannot be fully explained by consideration of intermetallic phases formed according to the equilibrium phase diagram of Fe-Al system. Hardening caused by quenched-in non-equilibrium vacancies was proposed to explain rise of hardness in quenched Fe-Al alloys [1]. In the present work the concentration of quenched-in vacancies in Fe-Al alloys with various Al content cAl ranging from 18 to 49 at.-% was determined by means of two techniques of positron annihilation spectroscopy (PAS): (i) positron lifetime spectroscopy was employed for investigation of samples with vacancy concentration less than 2 x 10-4 at.-1 representing a limit for saturated positron trapping; (ii) in samples containing more vacancies than 2 x 10-4 at.-1, the vacancy concentration was determined using back-diffusion measurement of monoenergetic slow positrons. It has been demonstrated that both these methods give mutually consistent results [2]. Non-equilibrium vacancies were detected in all alloys studied after quenching from 1000°C. The concentration of quenched-in vacancies strongly increases with increasing Al content from 10-5 at.-1 in the alloy with cAl = 18 at.-% up to 10-1 at.-1 in the alloy with cAl = 49 at.-%. Comparison of the vacancy concentration and the Vickers microhardness measured on quenched samples revealed that hardness indeed increases with increasing concentration of quenched-in vacancies. In alloys with cAl > 30 at.-% the concentration of quenched-in vacancies exceeds 10-4 at.-1 and the hardness was found to be proportional to square root of the vacancy concentration in agreement with solution hardening model proposed by Chang et al. [1]. Subsequent annealing of samples at 520 °C causes recovery of quenched-in vacancies. This is accompanied by a drop of microhardness, but only in alloys where the initial concentration of quenched-in vacancies was least 10-4 at.-1. Hence, vacancies have a measurable effect on hardness of Fe-Al alloys when their concentration becomes 10-4 at.-1 or higher. References [1] Y. A. Chang et al., Intermetallics 1, 107 (1993). [2] J. Čížek et al., Physica B (2012) doi:10.1016/j.physb.2011.12.122.

Quenched-in vacancies in Fe-Al alloys with Al content ranging from 24 to 49 at.% were investigated employing two complementary techniques of positron annihilation: slow positron implantation spectroscopy and positron lifetime measurements. It was found that quenched alloys exhibit a very high concentration of vacancies. Although the free positron component cannot be resolved in positron lifetime spectrum in majority of samples, the concentration of quenched-in vacancies can be still determined from the positron diffusion length measured by a variable energy positron beam. The lowest concentration of vacancies was found in a stoichiometric (SM) Fe3Al alloy. The concentration of defects increases with increasing degree of non-stoichiometry with respect to Fe3Al, i.e. in alloys with under-SM and over-SM Al concentration. However, the increase in concentration of quenched-in defects is more pronounced in Al-rich alloys, i.e. alloys containing more than 25 at.% of Al.