Unveiling the fine microstructure of nanoscale composite particles embedded in brittle Si phase in an Al-Si-Cu-Mg alloy

•The particles within Si phases in an Al-Si-Cu-Mg alloy are composed of Al phase and AlCu phase.•The AlCu phase possesses a lattice constant of a = 0.30 nm and the space group of Pm3̅m.•The AlCu phase prefers to grow at the interface between Al and Si phases. Three types of orientation relationship are found. [Display omitted] Embedded nanoscale Al particles are promising for improving the toughness of large eutectic Si-phase particles in Al-Si-based alloys. However, the composition and fine structure of such Al particles in Al-Si alloys with Cu addition remain poorly known. Here, through advanced aberration-corrected transmission electron microscopy and electron tomography, we reveal that the void-free particles are composed of a pure Al phase (face-centered cubic structure) and an AlCu phase with three types of crystallographic orientation relationships. The AlCu phase, which possesses a lattice constant of a = 0.300 nm and the space group of Pm3̅m, grows at the interface between Al and Si phases. Our study suggests that these Al(Cu) composite particles are directly solidified from the melt during solidification, rather than indirectly precipitated from a Si-Al-Cu solid solution. Heterogeneous nucleation of the AlCu phase on Si surface planes plays a critical role in forming these Al(Cu) particles embedded inside the large brittle Si particles.