| Modulated ordered structures, or long period superlattices (LPS), in the fcc-based binary alloy systems Ag-Mg and Cu-Pd have been studied by high resolution electron microscopy and electron diffraction. In the Ag-Mg system, several new modulated phases have been found. A preliminary free energy model, incorporating ideas and features found in the well-studied Axial Next-Nearest Neighbor Ising (ANNNI) model, is discussed and applied to the Ag;In addition, Cu-Pd samples of compositions varying from 16 to 26 at.% Pd were irradiated in situ in a 1.5 MeV electron microscope at various temperatures with the intent of enhancing kinetics at low temperatures (between 300 and 500 K). However, the alloy did not evolve toward the expected equilibrium structures at these temperatures. Instead, irradiation produced steady-state short-range order (SRO) which, for specimens of 18% or more Pd, is characterized by diffuse intensity at (1, ;An attempt is made to understand the stability under high energy electron irradiation of the modulated SRO state as compared with unmodulated order employing a mathematical technique recently developed by Martin (1984). |
