THE DEFOCUS CONTRAST OF A THETA' PRECIPITATE IN AL-4WT-PERCENT-CU - FRESNEL FRINGE ANALYSIS APPLIED TO AN ATOMICALLY ABRUPT INTERFACE

The through-focal contrast of a theta' precipitate in Al-4wt%Cu is exa mined. The aim of the work presented here is to establish whether Fres nel contrast analysis can be used to characterise an atomically abrupt interface. Simulations are presented which illustrate the sensitivity of variations in image contrast, at a relatively low resolution and a s a function of the defocus, to atomically localised changes on compos itional spread. The effects of dynamical contributions to the contrast variations are also considered in this context. Preliminary experimen tal data are analysed and the problems associated with matching experi ment and theory are discussed.