Grain boundary sites in fcc metals studied by PAC

Abstract

Perturbed angular correlation of gamma rays (PAC) is applied to study the atomic structure of grain boundaries (gb) in fcc metals using nuclear quadrupole interactions.¹¹¹In probes were diffused in samples of Au, Cu, Ni and Pt which had first been annealed to form high-angle gbs. Diffusions were carried out at such low temperatures that only gb diffusion should occur (Harrison's type-C kinetic regime), and measurements were made after removing any residual surface activity. For three Pt samples, a unique signal attributed to a gb site was detected with a site fraction of about 50%, coupling frequency ω₁=80.0(2) Mrad/s and electric-field-gradient asymmetry parameter η=0. A Pt sample of 99.9995% purity exhibited much less inhomogeneous signal broadening than 99.998% pure samples, indicating the level of purity necessary to observe the gb sites via quadrupole interactions. The broadening is attributed to gb segregation. For Au, Cu and Ni, only ill-defined, non-unique signals were detected, apparently because of gb segregation. The small broadening in the high-purity Pt sample indicates a high degree of uniformity in the crystal structure of the high-angle gb sites. The large site fraction of the 80 Mrad/s signal demonstrates the dominance of a single site for Cd probes in high-angle gbs.