Strong and stable ultrafine-grained Al-Mg alloys via polymer-derived in situ nanoparticles

Ultrafine-grained Al-Mg alloys provide immense promise as lightweight metallic materials for load-bearing applications, contributing significantly to energy efficiency and CO2 emission reduction. However, for prevalent bulk ultrafine-grained Al-Mg alloys fabricated by a combined mechanical alloying and thermomechanical consolidation, the presence of remaining polymers, which are used as a process control agent during milling, jeopardizes the mechanical performance of the consolidated materials and thus significantly limit their broad applications. An ideal solution to overcome this critical issue is to turn these polymers into useful nanoparticles, enabling simultaneous improvements in strength and grain size stability. In this study, we present an approach to currently achieve strength enhancement and grain size control by incorporating polymer-derived in situ nanoparticles. Through a combined process involving polymer-assisted mechanical alloying, extrusion at the eutectic temperature, and annealing-driven solid-state phase transformation, we achieved a strong and stable ultrafine-grained Al-Mg alloy. After post-heat treatment at 440 °C (i.e. 0.76 Tm; Tm is the absolute melting temperature of pure Al) for 24 h, the compressive yield strength exhibited minimal reduction from ∼1.3 GPa for the as-extruded alloy to ∼1 GPa for the as-annealed alloy. Similarly, the average grain size only experienced a slight increase, from 106 to 145 nm after post-annealing. These findings offer practical insights into the design of mechanically robust and thermodynamically stable ultrafine-grained metallic materials via a new strategy of polymer-derived in situ nanoparticles.