Effect of Alloy Purity on Fracture Behavior of Overaged 7000 Alloy Plates

The article deals with the relations between microstructural parameters, microscopic fracture mechanisms, and resultant plane-strain fracture toughness, K-IC, of hot-forged 7000 alloy plates in overaged condition. Three industrially produced alloys with the (Zn + Mg + Cu) content of similar to 11.5 mass% and different contents of (Fe + Si) impurities (from 0.23 to 0.37 mass%) were used. The variation in alloy purity generated differences in the geometric characteristics of coarse intermetallic phase particles, exerting significant effect on fracture behavior of these alloys. To evaluate the fracture mechanism, characterization of the broken K-IC test specimen surfaces was performed using scanning electron microscopy. Three micromechanisms governing fracture process were identified. The extent of primary voiding at coarse intermetallic phase particles increases systematically with the increase of (Fe + Si) level, whereas the extent of intergranular/transgranular fracture decreases. This observation was confirmed by a quantitative description of fractures by means of a profilometric method. It was found that morphological parameters of the fracture profiles clearly differentiate between different fracture modes and offer conclusive proof that coarse voiding at fractured intermetallic phase particles dominates fracture at low alloy purity. A higher fraction of coarse voiding is shown to result in an overall reduction of fracture toughness.