The aim of the present work is to determine the role of intermetallic (IM) phases in the
fatigue crack propagation behavior of hot-forged Al-Zn-Mg-Cu alloys in T73 condition. To
generate differences in the volume fraction and coarseness of various IM particles, the (Fe+Si)
impurity level is varied from 0.23 to 0.37 mass%. The fatigue crack propagation tests are conducted
in air at ambient temperature and a stress ratio R of 0.1. Characterization of the fatigue fracture
surfaces is performed using scanning electron microscopy (SEM) and energy-dispersive X-ray
spectroscopy (EDS). Quantified IM particles data for each alloy are then related to the fatigue
properties and fractographic analysis results. It was found that almost all particles of the Fecontaining
phases (primarily (Cu,Fe,Mn)Al3 and Al7Cu2Fe) are broken and not effective in
hindering fatigue crack propagation. On the other hand, the Mg2Si and soluble phase particles
smaller than those of the Fe-containing phases contribute beneficially to fatigue life. These particles
increase the tortuosity of the crack path and retard the crack growth rate. The crack growth rate
decreases as the volume fraction of coarse Fe-containing particles increases, because more
secondary cracks are produced decreasing the effective stress intensity at the main crack tip.
Effect of Aging Treatment on Pitting Corrosion and Corrosion Fatigue Crack Propagation Behavior of Oil-Grade Alloy 718
