Oxide particle densities in internally oxidised Ag-Cd alloys: Part 2 Alloys with ternary additions

Abstract

The densities of oxide particles are studied as a function of depth below the surface of internally oxidised Ag–9·5 at.-%Cd alloys with additions of ∼0·2 or 2 at.-%Al, Mn, Ni, Sn, or Zn. Because of its low diffusivity and low free energy of oxide formation, Ni has no influence on particle density. Tin forms needlelike oxides and reduces the particle density as a result of geometric effects. Zinc has a marked effect on particle shape with platelike precipitates which coalesce during subsequent precipitation of CdO, but does not greatly affect particle density. Both Al and Mn increase particle density up to two orders of magnitude. Planar size distributions are presented which indicate that two types of nucleation site are present in the Mn containing alloys. In the alloy containing 0·15 at.-%Al the Al₂O₃ particles are too fine to act as nuclei for CdO precipitation; two types of band both with a narrow size distribution of CdO particles, but with slightly different size and largely different density are formed. Heterogeneous nucleation and depletion of the centre region of sample sheets, both of which result from the counterdiffusion of the oxidising elements, are shown to have a great influence on particle density, resulting in a more homogeneous distribution of particle densities over the sample cross-section than that predicted by available theories.

MST/900b