DIFFRACTION CONTRAST IMAGES OF SMALL STACKING-FAULT TETRAHEDRA IN FCC METALS

Transmission electron microscopy images of stacking fault tetrahedra (SFTs) are studied in detail both experimentally and by image simulation. Experimental dark-field diffraction contrast images of SFTs are examined under various conditions. The image contrast of stacking faults and stair-rod dislocations are simulated based on standard multi-beam dynamical theory, and the image contrast of very small SFTs (< 2.6 nm) is calculated by the multi-slice method. The correspondence between experimental and calculated results under systematically varied conditions is quite satisfactory. Stacking faults make a larger contribution to the total SFT image than stair-rod dislocations. The images are generally smaller than the SFT. The difference in size depends on many parameters such as deviation from the Bragg condition, location in the foil, and specimen thickness. However, the difference is always smaller than the fringe spacing in the stacking fault image, and is generally smaller at a larger deviation from the Bragg condition. SFTs smaller than the fringe spacing are sometimes indistinguishable in contrast. The diffracting vector of 200 achieves higher visibility than any other diffracting vectors. The visibility ratio is higher at larger deviations from the Bragg condition. Multi-slice calculations suggest that SFTs smaller than 1 nm can be detected by weak-beam diffraction contrast methods. Suitable observing conditions for data acquisition in radiation damage studies are proposed.