The GP-zone to β" transformation in the Al-Mg-Si system

Abstract

GP-zones and the β"-Mg5Si6 phase are essential strengthening phases of the Al-Mg-Si system. The GP-zones nucleate in a very high number during heat-treatments and transform into fewer and coarser β"-particles. We present the structure of GP-zones in the Al-Mg-Si system and relate it to that of β". The GP-model was derived by analysis of exit wave images, comparison of inter-atomic distances and image simulations. It is a centred super-cell on the Al-lattice with a→GP=[32̄0]·aAl (14.60Å), b→GP=[001]·aAl (4.05 Å), →Gp=[130]·aAl (6.40 Å). Both phases have the same monoclinic space group (C2/m). The a- and c- parameters of β" are 15.16 Å and 6.74 Å, respectively, corresponding therefore to a slightly expanded super-cell. We show that the structure of β" may be obtained from that of the GP-zone by a translation b/2 of the Mg (000) atoms.