Precipitation behavior and mechanical property anisotropy of Al–Zn–Mg–Cu alloy forgings

The grain structure, precipitation behavior, and tensile property anisotropy of Al–6.28Zn–2.11Mg–2.28Cu (wt. %) alloy, fabricated by hot forging, solution-treating, and two-step aging processes, were investigated. The solution-treated alloy had partially recrystallized grains with sizes of 145 × 83 × 68 μm (longitudinal (L) × long transverse (LT) × short transverse (ST) directions). The L direction represents the <111> texture, whereas the LT and ST directions represent the <101> texture. The ST direction had a higher intensity than the LT direction in the <001> and <113> textures. D0₂₃-structured Al₃Zr precipitates were present in the solution-treated alloy and acted as preferential nucleation sites for η precipitates during aging. First aging at 121 °C resulted in the formation of GP zones and η′ precipitates in the Al matrix, and the following secondary aging at 177 °C induced the growth of GP zones and η′ to η transformation. Yield strength modeling indicated that the change in η-type precipitates owing to aging played a crucial role in the yield strength of Al–Zn–Mg–Cu alloy forgings. The ST direction exhibited a slightly higher yield and ultimate tensile stress, and significantly lower fracture elongation than the LT direction. The degradation of ductility in the ST direction was owing to the presence of the <001> and <113> textures, which caused a drastic decrease in the strain-hardening rate and subsequent necking and failure without post-uniform elongation.