Crystal structure of the beta '' phase in an Al-1.0mass%ME2Si-0.4mass%Si alloy

The crystal structure of the beta " phase in an Al-1.0mass%Mg2Si-0.4mass%Si alloy aged at 423 K for 2400 ks has been investigated by high resolution t ransmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and micro beam diffraction (MBD) techniques and energy dispersive X-ray spectroscopic (EDX) analysis. The aging treatment at 423 K for 2400 k s was a specific aging condition such that the beta " phase predominantly p recipitates, The beta " phase has a monoclinic lattice with the lattice par ameters a = 0.77 +/- 0.02 nm, b = 0.67 +/- 0.01 nm, c = 0.203 nm, and gamma = 75 +/- 0.5 degrees. Its space group is P-2/m. The crystallographic orien tation relationship is found as follows: (001)(beta ")//(001)(m) and 100(be ta ")//[310](m). The beta " phase has a needle shape with its longitudinal direction parallel to the direction of [001](beta "). The chemical composit ion of the beta " phase is Si:Al:Mg = 6:3.1. (C) 1999 Elsevier Science S.A. All rights reserved.