ABSTRACT
This chapter is concerned with the analysis of the geometry of small point-defect clusters such as dislocation loops and stacking-fault tetrahedra (SFT) which are imaged in diffraction contrast due to their elastic displacement fields. The defect structure-dislocation loop geometry in the present case-can determine such material properties as mechanical hardening by pinning of gliding dislocations. Knowledge of the defect structure can also give evidence as to how these defects were produced and may suggest ways to encourage or suppress their formation. In the present chapter, we are concerned with methods for determining the Burgers vectors, habit-planes, shapes and detailed morphologies of dislocation loops. We postpone discussions of how to determine the vacancy or interstitial nature of dislocation loops, and how to obtain quantitative measures of cluster sizes and number densities to chapters 4 and 5. We shall discuss two approaches for determining loop geometries. The Burgers vectors and habit-planes of dislocation loops of diameter smaller than about 10 nm are often best analysed using the black-white contrast method. This technique makes use of the characteristic image symmetries shown by clusters of different types when imaged under strong two-beam diffraction conditions. Black-white contrast is sensitive to the longrange strain fields of the clusters, and so gives relatively little information on features such as loop shapes. The second approach we shall discuss is weak-beam microscopy. Weak-beam microscopy is largely complementary to black-white analysis. It is sensitive to local strain fields and so is often useful for analysing clusters of complex shape or geometry, especially clusters larger than about 5 nm. It may be the only way to image and analyse successfully very small clusters lying close to the foil centre. The application of these methods will now be discussed in turn.
