Vacancy Mobility in Nickel Aluminide versus Composition

The fractional concentration of Ni-vacancies in NiAl at high temperature has been determined for compositions between 50 and 53 at.% Ni from measurements using perturbed angular correlation of gamma rays (PAC). The vacancies were detected by quadrupole interactions induced at nearby In/Cd impurity probes present on the Al sublattice in high dilution. One set of measurements was made at high temperature. A second set made after rapid quenching exhibited an enhancement of Ni-vacancy site fractions that is attributed to diffusion and trapping of vacancies during quenching. The composition dependence of the enhancement was analyzed assuming that the enhancement is proportional to the root-mean-square diffusion length during quenching. The experimental dependence on composition is found to be consistent with bulk diffusion data. The methodology developed clarifies how local environments of impurity probes are modified during rapid quenching. In addition, it is shown that there is an enormous increase in the mobility of vacancies with increasing deviation of the composition from stoichiometry. Diffusion mechanisms that can explain the data trends are discussed.