Structure and morphology of S-phase precipitates in aluminum

This work presents a crystallographic and morphological analysis of S-phase precipitation in Al alloys. Using quantitative high resolution electron microscopy, four models for the crystal structure of the S-phase (Al2CuMg) in Al-based alloys are critically evaluated, and a new model is proposed. This model is identical to that of Perlitz and Westgren, but with an exchange of Cu and Mg. Two distinct precipitate morphologies are observed. Both are laths elongated along (100) directions common to the matrix and the precipitate and lie on {021} planes of the matrix. Type I precipitates have interfaces of the type (021)(Al)parallel to(001)(s) while type II precipitates have interfaces of the type (021)(Al)parallel to(043)(s), i.e. the two types differ in the S-phase lattice plane that is conjugate to the {021}(Al) habit plane. The interface plane of type I precipitates tends to be atomically flat containing only growth ledges while that of type II precipitates is stepped, The orientation relationship of the two types of precipitate differs by a rotation of about 5 degrees around the lath axis. The difference between the two types of precipitate is discussed in terms of their lattice correspondence, and type II precipitates are shown to follow an invariant line strain. Moire analysis of lattice distortions revealed that {020}(Al) planes remain undistorted while {002}(Al) planes suffer significant shear during S-phase nucleation. Published by Elsevier Science Ltd on behalf of Acta Metallurgica Inc.