Ultrastrong and ductile Al–Mg alloy matrix composites via composition-modulated precipitation induced by intragranular ceramic nanoparticles

Nano-precipitation is critical in achieving high yield strength and strain hardening capacity in aluminum alloys and their composites, while Al–Mg alloys and their composites are generally believed not to be strengthened significantly by precipitations. This study reveals that a coherent composition-modulated precipitate χ deviated from the conventional precipitation sequence forms near intragranular ceramic nanoparticles (ICNPs) in Al–Mg alloy matrix composites, providing precipitation strengthening of ∼120 MPa and activating early plastic relaxation around ICNPs. Thus, an Al–5Mg alloy (wt.%) reinforced with 1.5 wt.% carbon nanotubes containing χ exhibits ultrahigh tensile yield strength of 653.2 MPa with uniform elongation of 8.9 %. χ is composed of alternating domains with different content of Mg and derives from disordering decomposition of metastable partially ordered δ'', as revealed by the first-principles calculations. This study subverts the understanding on the weak precipitation strengthening in Al–Mg alloys and their composites, and enlightens exploiting superior strength and ductility via ICNPs induced exotic precipitation strengthening.