Effect of compositional variations on fracture toughness in precipitation hardened Al-alloys

To provide an understanding of how compositional variations affect the microstructural parameters associated with the coarse intermetallic (IM) particles and in turn the fracture toughness in AA 7000 aluminum forgings, the microstructural and fractographic analysis together with mechanical tests were carried out using three industrially produced Al-Zn-Mg-Cu alloys with different contents of impurities (Fe+Si). Light optical microscopy and image analysis were used to quantify the volume fraction, size and distribution of all soluble and unsoluble coarse (> 0,1μm) IM particles identified in the corresponding R-C and L-R planes for T73-type heat treatments by selective etching and energy dispersive X-ray spectroscopy analysis. These quantitative data were correlated to plain strain fracture toughness, K1c, with the results being used to produce useful information on alloy design and thermomechanical processing via microstructural control. The simultaneous scanning electron microscope observation of fracture surface features and estimation of area fractions of different fracture modes in the plastic zone segments of test specimen showed that multiple failure mechanisms occur with the coarse voiding at intermetallics becoming more important as the fraction of coarse IM particles increases.Quantitative assessment of the relevant microstructural and fractographic parameters will be utilized for developing and verifying a multiple micro mechanisms-based model for fracture toughness. (Original)