Interpretation of vacancy migration, trapping, and clustering in fcc metals as observed through perturbed angular correlations and distributions ofγrays

In the past six years perturbed angular correlations (PAC) and perturbed angular distribution (PAD) of $\gamma$ rays have been widely applied to study the migration, trapping, and clustering of lattice defects in metals. Essential features of these experiments are distinctive precessional signals arising in the nuclear hyperfine interaction. These signals serve as convenient labels for specific lattice defects bound to $\gamma$-ray-emitting probe atoms, and under favorable circumstances provide information on the lattice symmetry of the defects. Because we believe that the maximum information is obtained from such experiments if systematic comparisons are made between similar metals, we describe here our survey and interpretation of PAC and PAD data for seven fcc metals (Ag, Al, Au, Cu, Ni, Pd, Pt) using ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$ probe. We consider defect production by irradiation, quenching, and ion-implantation, and note that the same defect types recur in all three methods. We include a total of 19 distinctive states in our discussion, and through a series of independent, comparative observations we argue that these states involve vacancy defects bound to the ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$ probe. On the basis of the observed frequencies, annealing behavior, and electric-field-gradient symmetry we divide vacancy states into four classes. For three of these classes structural assignments can be made. These include the nearest-neighbor monovacancy observed in five metals, divacancies or faulted loops in the {111} plane observed in six metals, and a tetrahedral cluster seen only in Ni. For the nearest-neighbor monovacancy the data permit estimation of migration enthalpies. For the monovacancy the data also permit an interpretation of the observed electric field gradients. Though some of the observed states remain with undetermined structure, we believe our comparative analysis provides an excellent foundation for further detailed study of small vacancy clusters under a variety of metallurgical conditions.