Dislocation mechanisms and mechanical behavior of highly concentrated Al-Mg (6.6 and 9.3 at.%) binary alloys

Al-Mg alloys are promising structural materials due to their strong solid solution strengthening, high strain-hardening capacity, and good formability. To explore the strengthening potential of higher Mg concentrations, this study investigates a naturally aged Al-9.3 at.% Mg alloy exhibiting spinodal decomposition and compares it to a solid-solution Al-6.6 at.% Mg reference alloy. Atom probe tomography (APT) and high-resolution scanning transmission electron microscopy (STEM) reveal spinodal modulations in Al-9.3 Mg, with a dominant wavelength of 11.7 nm and a Mg fluctuation amplitude of ∼3.6 at.%. These modulations align along the <100> direction and generate periodic lattice distortions at the coherent, compositionally diffuse interfaces of Mg-rich and Mg-lean regions, arising from the significant atomic size mismatch (21 %) between Al and Mg atoms. Compared to AlMg6.6, the AlMg9.3 alloy exhibits significantly higher yield and ultimate tensile strengths, while maintaining similar ductility (∼32 %). Al-6.6 Mg exhibits frequent cross-slip on {111} planes, whereas Al-9.3 Mg displays wavy slip lines, and dislocation structures indicative of jogs, loops, and dipoles. The enhanced work hardening behavior observed in Al-9.3 Mg is attributed to the interactions between dislocations and spinodal structures, which hinder dislocation motion and promote dislocation accumulation. This study reveals the underlying dislocation mechanisms in spinodal modulated structures and the corresponding strengthening effects, shedding light on the development of lightweight and high-strength Al-Mg alloys with elevated Mg contents.