It was recalled that particular defect structures, so-called lamellar faults, appeared during the directional solidification of Al-Al2Cu(θ) lamellar eutectic composites. Each fault corresponded to the terminations of lamellae. These terminations consisted of a continuous succession of 3 types of ribbon-like interfaces, among which were 2 low-angle boundaries: one was located in the face-centered cubic phase and the other in the θ intermetallic (C16, a = 0.6063, c = 0.4872nm). High-resolution transmission electron microscopy of a θ/θ low-angle tilt boundary, with the electron beam parallel to <113>θ showed that the core structures of the dislocations were undissociated in spite of the large magnitude of the possible Burgers vectors. The dislocations were of mixed-type, and were oriented along the <113>θ; the direction which was the closest to the growth axis direction of the eutectic. An unusual defect in the tilt boundary consisted of a dense group of 3 dislocations, separated by nanometric distances. This defect acted like a single dislocation at far-field distances. The mechanical equilibrium of such a defect was studied by using isotropic and anisotropic elasticities. It was strongly suggested that 2 of these

dislocations formed a dipole; thus indicating that the boundary did not have a completely relaxed structure.

HRTEM Observation of a <113>θ Low Angle Tilt Boundary in the Al-Al2Cu(θ) Eutectic Composite. C.H.Belgacem, M.Fnaiech, M.Loubradou, S.Lay, R.Bonnet: Physica Status Solidi A, 2002, 189[1], 183-96