Influence of Iron and Silicon on Toughness and Fracture Characteristics of Al–Mg–Mn and Al–Zn–Mg Alloys

Abstract

Experiments were performed on Al–Mg–Mn and Al–Zn–Mg alloy plates to investigate the influences of iron and silicon on notch toughness and tear resistance and their fracture characteristics. Notch toughness was evaluated from the ratios of notch-tensile strength to tensile strength (notch-tensile ratio) and notch-tensile strength to yield strength (notch-yield ratio) and the Charpy impact value, and the tear resistance was determined from the crack initiation and propagation energies, particularly by the latter, in a modified Navy tear test.The notch toughness and tear resistance of fully annealed Al–Mg–Mn and naturally and artificially aged Al–Zn–Mg alloy plates were markedly reduced with increasing amounts of iron and silicon. Iron, silicon and iron + silicon, when these contents were equal, had similar influence on the notch toughness and the tear resistance. In a strict sense, however, the reduction of toughness due to silicon was most remarkable.As the amounts of iron and silicon were increased, the dimple pattern associated with ductile fracture gave way to a quasi-cleavage pattern associated with a relatively brittle fracture, and an intergranular rupture along the fibrous grain structure was observed. Fine lamellar cracks occurred near second phases containing iron and silicon. From these observations, it seems very likely that the crack initiation and propagation in the Al–Mg–Mn and Al–Zn–Mg alloys are closely related to the distribution of iron and silicon. However, it is necessary to consider some supplementary mechanisms, if iron and silicon are contained separately.