Characterization of Mg-rich clusters in the C36 phase of the Mg–5Al–3Ca–0.7Sr–0.2Mn alloy

Highlights: • Disk-shaped Mg-rich clusters were observed within the C36 eutectic phase. • The clusters were identified as the Mg-rich C36-(Mg,Al)₂(Mg,Ca) compound. • The diameter of the clusters varies from a few nanometers to 150 nm. • The clusters embedded in the C36 phase exhibit translational moiré fringes. • The Mg atoms can substitute both Al and Ca atoms in the C36 phase. - Abstract: In this paper, characterization of clusters embedded in the C36 eutectic phase using TEM, STEM and XRD methods is presented. Disk-shaped Mg-rich clusters with sizes less than 150 nm were observed in the sand-cast Mg–5Al–3Ca–0.7Sr–0.2Mn alloy. The composition of the C36 eutectic phase was 24.1Mg–47.2Al–27.8Ca–0.9Sr and that of the Mg-rich clusters was 53.1Mg–33Al–13.1Ca–0.8Sr in at.%. The lattice parameters of the C36 phase were determined to be a₀ = 5.827 Ǻ and c₀ = 18.834 Ǻ, whereas for the clusters they were determined to be a₀ = 5.586 Ǻ and c₀ = 18.834 Ǻ. The chemical compositions of the eutectic phase and clusters, and the decrease of the lattice parameters with increasing magnesium content in the C36 phases suggest that Mg atoms occupy both Al and Ca sites in the C36 phase