Diffraction contrast from elastic shear strains due to coherent phases

Abstract Electron transmission microscopy and diffraction reveal that the precipitation of {100} disc-shaped G.P. zones in a copper-2% beryllium alloy is accompanied by the appearance of striations parallel to the traces of {110} planes in the microstructure and < 110 > relrods normal to these planes in diffraction patterns. Using the kinematical theory of diffraction, it is shown that these striations are a strain contrast effect arising from elastic shear strains induced by conditions of tetragonal coherency in agreement with a model proposed by Tyapkin. The high elastic anisotropy of copper is given as the reason for this type of strain distribution adjacent to the disc-shaped particles. This model is also applicable to the initial stages of ordering in stoichiometric CuAu, CoPt. NiPt and Ni3V.