Crystalline Defects in a B.C.T. Al2Cu(?) Single Crystal Obtained by Unidirectional Solidification along [001

Abstract

A directionally solidified Al2Cu(θ) crystal (body centred tetragonal lattice, a = 0.6063 nm, c = 0.4872 nm) grown along the c axis has been observed by transmission electron microscopy (TEM) to investigate the solidification defects. In a first step, using two-beam TEM, matrix dislocations with large Burgers vectors 1/2〈111〉, [001], and [100] were positively identified, as well as a rectangular spiral source with a Burgers vector b = 1/2[] acting along the (2) plane. These defects were identified from the comparison between sets of experimental and theoretical images. It is shown that the topological contrasts of these dislocations can 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 has been investigated by high resolution TEM. Its structure can be described as a succession of two consecutive mixed dislocations b = 1/2[111] and 1/2[11], which gives for the pair an edge resultant Burgers vector [110]. A quite unusual core splitting is 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 have been carefully determined at a near atomic scale using a dedicated simulation program using anisotropic elasticity theory. Large planar faults along (001) are also sometimes noticed. These results stress that, in the characterisation of crystalline defects in crystals with large lattice parameters, numerical methods provide an important advantage over classical methods based on qualitative arguments only.