Formation of giant atomic clusters in the β-Samson (β-Al₃Mg₂) phase of the Al-Mg alloy system

The crystallographic features of the (α-Mn → α-Mn + β-Samson) reaction in Al-Mg alloys around the composition of 48 at.% Mg have been investigated mainly by transmission electron microscopy to understand the formation of giant atomic clusters in the β-Samson structure. It was confirmed, for instance, that the state of an Al-48 at.% Mg alloy annealed at 573 K for 24 h could be identified as the (α-Mn + β-Samson) coexistence state, and that metastable states were found locally in α-Mn/β-Samson boundary regions. An analysis of electron diffraction patterns obtained from the coexistence state indicated that the orientation relationship of (3¯ 30)_α//(8¯ 80)_β-S and [110]_α//[110]_β-S was established between α-Mn and β-Samson regions, together with a lattice contraction of about 10% along the [001]_α direction and an expansion of about 0.4% along the [1¯ 10]_α direction in the (α-Mn → β-Samson) structural change. High-resolution electron micrographs of the coexistence state also suggested that some atoms at the Mg (I) and Mg (II) sites in the distorted α-Mn structure could be converted to those at the Mg (23) site of the β-Samson structure as the center site of the Samson cluster. Furthermore, both the features of giant clusters in the β-Samson structure and their formation from the α-Mn structure were discussed on the basis of the experimental data obtained in this study.