Effects of Cr Addition on the Microstructure and Mechanical Properties of an Al-Si-Cu-Mg Alloy.

The effects of chromium (Cr) addition ranging 0.1-0.3 wt.% on the microstructure and mechanical properties of Al-7Si-4Cu-0.25Mg (wt.%) alloy have been investigated. The cast Cr-free alloy consisted of α-Al, eutectic Si, Q-AlMgCuSi and θ-AlCu phases. Doping of Cr resulted in the appearance of a polyhedron-shaped α-AlCrSi phase with a cubic structure. The AlCrSi particles were found to embed with AlCu blocks and bring about size reduction for the AlCu blocks. The area fraction of AlCrSi monotonously increased with Cr content. After T6 treatment, the AlCu blocks almost fully dissolved and transformed to θ'-AlCu precipitates in the Cr-containing alloys. TEM observation revealed relatively large-sized θ' precipitates attached to AlCrSi dispersoids. The Cr-containing alloys showed impressive mechanical properties, with the peak strength up to 452 MPa at room temperature. The ductility exhibited an increasing trend with Cr content, but the strength dropped dramatically when the Cr content reached 0.3 wt.%. It is suggested that the strength contribution from the AlCrSi phase is limited, especially at an elevated temperature.