Influence of misfit and interfacial binding energy on the shape of the oxide precipitates in metals; Interfaces between Mn3O4 precipitates and Pd studied with HRTEM

Transmission electron microcopy (TEM) revealed Mn3O4 precipitates with two types of dominant shape in Pd-3at.% Mn that was internally oxidized in air at 1000 degrees C. One type is octahedrally shaped and bounded by {111} pla nes of the Mn3O4. These observations were compared with earlier observation s in the Ag/Mn3O4 system and the octahedrons show a relatively larger trunc ation by (002) in Pd than in Ag. Further, the second type of precipitate sh ape, comprising about 1/3 of all of the precipitates in Pd, was not observe d in Ag. it corresponds to a plate-like structure, showing an orientation r elationship where the tetragonal axes of Mn3O4 are parallel to the cube axe s of Pd, with the c-axis of Mn3O4 as habit plane normal. High-resolution TE M observations revealed the presence of a square misfit dislocation network with line direction [110] and Burgers vector 1/2[110] at these interfaces with (002)Mn(3)O(4)parallel to{200}Pd. The general conclusions of the prese nt analysis are: (1) the anisotropy in interface energy for oxide precipita tes 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-cohe rent interfaces is important only if sufficient anisotropy in mismatch is p resent and if the matrix is sufficiently stiff; and (3) the stronger coupli ng strength due to electronic binding effects across the interface in Pd co mpared with Ag is responsible for formation of the dislocation network stru ctures at larger misfit. (C) 2000 Acta Metallurgica Inc. Published by Elsev ier Science Ltd. All rights reserved.