Stir cast and extruded Al-7Si-0.3Mg alloy containing iron and beryllium

Stirring an Al-7Si-0.3Mg casting alloy during solidification inhibits the creation of the dendritic structure such that the primary alpha-Al forms as spheroids and other compact irregular shapes owing to thixotropic behaviour. The extruded stir cast alloy exhibits improved mechanical properties, particularly high elongation (15%) and impact energy (14.5 J) but with slightly lower ultimate tensile strength (270 MPa). Ir is suggested that the superior ductility and impact properties of the extruded stir cast alloy compared with the standard product is a result of the presence of alternately arranged bands of alpha-Al and eutectic silicon, such that cracks nucleated preferentially on silicon particles are arrested as they approach the adjacent alpha-Al. The presence of iron in the alloy results in the formation of coarse beta-FeSiAl5 phase associated with poor mechanical properties. However, trace additions of beryllium lead to formation of a new (Be-Fe)-BeSiFe2Al8 phase which mitigates solve of the deleterious effects of the iron. (C) 1997 The Institute of Materials.