The effect of compositional variations on the fracture toughness of 7000 al-alloys

Abstract

To provide an understanding of how compositional variations affect the microstructural parameters associated with coarse intermetallic (IM) particles and the fracture toughness in AA 7000 aluminum forgings, a microstructural and fractographic analysis as well as mechanical tests were carried out on three industrially produced Al-Zn-Mg-Cu alloys with different contents of impurities (Fe+Si). Light optical microscopy and image analysis were used to assess the volume fraction, size and distribution of all the soluble and insoluble coarse ( gt 0.1 mu m) 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. These quantitative data were correlated with the plain-strain fracture toughness, K-IC, with the results being, used to produce useful information on alloy design and thermomechanical processing via microstructural control. The scanning electron microscope observation of fracture surface features and an estimation of the area fractions of different fracture modes in the plastic zone segments of a test specimen showed that multiple failure mechanisms occurred with coarse voiding at the intermetallics becoming more important as the fraction of coarse IM particles increases. A quantitative assessment of the relevant microstructural and fractographic parameters will be utilized for developing and verifying a multiple micromechanisms-based model for fracture toughness.