Positron Lifetime Measurements in Nanostructured Ni-Al Samples
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Positron Lifetime Measurements in Nanostructured Ni-Al Samples S. Van Petegem1, D. Segers1, C. Dauwe1, F. Dalla Torre2, H. Van Swygenhoven2, M. Yandouzi3, D. Schryvers3 , G. Van Tendeloo3, J. Kuriplach4, M. Hou5 and E.E. Zhurkin6 1
NUMAT, Subatomic and Radiation Physics Dept., Ghent University, Proeftuinstraat 86, B-9000 Ghent, Belgium 2 Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland 3 EMAT, Centre for Electron Microscopy and Materials Science, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium 4 Dept. of Low Temperature Physics, Charles University, V Holesovickach 2, CZ-18000 Prague 8, Czech Republic 5 Physique des Solides Irradiés CP234, Université Libre de Bruxelles, Bd du Triomphe, B-1050 Brussels, Belgium 6 Dept. of Experimental Nuclear Physics, St. Petersburg State Technical University, Polytekhnicheskaya 29, 195251, St. Petersburg, Russia ABSTRACT Positron lifetime spectroscopy is an effective tool to study various types of defects in materials including nanostructured ones. The size of free volumes associated with defects can be estimated using the lifetime components found in the measured spectra. Positron lifetime experiments are performed on nanocrystalline Ni-Al samples synthesized by the inert-gas condensation technique. The samples are further characterized by means of X-ray diffraction, electron diffraction and microscopy techniques as well as density measurements. In the lifetime spectra we observe three lifetime components corresponding to different annihilation sites in the samples. These lifetimes are compared with the results of simulations of positron lifetimes in modeled Ni-Al samples obtained using molecular dynamics and Monte Carlo calculations. Finally, we present positron lifetime results for nanocrystalline Ni3Al samples which were produced or annealed at different temperatures.
INTRODUCTION Positron lifetime spectroscopy is a well-established technique to study open volume defects in solids [1]. The positron lifetime is primarily determined by the average electron density seen by the positron [2]. In a defect-free solid, the positron lifetime is a characteristic of the material. Positrons can get trapped into defects due to the formation of an attractive potential at open volume defects such as vacancies, vacancy clusters, dislocations, etc. When the positron is trapped into such a defect, its lifetime increases as a function of the defect size due to the locally reduced electron density. In large open volumes, positronium can be formed which is a bound state of a positron and an electron, analogous to a hydrogen atom. Nanocrystalline materials are a big challenge for positron lifetime spectroscopy due to the complexity of their structure. In this work, positron lifetime experiments in nanocrystalline Ni-Al samples with different composition are described. In these binary alloys, effects like segregation
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at the grain boundary interfaces and chemical ordering play an important role in the microstructure.
EXPERIMENTAL DETAILS Ni-Al nanocrystalline
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