Revealing the role of heterogeneous microstructure on fatigue crack propagation behaviors in T74 Al-Zn-Mg-Cu alloys

Abstract

Heterogeneous grain structures have been demonstrated to effectively balance the strength-ductility trade-off in Al-Zn-Mg-Cu alloys, but studies on their fatigue behaviors remain insufficiently explored. Herein, we examine the impact of bimodal grain structures on the grain boundary precipitate characteristics and the final fatigue crack propagation (FCP) behaviors of T74-tempered Al-Zn-Mg-Cu alloys. Three types of bimodal grain structures are constructed, comprising recrystallized coarse grains (CGs) embedded within non-recrystallized fine grains (FGs) in homogeneous (∼90% CGs), banded (∼50% CGs), and dispersed (∼20% CGs) configurations. Following aging, the recrystallized CGs with high-angle grain boundaries are predominantly associated with coarse grain boundary precipitates (GBPs) and wide precipitation-free zones (PFZs), while the non-recrystallized FGs exhibit finer GBPs and narrower PFZs. Multi-scale analysis of crack propagation behavior reveals that the FG regions have a limited ability to accumulate dislocations, while the CGs provide minimal resistance to crack propagation due to the presence of soft PFZs. Consequently, alloys with dispersed and homogeneous CG distributions demonstrate high fatigue crack growth rates. Interestingly, the interaction between the crack tip and the banded CGs awakens the stress dissipation in CGs, which effectively suppresses the driving force of crack initiation and propagation via significant crack blunting and deflection. Therefore, the alloy with ∼50% banded CGs distributed in FGs exhibits superior resistance to crack propagation compared with the other configurations. These findings highlight that optimizing heterostructure parameters is a promising strategy for enhancing fatigue crack resistance in age-hardened aluminum alloys.