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.

The measurement of stacking-fault energies of pure face-centred cubic metals

The weak beam technique is used to obtain a value for the stacking-fault energy of copper. The observed partial dislocation image spacings are compared with many-beam computational models and it is demonstrated that anisotropic elasticity theory breaks down for small partial dislocation spacings in copper.

The weak beam technique as applied to the determination of the stacking-fault energy of copper

The equilibrium shape of dissociated dislocations in thin foils of Cu-10 at.% Al is investigated by weak-beam electron microscopy. It is found that dislocations assume a truncated triangular shape caused principally by the reduction in line energy when Shockley partials rotate towards screw character. The effects of interaction image stresses and surface slip steps are generally small, but the former may be important in the case of spontaneous cross-slip of a dislocation at a surface.

Non-parallel dissociation of dislocations in thin foils

The fourfold dissociation of superdislocations in Ni3Al has been recorded on images that were formed using second-order reflections, and the dissociation distances of the Shockley partial dislocations that bound a complex stacking fault (d CSF) have been measured. Comparisons with computer simulated (2g · 5g) images highlight the presence of supplementary intensity peaks when d CSF is less than 3.0 nm. These simulations also indicate that the distance between the weaker pair of intensity peaks can be used to measure d CSF if the experimental observations are corrected for the image shift that occurs during the microscopy. Experimental observations confirm the presence of the supplementary peaks, and measured values of d CSF were found to be larger in binary Ni3Al than in the boron-modified alloy. These measured values have been corrected through comparison to image simulations, and the corresponding complex stacking fault and antiphase boundary energies have been calculated using anisotropic elas...

Measurements of antiphase boundary and complex stacking fault energies in binary and B-doped Ni3Al using TEM

SUMMARY

The weak-beam method of electron microscopy enables the high resolution capabilities of the electron microscope to be used in the study of lattice defects. In this paper, the principles of the method are outlined and the experimental procedures for obtaining weak-beam images are discussed. The experimental diffraction conditions necessary for obtaining quantitative results and the limitations of the method are summarized, and applications of the method to various problems in defect studies are reviewed.

The principles and practice of the weak-beam method of electron microscopy

It is pointed out, and confirmed by calculation in a specific case, that the effects of the column approximation in calculating weak beam images of defects can be greatly reduced if the weak beams used make small angles with the Bragg planes.

C. H. Sworn

Papers

Column approximation effects in high resolution electron microscopy using weak diffracted beams