A directionally solidified Al2Cu(θ) crystal (body centered tetragonal lattice, a = 0.6063nm, c = 0.4872nm) grown along the c-axis was observed by transmission electron microscopy to investigate the solidification defects. In a first step, using 2-beam transmission electron microscopy, matrix dislocations with large Burgers vectors ½<111>, [001], and [100] were positively identified, as well as a rectangular spiral source with a Burgers vector b = ½[¯1¯1¯1] acting along the (¯1¯12) plane. These defects were identified from the comparison between sets of experimental and theoretical images. It was shown that the topological contrasts of these dislocations could be highly affected by the elastic relaxation at both free surfaces of the thin foil. In a second step, a (110) facetted low angle tilt boundary with the c axis was investigated by high resolution TEM. Its structure could be described as a succession of 2 consecutive mixed dislocations b = ½[111] and ½[11¯1], which gave for the pair an edge resultant Burgers vector [110]. A quite unusual core splitting was observed for one of these mixed dislocations since narrow faults extend on the (110) boundary plane. Around this complex core, the relative positions of the partials were carefully determined at a near atomic scale using a dedicated simulation program using anisotropic elasticity theory. Large planar faults along (001) were also sometimes noticed. These results stressed that, in the characterization of crystalline defects in crystals with large lattice parameters, numerical methods provided an important advantage over classical methods based on qualitative arguments alone.

Crystalline Defects in a BCT. Al2Cu(θ) Single Crystal Obtained by Unidirectional Solidification along [001]. R.Bonnet, M.Loubradou: Physica Status Solidi A, 2002, 194[1], 173-91