When Mechanics Rewrites Chemistry: Divergent Pathways of Hardening and Softening in High Strength Al-Alloys.

High-strength 7xxx Al alloys are the backbone of lightweight structural engineering, yet their broader application is hindered by limited ductility. This deficiency marks a breakdown of traditional strengthening paradigms where microstructure features designed for peak strength trigger localized softening, creating a fundamental mechanism trade-off. Here, we reveal that plastic flow activates two divergent mechano-chemical pathways during deformation, namely, deformation-driven hardening and shear-induced softening that govern strain localization and ductility. In the solution-treated state, strain-induced nanoscale solute clustering promotes homogeneous dislocation storage and sustained work hardening. Conversely, in the peak-aged state, localized shear drives precipitate dissolution, creating precipitate-free channels. These soft channels not only amplify strain localization but also act as preferential sites for the ingress of environmental species, which further weakens the lattice and accelerates catastrophic failure. Leveraging these findings, we show that mechanical response can be engineered by balancing homogeneous plastic flow against shear-driven local chemical phase dissolution and the associated soft channels. This study provides critical mechanistic insights into the dynamic mechanochemical response of high-strength Al alloys, offering physical guidelines for future microstructural tailoring to suppress severe deformation localization.