An experimental study of anti-phase boundaries contrast of Fe-Si alloys in superlattice reflexions

Thin foils of ordered alloys of different composition are studied by transmission electron microscopy. The contrast on antiphase boundaries 1/4a ₀ ^′ 〈111〉, 1/2a ₀ ^′ 〈100〉 is compared with contrast theories. It is shown that the dependence of the contrast profile on the deviation parameter from the Bragg reflexion position and the thickness of the foil for oblique boundaries is qualitatively in agreement with the theory. On nearly perpendicular boundaries the observed dependence of contrast is qualitatively the same for higher silicon contents. For lower silicon contents, however, only dark contrast (in the dark field image) is observed for all values of the deviation parameter and foil thickness. This behaviour of nearly perpendicular boundaries at lower silicon contents is explained by the existence of a disordered layer of finite thickness in the boundary. The disordered layer also causes the anomalous contrast of 1/2a ₀ ^′ 〈100〉 boundaries in S₂ reflexions for which zero contrast is predicted by the theory. In contrast to the idealized model of a boundary of this type with disordered second nearest neighbours only, the experiments show also some disorder of the first nearest neighbours. Finally, examples of complicated fringe contrast are shown which can possibly be interpreted as many-beam cases with superlattice and fundamental reflexions excited simultaneously.