Origin of the anisotropic fracture toughness in Al-Zn-Mg-Cu thick plate with isotropic yield strength

Achieving isotropic damage tolerance in thick plates of high-strength aluminum alloys remains a critical challenge. This study investigates the strength and toughness of a rolled thick plate of an Al-Zn-Mg-Cu alloy. The results show that while yield strength (∼ 600 MPa) is isotropic due to homogeneous nanoscale η^′ precipitates, fracture toughness varies significantly across six orientations from 30.4 to 42.5 MPa∙m^1/2. Multiscale characterization reveals that fracture behavior is governed by directional mesoscale weak interfaces, including pancake grain boundaries decorated with precipitate-free zones and rolling-direction particle stringers. A geometric accessibility index is introduced to quantify the relationship between crack orientation and the likelihood of accessing low-resistance pathways. These findings demonstrate that mitigating toughness anisotropy requires disrupting mesoscale connectivity, a strategy distinct from the nanoscale precipitation engineering typically employed to enhance strength.