New type of precipitate in Mg-rare-earth alloys

This paper describes the crystallographic features of a platelet-shaped precipitate that forms in the early stage of age hardening of Mg alloys containing rare-earth elements in the Ce subgroup. Transmission electron microscopy combined with computer simulation revealed that the precipitate, the habit plane of which is a {1100} matrix prism plane, is morphologically analogous to Guinier-Preston zones in Al-Cu alloys, that is two dimensional, but possesses an ordered structure based on the D019 superlattice. The precipitate also contains periodically arranged atomic steps that are similar to antiphase boundaries in ordered alloys. These periodic steps result in a split of superlattice reflections similar to that observed in diffraction patterns obtained from a long-period superlattice such as CuAu II.