An investigation of vacancy-like defects in stoichiometric NiAl by means of positron annihilation measurements

Vacancy-like defects in stoichiometric NiAl are investigated by means of po sitron lifetime and Doppler-broadening measurements after quenching from hi gh temperatures and proton or electron irradiation. The lifetime of the ann ealed samples, tau = 178 ps, confirms that remnant defects, most probably V acancies and vacancy complexes, are quenched-in during the production of th e samples. The decrease of the positron lifetimes and the S-parameter value s with increasing quenching temperature are explained with the increasing c oncentration of Al vacancies. The isochronal annealing of the samples, quen ched from 1600 degrees C, shows a distinct agglomeration of defects in the temperature range 400-800 degrees C. After proton and electron irradiation no change in the positron lifetime is observed. There is an annealing stage in the temperature range 900-1200 degrees C. The S-parameter after proton irradiation shows an additional annealing step at around 200 degrees C. Lon g-time annealing for 520 h at 1150 degrees C does not result in a remarkabl e reduction of the positron lifetime. (C) 1998 Acta Metallurgica Inc. Publi shed by Elsevier Science Ltd. All rights reserved.