Correlation of the aging characteristics and deformation behavior of A357 alloy

The mechanical properties, microstructures, and deformation behavior for a cast Al-7Si-Mg casting alloy A357 under various aging conditions have been investigated in the present paper. It is shown that the combination of stre ngth properties and elongation varies with aging parameters. Aging at 165 d egreesC gives the optimum balance of strength and ductility. Transmission e lectron microscopy (TEM) observation shows that, in the underaged condition s in which Guinier-Preston (GP) zones are formed, the tensile deformation i nvolves dislocations, which cut through the GP zones. Planar slip bands can be seen in the deformed substructures, and the slip bands coarsen with inc reasing aging temperature or time. In the overaged conditions in which P ph ases precipitate, the dislocations bypass precipitates and distribute unifo rmly, except some dislocation tangles around coarser precipitates. When the interaction mode between dislocations and particles changes from shearing to bypassing, the deformation homogeneity greatly increases for the casting s with fine p precipitates, which may explain the significant secondary elo ngation behavior. Based on these studies, a step overaging treatment has be en used to improve the overall tensile properties.