Relationships between Some Microstructural Features and the Fracture Toughness of an Al–Zn–Mg–Cu–Mn Forging Alloy

Abstract

The fracture toughness of three Al–Zn–Mg–Cu forgings of nominally identieal composition, except for their Fe contents of < 0.05, 0.30, and 0.74%, was found to increase with reduction in iron content. It was observed that the overload fracture in the alloy containing < 0.05% iron was intergranular, while in the other two alloys the fracture followed stringers of intermetallic particles. The scanning electron microscope was used to examine the fracture surfaces of the toughness testpieces and where the fatigue fracture changed to overload fracture there was evidence of both a stretched zone and of vertical cliffs. The variations in height of these cliffs were measured by stereo-techniques and the mean values were found to be a quarter of the mean grain-boundary spacing in the alloy containing <0.05% Fe and a quarter of the mean spacing of the intermetallic stringers in the other two alloys. The higher fracture toughness of the alloy containing < 0.05% Fe (20.9 MN/m^3/2) compared with the other two alloys (15.4 and 14.2 MN/m^3/2) was associated with a greater cliff height. It was concluded that when either grain boundaries or stringers of intermetallic particles constituted preferred paths for the overload crack an increase in spacing of such features, as measured in the short-transverse direction, resulted in an increase in the short-transverse fracture toughness.