Slow removal of vacancies in B2-Ni52Al48 upon long-term low-temperature annealing -: art. no. 224102

Due to the high concentration and low diffusivities of thermal vacancies in B2-NiAl, these vacancies are thought to be only then fully removed when long-term annealing at low temperatures is employed [H.-E. Schaefer , Intermetallics 7, 277 (1999)]. In the present paper the annealing of vacancies in Ni52Al48 in the temperature range from 635 K to 685 K on a one-year time scale is specifically studied by positron annihilation spectroscopy. The positron lifetime tau(V)=(179+/-1) ps in as-prepared Ni52Al48 demonstrates the presence of remnant single vacancies. The decrease of the concentration of these vacancies with annealing time demonstrates that they can be removed by low-temperature long-term annealing leading to vacancy clusters. From the annealing a vacancy migration enthalpy of H-V(M)=(2.0+/-0.1) eV has been determined for Ni52Al48. The remnant single vacancies are detected to be located on the Ni sublattice of ordered NiAl. The present results are expected to be of interest as an exemplary case that removal of vacancies and therefore thermal equilibrium in complex solids may be achieved only after long times.