TEM investigations on the structure of antiphase boundaries in D03 ordered Fe3Al

Antiphase boundaries (APBs) in D03 long-range ordered Fe3Al were studied by transmission electron microscopy. It was found that the APBs gave rise to a diffraction contrast not only in superlattice reflections but also in fundamental ones. This indicates that the total fault displacement R across the APBs is not exactly equal to the basic displacement R 0 but involves an additional one r so that R = R0 + r. The basic displacement R0 is either R0 = 1/4(111) for the nearest-neighbor APBs or R0 = 1/2(111) for the next nearest-neighbor APBs. R0 was responsible for the normal, strong APB contrast when imaged with superlattice reflections. Anomalous APB contrast imaged with fundamental reflections can be attributed to the additional non-crystallographic displacement r. The APB contrast caused by r was generally weak and asymmetric in the dark field image. While the nearest-neighbor APBs were clearly visible when imaged with fundamental reflections, the next nearest-neighbor APBs gave no observable anomalous contrast. The additional displacement r is therefore dependent on R 0. Its magnitude is smaller for R 0 = 1 /2(111) than for R 0 = 1 /4(111). Furthermore, the direction of the additional displacement r across the nearest-neighbor APB varies with the orientation of the domain boundaries. The main component of the fault vector r was determined to be perpendicular to the fault plane. The nearest-neighbor APB is of extrinsic character. The lattice near the APB is dilated.