Precipitation of aluminum in the silicon phase contained in w319 and 356 aluminum alloys

Solidification of Al-Si alloys typically yields regions of a eutectic mixture of aluminum and silicon phases. In 319-type and 356 aluminum alloys, we have determined that a small percentage (10 to 25 pct) of the silicon phase contains small (10 to 15 nm) equiaxed precipitates of aluminum as identified by electron diffraction with an orientation relation to the silicon matrix: $$[001]_{Si} \parallel [111]_{Al} ;[111]_{Si} \parallel [001]_{Al} ;and(110)_{Si} \parallel (1\bar 10)_{Al} $$ . These precipitates are typically distributed in bands in the silicon and their morphology is insensitive to solidification time and subsequent heat treatments. Energy-dispersive X-ray (EDX) analysis showed that, depending on the alloy, these precipitates contained aluminum, copper, and traces of oxygen.