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

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.