The role of silicon in the formation of the (Al5Cu6Mg2) sigma phase in Al-Cu-Mg alloys

The role of silicon in the precipitation of the sigma phase (Al5Cu6Mg2) has been investigated through comparative studies on Al-3.63Cu-1.67Mg (wt%) an d Al-3.63Cu-1.67Mg-0.5Si alloys. Both alloys were extensively examined afte r solution treating at 525 degrees C for 2.5 h followed by ageing at 265 de grees C for times up to 650 h. Limited studies were also undertaken on both alloys after ageing at 200 and 305 degrees C. Precipitation of sigma was o bserved in Al-3.62%Cu-1.66%Mg-0.5%Si for all ageing conditions studied but was absent in Si-free Al-3.62%Cu-1.66%Mg. In addition, S' and theta' phases were observed in both alloys. The volume fraction of sigma phase in the Si containing alloy was substantially reduced by a pre-age stretch followed b y ageing for 24 h at 265 degrees C with S' being the dominant precipitate t ype. The volume fraction of sigma phase in the Si containing alloy was lowe r after ageing 24 h at 200 degrees C than after 24 h at 265 and 305 degrees C. Peak hardness was higher for the Si free alloy on ageing at 200 and 265 degrees C, but the Si free alloy softened more rapidly, reflecting the mor e rapid coarsening kinetics of S' compared with sigma. (C) 2000 Kluwer Acad emic Publishers.