Papers by Author: M. Vratnica

Abstract: 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.

Abstract: The crack growth resistance of the Al-Zn-Mg-Cu alloy forgings in overaged condition was investigated with three industrially produced alloys, which showed differences in the microstructures governed by compositional variations. Fatigue-crack propagation experiments were conducted at ambient temperature and variations in crack growth rates (da/dN) as a function of applied stress intensity range (ΔK) were related to the characteristics of microstructures, including coarse intermetallic (IM) particles and precipitates. It appears that the crack growth rate increases systematically with an increase of the impurity level, which in turn increases the amount and size of large Fe- and Si-containing IM particles while decreases their spacing. That degradation in resistance to crack growth was attributed to the acceleration of the crack initiation and propagation by coarse IM particles were confirmed by in-situ SEM observation of the fracture process. The observed anisotropy in fatigue behavior was caused by the anisotropy in coarse IM particle orientation.