Influence of Interfacial Binding Energy and Misfit on the Shape of the Oxide Precipitates in Metals

Transmission electron microscopy revealed Mn3O4 precipitates with two types of dominant shapes in Pd-3at.%Mn that was internally oxidized in air at 1000°C. One type is octahedrally shaped and bounded by {111} planes of the Mn3O4. These observations were compared with earlier observations in the Ag/Mn3O4 system. The octahedrons show a relatively larger (002) truncation in Pd than in Ag. Further, the second type of precipitate shape, comprising about ⅓ of all the precipitates in Pd, was not observed in Ag. It corresponds to a plate-like structure. HRTEM observations revealed the presence of a square misfit dislocation network with line direction <110> and Burgers vector ½<110> at these interfaces with (002)Mn3O4//{200}Pd. The general conclusion of the present analysis is (i) anisotropy in the interface energy for oxide precipitates in a metal matrix is substantial due to the ionic nature of the oxide, giving well defined shapes associated with the Wulff construction, (ii) the influence of misfit energy on the precipitate shape as bounded by semi-coherent interfaces is important only if sufficient anisotropy in the mismatch is present and if the matrix is sufficiently stiff, and (iii) the stronger coupling strength due to electronic binding effects across the interface in Pd compared to Ag is responsible for the formation of the dislocation network structures at larger misfit.