Crystal structure of the β″ phase in an Al-1.0mass%Mg₂Si-0.4mass%Si alloy

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 100_β″// [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.