Constitutional vacancies in CoAl studied by positron lifetime spectroscopy

The positron lifetimes in the B2 intermetallic compounds Co100-xAlx (X=46.9 to 54.1) water-, air- and furnace-cooled from high temperatures have been measured to reveal their defect structures. Irrespective of cooling rate, the positron mean lifetime increases from 162 to 179 ps as the Al concentration increases from 46.9 to 54.1at%. The minimum and maximum values, respectively, agree with the positron lifetimes in the Al-vacancy and Co-vacancy calculated with the DV-X a method. A water-quenched Co54Al46 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 shows the existence of constitutional vacancies not only on the Al-rich side but also on the Co-rich side of the stoichiometry. On the borders of CoAl single phase region vacancies are exclusively formed on the sites assigned to deficient atomic species, in order to compensate for the deviation of stoichiometry, but in the vicinity of stoichiometric composition the number of vacancies on the site assigned to surplus atomic species is comparable with that of vacancy on the site assigned to deficient atomic species.