Abstract
The effects of composition and processing on microstructure and fracture behavior of Al-Zn-Mg-Cu (7075 type) alloys were investigated. The microstructural variations involved changes in second phase particle size and distribution and in grain structure. Second phase particles in three size ranges were investigated: (1) coarse insoluble particles involving Fe at various levels, (2) finer intermediate particles involving Cr or Zr varied in size by choice of homogenizing practice, and (3) fine aging precipitates varied in size by choice of aging practice. The use of high temperature, long-time homogenizing practices to produce coarse intermediate particles also resulted in recrystallization and grain growth, particularly in the Zr-bearing Al-Zn-Mg-Cu alloys. Grain size and shape were found to strongly affect toughness; plane stress fracture toughness [of 12-in. (0.30 m) wide center notched panels] at a given yield strength level differed by a factor of three between a fully recrystallized, coarse equiaxed grain structure to an unrecrystallized lamellar structure. Smaller effects of second phase particles distribution were found in the Al-Zn-Mg-Cu alloys. The trends observed in a fatigue crack growth rate study of the same materials will be discussed.
Published Version
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