TEM investigations on the structure of antiphase boundaries in D0(3) ordered Fe3Al

Antiphase boundaries (APBs) in D0(3) long-range ordered Fe3Al were studied by transmission electron microscopy. It was found that the APBs Save rise t o a diffraction contrast not only in superlattice reflections but also in f undamental ones. This indicates that the total fault displacement R across the APBs is not exactly equal to the basic displacement R, but involves an additional one r so that R = R-0 + r. The basic displacement R-0 is either R-0 = 1/4[111] for the nearest-neighbor APBs or R-0 = 1/2[111] for the next nearest-neighbor APBs. R, was responsible for the normal, strong APE contr ast when imaged with superlattice reflections. Anomalous APE contrast image d with fundamental reflections can be attributed to the additional non-crys tallographic displacement r. The APE contrast caused by r was generally wea k and asymmetric in the dark held image. While the nearest-neighbor APBs we re clearly visible when imaged with fundamental reflections, the next neare st-neighbor APBs Save no observable anomalous contrast. The additional disp lacement 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 addit ional displacement r across the nearest-neighbor APE varies with the orient ation of the domain boundaries. The main component of the fault vector r wa s determined to be perpendicular to the fault plane. The nearest-neighbor A PE is of extrinsic character. The lattice near the APE is dilated. (C) 1998 Elsevier Science S.A. All rights reserved.