Mn3O4 precipitates in Ag and Cu studied with HRTEM: orientation relations and tetragonal twinning

Internal oxidation of Ag-3at.%Mn resulted in Mn3O4 precipitates and of Cu-1at.%Mn gave rise to MnO and a minor amount of Mn3O4 precipitates. In Cu Mn3O4 transformed out of MnO, whereas in Ag Mn3O4 developed directly. Predominantly, precipitates with parallel topotaxy and an octahedron shape due to 8 (111) oxide facets are formed. Due to the tetragonality of Mn3O4 a tilt of 3.8 degrees can exist between the (111) of Mn3O4 and Ag on all 8 interfaces, but it appeared more favourable to align (ill) parallel for one pair of facets. Starting from a parallel topotaxy for MnO and Cu, the transformation out of MnO and the Pain-strain lattice correspondence with MnO determines largely for Mn3O4 the orientation relation with Cu. Tetragonal twinning is observed in Mn3O4 precipitates in both Ag and Cu. Coexistence in precipitates of MnO and tetragonal twinned Mn3O4, is frequently observed in Cu. Probably, tetragonal twinning reduces the misfit between Mn3O4 inclusions and the cubic matrix. This is verified by linear elasticity calculations pertaining to the inclusion problem