Orientation relationships and lattice matching for the S phase in Al–Cu–Mg alloys

The form and crystallography of precipitate S phase have been examined in overaged Al–Cu–Mg alloys of low Cu:Mg atomic ratio using high-resolution electron microscopy and diffraction. Orthorhombic S takes the form of laths elongated parallel to [1 0 0] S//[1 0 0] α in face-centred cubic (fcc) matrix α solid solution and, in any given sample, may adopt a continuous or near-continuous range of orientation relationships, distinguished by rotation of up to 7° about these common axes between two discrete limits defined by the relationships: ( 0 0 1 ) S / / ( 0 2 1 ) α , [ 1 0 0 ] S / / [ 1 0 0 ] α and ( 0 2 ¯ 1 ) S / / ( 0 1 4 ) α , [ 1 0 0 ] S / / [ 1 0 0 ] α Such variation in orientation is accompanied by accommodating changes in interface orientation and structure, and by a systematic variation in the lattice parameters of the S phase. It is demonstrated that these changes are, in turn, associated with, at least local, changes in matrix α lattice parameter, likely to be attributable to heterogeneous coherent nucleation in association with pre-existing defects and/or solute or solute/defect clusters.