Positron lifetime study on vacancy behavior in bcc-base intermetallics

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The bcc-base intermetallic compound, Co_2TiAl, is expected to be useful as a coating material on refractory material, because it maintains L2_1-type ordered structure up to high temperature and has excellent oxidation resistance. The Co_2TiAl phase has a wide composition range, extending across the stoichiometric composition and hence constitutional defects have to be naturally introduced at non-stoichiometric compositions. However, the defect structure has not been revealed yet. In this work we have measured the positron lifetimes in Co-rich and Al-rich Co_2TiAl water-quenched or furnace-cooled from 1273K. The positron mean lifetime in the water-quenched Co_<50.8>Ti_<24.0>Al_<25.2> is about 160ps and nearly equal to that in the furnace-cooled Co_<50.8>Ti_<24.0>Al_<25.2>, but is much larger than about 120ps of the first-principles calculated positron lifetime in the perfect lattice, Co_<50>Ti_<25>Al_<25>. For both the water-quenched and furnace-cooled specimens, χ^2q^<-1>, which is proportional to the square of the difference between the experimentally obtained spectrum and the fitted decaying function of one lifetime component, is close to unity, which can be interpreted as due to the saturation trapping of positron by vacancy. These results clearly show that many vacancy of more than 10^<-4> atomic fraction is introduced in Co-rich and Al-rich Co_2TiAl, in order to compensate the deviation from the stoichiometry.