Abstract
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-Al(5)Mg(8)Cu(2)Si(6) and θ-Al(2)Cu phases. Doping of Cr resulted in the appearance of a polyhedron-shaped α-Al(13)Cr(4)Si(4) phase with a cubic structure. The Al(13)Cr(4)Si(4) particles were found to embed with Al(2)Cu blocks and bring about size reduction for the Al(2)Cu blocks. The area fraction of Al(13)Cr(4)Si(4) monotonously increased with Cr content. After T6 treatment, the Al(2)Cu blocks almost fully dissolved and transformed to θ'-Al(2)Cu precipitates in the Cr-containing alloys. TEM observation revealed relatively large-sized θ' precipitates attached to Al(13)Cr(4)Si(4) 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 Al(13)Cr(4)Si(4) phase is limited, especially at an elevated temperature.