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The weak-beam technique has been used to observe two- and four-fold dissociations in Fe-Al alloys, having compositions between 26 and 36 at.% Al, with B2-type and with DO3-type long-range order. From the orientation-separation curves, the antiphase boundary energies have been determined as a function of composition for both types of order.

The stacking-fault energies of a number of copper alloys with electron-atom ratios less than or equal to 1·10 have been measured from observations of single dissociated dislocations imaged using the weak-beam method of electron microscopy. The results are compared with previous determinations made using the extrapolation of data from strong-beam node images and from the analysis of the onset of dynamic recovery during constant strain-rate testing.

The weak-beam technique of electron microscopy (Cockayne, Ray and Whelan 1969) has been used to study constrictions found on extended dislocation lines in a copper-silicon alloy. The nature of these constrictions has been determined using in situ heating of the alloy.

Using standard imaging techniques of electron microscopy it is not possible to resolve the partial dislocations having the equilibrium separation for dissociated dislocations in the pure face-centred cubic metals. Consequently the stacking-fault energy γ cannot be determined from direct measurements of the separations of the partials. In this paper it is demonstrated that such determinations are possible using the weak-beam method of electron microscopy. The separations of Shockley partial dislocations in silver and copper have been measured, as a function of dislocation line orientation, using this method. From these measurements, the values γsilver= 16·3+1·7 erg cm-2 and γcopper= 41±9 erg cm-2 have been obtained from analyses based on anisotropic elasticity theory.

A method for determining the position of dislocation lines accurately from dislocation images is reported and applied to the case of measuring the separation of partial dislocations. A theoretical analysis of the method is given, and the predictions verified by many-beam computed image profiles. The separation of partial dislocations in Cu + 10at. % Al has been determined using the method. The separation was found to be 120±20 Å. Other applications of the method are suggested.

The 'weak-beam' technique of Cockayne, Ray and Whelan (1969) for imaging dislocations in the electron microscope has been applied to dislocations in silicon. These are shown to be dissociated into partial dislocations, the dissociation width corresponding to a stacking-fault energy of about 55 erg cm-2.

The weak-beam technique of electron microscopy (Cockayne, Ray and Whelan 1969) has been used to image the fourfold dissociation of superlattice dislocations in a DO3-ordered Fe-26 at.% Al alloy. Values of 77±12 and 85±16 erg cm-2 have been obtained for the first and second nearest-neighbour antiphase boundary energies, respectively.

The 'weak-beam' technique of Cockayne, Ray and Whelan (1969) for resolving very closely spaced dislocations in the electron microscope has been applied to the dissociation of BZ-type superlattice dislocations in highly ordered Fe-35 at. % Al. By plotting the separation of the partials as a function of orientation a value of 105±15 erg om-2 has been obtained for the antiphase boundary energy.