Crystallography of embedded particles in Al–Mg–Zn alloys. Symmetry analysis

Abstract

Following a proper heat treatment, the alloy system Al-Mg-Zn shows a great wealth of precipitate particles forming coherent ( 0 crystals) and incoherent ( crystals) boundaries with the Al matrix. Both the matrix crystal and the precipitate crystals are holohedral, as their point groups correspond to their metric symmetries (m3m and 6/mmm). On the basis of published orientational relationships for a principal variant of every known precipitate family, the full sets of orientational variants are deduced by the concepts of intersection groups, H , and variant generating sets, V . The intergrowth symmetry principle has been visualized by stereographic projections. Special attention has been given to patterns of superimposed lattice nodes in reciprocal space and the implications of overlap in terms of observable reflections. It has been uncovered, by artificially reducing the point group symmetry of the coherent 0 phase, where V is a proper subgroup of the matrix holohedral, that twin laws for merohedry are revealed when V is factorized into a weak direct product.

An atomic scale structural investigation of nanometre-sized η precipitates in the 7050 aluminium alloy

“…It is appropriate to conclude that the resulting plate-like morphology of η' (or η2) would be associated with the growth along the preferential directions with a low-strain energy on the hexagonal plane of this embryo of di-hexahedron crystal, so that the low strain-energy of the interface can be maintained [7]. It should be noted that although a number of investigations [1,35,36,[38][39][40] have employed the above crystallographic method to determine the morphologies of precipitates [29,30], few of those works have connected them with the variable compositions of alloys, which lead to changes in lattice parameters to some extent (i.e., the lattice misfit).…”

Average crystal structure(s) of the embedded meta stableη′-phase in the Al–Mg–Zn system

“…Care still had to be taken to avoid measurements of those η -reflections that exhibit overlap with Al-reflections. A full discussion of reflection overlap and other features of the reciprocal space within this system will be given elsewhere [11].…”