Molecular Dynamics Study on the Inverse Hall‐Petch Relationship in Nano‐Polycrystalline Al‐Zn‐Mg Alloys

ABSTRACT Nanocrystalline Al‐Zn‐Mg alloys are pivotal for next‐generation aerospace and automotive applications that require superior strength‐to‐weight ratios. However, optimizing their mechanical properties at the atomic scale remains a challenge because classical strengthening theories break down. This study employs molecular dynamics simulations to elucidate the anomalous mechanical response and deformation mechanisms of polycrystalline Al‐Zn‐Mg alloys under tensile loading. We identify a critical grain size threshold of 16.3 nm, below which the material exhibits an inverse Hall‐Petch relationship. Unlike coarse‐grained counterparts, the mechanical behavior in this regime is governed by a dislocation annihilation‐dominated softening mechanism and grain boundary instability, rather than traditional pile‐up hardening. Furthermore, our results demonstrate that while thermal processing temperatures critically affect grain boundary stability, high strain rate loading can induce the formation of Lomer‐Cottrell locks, thereby reactivating work hardening capacity even at the nanoscale. These findings not only delineate the theoretical limits of grain‐refinement strengthening in Al‐Zn‐Mg systems but also provide essential guidelines for optimizing thermomechanical processing and designing alloys for high‐strain‐rate forming applications.