INFLUENCE OF MISFIT AND INTERFACIAL BINDING ENERGY ON THE SHAPE OF THE OXIDE PRECIPITATES IN METALS; INTERFACES BETWEEN Mn_3O_4 PRECIPITATES AND Pd STUDIED WITH HRTEM

摘要

Transmission electron microcopy (TEM) revealed Mn_3O_4 precipitates with two types of dominant shape in Pd-3at./100 Mn that was internally oxidized in air at l000deg. C. One type is octahedrally shaped and bounded by { l l l } planes of the Mn_3O_4. These observations were compared with earlier observations in the Ag/Mn_3O_4 system and the octahedrons show a relatively larger truncation by (002) in Pd than in Ag. Further, the second type of precipitate shape, comprising about I/3 of all of the precipitates in Pd, was not observed in Ag. It corresponds to a plate-like structure, showing an orientation relationship where the tetragonal axes of Mn_3O_4 are parallel to the cube axes of Pd, with the c-axis of Mn_3O_4 as habit plane normal. High-resolution TEM observations revealed the presence of a square misfit dislocation network with line direction (110) and Burgers vector l/2 at these interfaces with (002)Mn_3O_4||{200}Pd. The general conclusions of the present analysis are: (l) the anisotropy in 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; (2) the influence of misfit energy on the precipitate shape as bounded by semi--coherent interfaces is important only if sufficient anisotropy in mismatch is present and if the matrix is sufficiently stiff; and (3) the stronger coupling strength due to electronic binding effects across the interface in Pd compared with Ag is responsible for formation of the dislocation network structures at larger misfit.