FACTORS AFFECTING STACKING-FAULT CONTRAST IN TRANSMISSION ELECTRON-MICROSCOPY - COMPARISONS WITH IMAGE SIMULATIONS

Factors that affect stacking fault contrast in transmission electron m icroscopy have been studied with the aid of a many-beam program for ca lculating computer simulated images. In the first part of this work, s imulated images were used to verify the standard methods that are gene rally used to identify the intrinsic/extrinsic nature of stacking faul ts. These methods were found to be valid when the fault had an average inclination in the thin foil, but geometrical exceptions to these rul es were also observed. A geometric parameter that accounts for the ori entation of the fault with respect to the diffraction vector (g) and t he beam direction (B) has been proposed. In the second part, the effec t of the strain field of the bounding dislocations on stacking fault c ontrast has been studied, and it was found that these effects can be s ignificant when the dissociation distance is small and the Burgers vec tors of the bounding dislocations are large. A comparison of experimen tal and simulated images of stacking fault dipoles in TiAl are present ed in order to underscore the influence that the bounding dislocations can have on the stacking fault contrast. The advantages to be gained by comparing experimental observations with simulated images have been illustrated.