Electron diffraction and weak-beam microscopy.

Electron diffraction patterns formed in the TEM provide a great deal of quantitative information which can be used to help interpret associated images. For example, when correctly indexed, they can be used not only to identify the crystalline phase under examination, but also to determine the phase orientation with respect to the direction of the incident electron beam. Electron diffraction patterns can be used to determine the line direction of dislocations, as well as the orientation of lamellae relative to crystallographic planes in the matrix. They may also contain fine structure, due to short-range and long-range ordering, or resulting from defects such as grain boundaries. Weak-beam microscopy has been used to study a variety of problems, including the nature of small precipitates and defect clusters, either in the matrix or along dislocations, dislocation interactions, stacking faults and dislocations in grain boundaries. The theoretical background of weak-beam TEM is introduced and the application of the weak-beam technique to the study of dissociated dislocations and low-angle grain boundaries is emphasized. -J.M.H.