High-resolution analytical electron microscopy study of isothermal plate-shaped products in some β-phase alloys

The isothermal plate-shaped products in β-phase Cu-Zn-Al, Cu-Zn-Au, and Ag-Cd alloys have been studied from the viewpoints of morphology, crystal structure, and composition using a high-resolution analytical electron microscope equipped with a field-emission electron gun. It is shown that thin plate-shaped products, at early stages of the isothermal formation, exhibit crystallographic properties characteristic of martensites formed at subzero temperatures but that their compositions were definitely different from those of the surrounding matrices. Therefore, atom diffusion is conceivable to be involved in the plate formation from the nucleation stage and to occur across the coherent plate/matrix interfaces. In order for nucleation of the isothermal transformation to occur, two kinds of reactions should be thermally activated at intermediate temperatures above T_o for the martensitic transformation: (1) atom diffusion to achieve the observed composition differences between plates and matrices and (2) martensitic transformation in hypothetical alloys with compositions less than those of the original ones by the observed composition differences. The incubation periods experimentally observed should be associated with these two thermal activation processes. This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18–22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.