M. Guymont

Crystallographic description of coincidence‐site lattice interfaces in homogeneous crystals

Coincidence-site lattice interfaces (CSLI) are frequently observed in crystals where a rigid framework remains invariant on both sides of the interface. They also seem to minimize the interface energy, for example, in metals where, empirically, the greater the density of the coincidence-site lattice the more stable the grain boundary becomes. Group-theory considerations allow the determination of all the possible interface operations which leave a given sublattice invariant. A classification of these CSLI with respect to the number of equivalent sublattices they leave invariant is a guide for the prediction of the most stable type of interfaces with respect to the sublattice considered. Examples from different types of crystals illustrate the method, which also applies for translation boundaries, twins and grain boundaries.

Characterization of two different types of long‐period ordered alloys by high‐resolution electron microscopy

High-resolution electron microscopy can be used to differentiate between two structural models of long-period ordered alloys. These models differ in the nature of the disorder as seen in the stacking irregularities which can have planar boundaries as exemplified by Ag3Mg or wavy boundaries as in AuCu II. In the composition range 22-27 at. % Mg, Ag3Mg is built up with a regular arrangement of two kinds of structural layers, 1 or 2 LI 2 cells in thickness. Some stacking disorder exists, but this alloy can be locally described using a space-group notation and has the character of being an infinitely adaptative structure. High-resolution images have been used to show the perfect planarity of the boundaries in Ag3Mg, thus demonstrating the way in which disorder is accommodated in this alloy.

Etude des phases ordonnées à longue période irrationelle. Les alliages AuCu II et AuCu–Zn

It is shown, by electron diffraction and high-resolution microscopy, that AuCu II is a typical example of an irrational long-period alloy, contrary to Ag3Mg, for instance, which is a typical rational long-period alloy. Antiphase boundaries are not planar (as is the case for Ag3Mg) but rather fluctuating around a mean position, as shown by high-resolution images; this structure is well described by Jehanno & Perios model [J. Phys. (1964), 25, 966-974], which always involves some disorder localized in the boundaries themselves. The long period is always the result of an average of domains of different lengths, thus giving a statistical meaning to the irrationality. This structural behaviour is not confined to alloys with large values of the mean length of domains, as is demonstrated by the study of AuCu-Zn.

The analysis of a defect in ordered Ag3Mg

The high resolution image of an apparently displacive defect in the long period ordered structure of Ag3Mg is analysed in terms of a simplified substitutional model. It is demonstrated by image simulation that the characteristics of the image could be described equally well on the basis of progressive relative substitution of Ag for Mg on two of the four sublattices in the structure.

Structural study of CdY2S4

Le compose du titre cristallise dans le systeme cubique, groupe Fd3m et sa structure est affinee jusqua R=0,045. Le cristal peut etre decrit a partir de domaines a structure NaCl desordonnee, correspondant a la formule Cd 2 Y 4/3 S 4 , en croissance coherente avec des domaines orientes de structure spinelle et de formule CdY 2 S 4

Structure of long period ordered Ag3Mg

The structure of ordered Ag3Mg is described as a discontinuous series of well defined long period phases, stable in small concentration ranges, corresponding to discontinuous changes of the long period. Each phase is a realization of Fujiwara’s model. High resolution electron microscopy has condirmed this interpretation.

Rational and irrational long period structure in ordered alloys

Among long period ordered alloys, electron diffraction and high resolution experiments lead to distinguish two classes of ordering well‐defined structures, probably corresponding to different ordering methanisms. Our description of both classes of structure is based on two typical cases: ordered Ag3Mg and AuCu II.