TEMPERATURE-DEPENDENT VOID-SHEET FRACTURE IN AL-CU-MG-AG-ZR

Previous research showed that tensile fracture strain increases as tem perature increases for AA2519 with Mg and Ag additions, because the vo id-sheet coalescence stage of microvoid fracture is retarded. The pres ent work characterizes intravoid-strain localization (ISL) between pri mary voids at large constituents and secondary-void nucleation at smal l dispersoids, two mechanisms that may govern the temperature dependen ce of void sheeting. Most dispersoids nucleate secondary voids in an I SL band at 25 degrees C, promoting further localization, while dispers oid-void nucleation at 150 degrees C is greatly reduced. Increased str ain-rate hardening with increasing temperature does not cause this beh avior. Rather, a stress relaxation model predicts that flow stress and strain hardening decrease with increasing temperature or decreasing s train rate due to a transition from dislocation accumulation to diffus ional relaxation around dispersoids. This transition to softening caus es a sharp increase in the model-predicted applied plastic strain nece ssary for dispersoid/matrix interface decohesion. This reduced seconda ry-void nucleation and reduced ISL at elevated temperature explain ret arded void sheeting and increased fracture strain.