Investigation of microstructures and mechanical properties of sitrcast and extruded al-7si-0.3mg cast alloy containing iron and beryllium

摘要

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 due to thixotropic behavior. The extruded stircast alloy exhibits improved mechanical properties, particularly high elongation (15 percent) and impact energy (14.5 J) but with slightly lower ultimate tensile strength (270 MPa). Fatigue properties of stircast and extruded alloy (containing 0.44 percent iron) are superior to sand cast, modified low (0.29 percent) iron alloys. The superior ductility, impact and fatigue properties of the extruded stircast alloy as compared to the conventionally cast alloy are attributable to the presence of alternately arranged bands of #alpha#-Al and eutectic silicon, because of which the cracks nucleated preferentially on silicon particles and arrested as they approach the adjacent #alpha#-Al. The presence of iron, the most commonly found impurity in the alloy, results in the formation of coarse #beta#-FeSiAl_5 phase associated with poor mechanical properties. However, trace additions fo beryllium lead to formation of a new (Be-Fe)-BeSiFe_2Al_8 phase which mitigates some of the deleterious effects of the iron.