Multiple ribbons in face-centred cubic materials. I. Geometry and properties of double ribbons

The geometry of double ribbons in f.c.c. materials is discussed. It is shown that two different types of double ribbon can exist and that each of these can be present in two different defect configurations which are referred to as the elongated-node type and the parallel-interaction type. The properties of these two configurations differ because the former must involve the presence of jogs while the second need not; if present these jogs then affect the formation and movement of the double ribbons. Images of the defects are compared for weak-beam and conventional strong-beam diffraction conditions. It is also shown that double ribbons can exist which do not contain an extrinsic stacking fault and that double ribbons can be stable in the edge orientation.