A microstructural study on the ductile fracture behavior of annealed AA7075 aluminum

Abstract In high-strength aluminum alloys such as AA7075, impurities introduced during casting, particularly second-phase particles, have traditionally been implicated in accelerating ductile fracture due to their association with pronounced plastic shear band formation during deformation. In contrast, alloys such as Al–Mg, Al–Zn–Mg, and Al–Zn–Mg–Cu exhibit minimal particle decohesion or cracking, which has little influence on their fracture behavior; this reduced effect is linked to matrix softening induced by magnesium solutes through dynamic strain aging (DSA). Ductile fracture commonly initiates within shear bands during the later stages of plastic deformation, especially once necking occurs due to shear localization often promoted by these particles. Void growth is minimal prior to necking, but once coalescence begins, the voids expand rapidly and reach exponential growth up to final fracture. Recent in situ scanning electron microscopy (SEM) studies on AA7075-O alloys have observed some instances of particle cracking and occasional decohesion; however, these events are not considered the primary drivers of failure. Instead, the experimental observations provide strong evidence that shear failure, specifically intervoid shearing, is the dominant ductile fracture mode under tensile loading. To quantify this mechanism and separate it from secondary effects, the detailed finite element (FE) modeling using representative volume elements (RVEs) was conducted. Our RVE simulations demonstrate that intervoid shearing governs fracture behavior in tension, even when particle cracking or decohesion events are present, confirming that these effects are secondary. Furthermore, RVE simulations under varied strain paths reveal the influence of stress triaxiality: The Rice and Tracy parameter increases from uniaxial tension to plane strain and equibiaxial loading, which corresponds to an increase in fracture strain, highlighting the interplay between loading conditions and ductile failure mechanisms. This combined experimental and modeling approach clarifies the relative importance of microstructural mechanisms in governing ductile fracture in high-strength aluminum alloys and emphasizes the critical role of shear localization and intervoid shearing across different stress states.