Probing the micro-mechanism of precipitate-strengthened alloys with precipitate free zone: An experimental and theoretical study

Precipitate free zone (PFZ) consistently forms near the grain boundaries (GBs) in precipitate-strengthened alloys, significantly weakening the materials because of their intrinsic softness compared to the bulk. However, the influence of PFZ near GBs on deformation mechanism remains largely unrevealed. Here, we systematically investigate the effects of PFZ on the macroscopic mechanical behavior and the microstructure deformation mechanism of the modelled precipitate-strengthened Al-Zn-Mg-Cu alloy, using a combination approach of experiments, molecular dynamics (MD) simulations, and theoretical modeling. Four Al-Zn-Mg-Cu alloys with highly different PFZ widths are prepared by tailoring the quenching media and applying the new deformation heat- treatment process proposed by us. Experimental characterizations demonstrate that severe dislocation accumulation occurs at the interface between PFZ and bulk. Meanwhile, MD simulations further reveal that PFZ is prone to plastic deformation during tensile process, contributing to the softening of materials. The PFZ exhibits significant strain concentration, leading to the preferential formation of dislocations within PFZ rather than at GBs. It is found that the level of strain concentration and the degree of dislocation accumulation are not sensitive to the PFZ width. Based on these mechanisms, a PFZ-dependent strength model is developed to quantitatively evaluate the influence of PFZ on tensile strength by considering dynamic strengthening of PFZ. It is predicted that an increase in PFZ width greatly reduces the tensile strength, with a 21 % reduction observed when PFZ width reaches 268 nm, emphasizing the important impact of PFZ width on materials strength.