Structural and compositional study of precipitates in under-aged Cu-added Al-Mg-Si alloy

Area of Science:

• Materials Science
• Metallurgy
• Nanotechnology

Background:

• Al-Mg-Si alloys are crucial for various industrial applications.
• Understanding precipitate formation is key to optimizing alloy properties.
• The role of copper (Cu) in these alloys requires detailed atomic-level investigation.

Purpose of the Study:

• To characterize the atomic structure of precipitates in under-aged Cu-added Al-Mg-Si alloys.
• To elucidate the influence of Cu on precipitate formation and morphology.
• To correlate precipitate characteristics with observed changes in alloy hardness.

Main Methods:

• Atomically-resolved annular dark-field scanning transmission electron microscopy (ADF-STEM).
• Energy-dispersive X-ray spectroscopy (EDS) for elemental analysis.
• Microstructural analysis of precipitates in Cu-free and Cu-added alloys.

Main Results:

• Two precipitate types were identified: ordered β" (Mg5-xAl2+xSi4, x≈1) with Cu substituting Al, and disordered structures.
• Disordered precipitates comprised β" sub-unit cells, Cu-containing clusters, and Q' sub-unit cells.
• Morphologies of Cu-free and Cu-added precipitates were similar in density and size, but Cu addition introduced new Cu-related phases.

Conclusions:

• Ordered β" precipitates incorporate solute Cu atoms.
• Disordered precipitates are clusters of Q' phase, including Cu-containing structures.
• The increased hardness in Cu-added alloys is attributed to the precipitation of Cu-related phases like Cu sub-unit clusters and Q' sub-unit cells.