Electron Microscopy of Ordering in Alloys

Abstract

The aim of this contribution is certainly not to provide a general course in electron microscopy, other and more elaborated works can be consulted for this purpose, e.g. [1,2]. We will only try to clarify these aspects of electron microscopy which are useful in the study of ordered alloys or in the evolution towards ordering. First we will study the symmetry aspects of the ordered alloy and predict the number of orientation as well as translation variants of the ordered phase in the disordered matrix. Due to the usual lowering of symmetry upon ordering a number of orientation and translation interfaces will result in ordered alloys. Their number and internal relationship can be predicted based on simple symmetry rules. The two dimensional defects related to ordering can be identified from electron microscope observations and sometimes produce specific electron diffraction effects. High resolution microscopy combined with computer simulated images allow to obtain atomic scale information of ordering related defects. Important is that we can show that the minority atoms in a large range of alloys can be systematically imaged as bright dots; this provides us with an easy code to interpret sometimes complex high resolution images.