Temperature-dependent void-sheet fracture in Al-Cu-Mg-Ag-Zr

Previous research showed that tensile fracture strain increases as temperature increases for AA2519 with Mg and Ag additions, because the void-sheet coalescence stage of microvoid fracture is retarded. The present work characterizes intravoid-strain localization (ISL) between primary voids at large constituents and secondary-void nucleation at small dispersoids, two mechanisms that may govern the temperature dependence of void sheeting. Most dispersoids nucleate secondary voids in an ISL band at 25 degrees C, promoting further localization, while dispersoid-void nucleation at 150 degrees C is greatly reduced. Increased strain-rate hardening with increasing temperature does not cause this behavior. Rather, a stress relaxation model predicts that flow stress and strain hardening decrease with increasing temperature or decreasing strain rate due to a transition from dislocation accumulation to diffusional relaxation around dispersoids. This transition to softening causes a sharp increase in the model-predicted applied plastic strain necessary for dispersoid/matrix interface decohesion. This reduced secondary-void nucleation and reduced ISL at elevated temperature explain retarded void sheeting and increased fracture strain.