Positron Lifetime Study of Defect Structures in B2 Ordered Co-Al Alloys

Abstract

The positron lifetimes in the B2 intermetallic compounds Co 100−X Al X (X = 46.9 to 54.1) water-quenched, air-and furnace-cooled from high temperatures have been measured to reveal their defect structures. The positron mean lifetime depends entirely on the composition, irrespective of the cooling rate, and steadily increases from 161 to 180 ps as the Al concentration increases from 46.9 to 54.1 at%. A waterquenched Co 53.1 Al 46.9 was isochronally annealed for 900 s successively at 25 K intervals up to 1073 K, but its positron lifetime remained unchanged by the annealing. These results clearly show that vacancies are formed not only on the Co-sites but also on the Al-sites, and the total vacancy concentration is more than 10 −4 in both the Co-rich and the Al-rich CoAl. It was furthermore found that the population ratio of Al-vacancies to Co-vacancies gradually increases with the Co concentration, and the number of Al-vacancies is comparable with that of Co-vacancies in the vicinity of stoichiometric composition.