The effect of precipitate-free zone on mechanical properties in Al-Zn-Mg-Cu aluminum alloy: Strain-induced back stress strengthening

The presence of precipitate-free zone (PFZ) near grain boundaries in precipitation-strengthened Al-Zn-Mg-Cu alloys has complex impact on mechanical properties, and the mechanism remains to be elucidated in detail. In this work, two Al-Zn-Mg-Cu alloy sheets with different features of PFZ were obtained by quenching in different media after solution heat treatment, and their deformation behavior was investigated by tensile at room temperature with different strain rates. It was found that variations in the distribution of dislocations in grain boundary regions can occur at room temperature due to the change of strain rates. These variations decrease the strength and elongation in samples with narrow PFZ (about 15 nm) at a low strain rate, but increase the strength and elongation in the samples with wide PFZ (about 112 nm). When the samples with wide PFZ are deformed at a low strain rate, dislocations emitted by grain boundaries pile up at the interface between PFZ and grain interior and lead to high back stress, resulting in higher strength and ductility. Therefore, it is possible to achieve strength-ductility synergy in Al-Zn-Mg-Cu alloys by manipulating the features and deformation behavior of PFZ near grain boundaries.