Crystal structure of the β′′ phase in an Al–l.0mass%Mg2Si–0.4mass%Si alloy

Abstract

Abstract The crystal structure of the β′′ phase in an Al–1.0mass%Mg2Si–0.4mass%Si alloy aged at 423 K for 2400 ks has been investigated by high resolution transmission 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 ks was a specific aging condition such that the β′′ phase predominantly precipitates. The β″ phase has a monoclinic lattice with the lattice parameters a=0.77±0.02 nm, b=0.67±0.01 nm, c=0.203 nm, and γ=75±0.5°. Its space group is P2/m. The crystallographic orientation relationship is found as follows: (001)β′′//(001)m and 100β′′// [310]m. The β′′ phase has a needle shape with its longitudinal direction parallel to the direction of [001]β′′. The chemical composition of the β′′ phase is Si:Al:Mg=6:3:1.