Strengthening of magnesium (Mg) is known to occur through dislocation accumulation, grain refinement, deformation twinning, and texture control or dislocation pinning by solute atoms or nano-sized precipitates. These modes generate yield strengths comparable to other engineering alloys such as certain grades of aluminum but below that of high-strength aluminum and titanium alloys and steels. Here, we report a spinodal strengthened ultralightweight Mg alloy with specific yield strengths surpassing almost every other engineering alloy. We provide compelling morphological, chemical, structural, and thermodynamic evidence for the spinodal decomposition and show that the lattice mismatch at the diffuse transition region between the spinodal zones and matrix is the dominating factor for enhancing yield strength in this class of alloy.

Body-centred cubic magnesium-lithium-aluminium-base alloys are the lightest of all the structural alloys, with recently developed alloy compositions showing a unique multi-dimensional property profile. By hitherto unrecognised mechanisms, such alloys also exhibit exceptional immediate strengthening after solution treatment and water quenching, but strength eventually decreases during prolonged low temperature ageing. We show that such phenomena are due to the precipitation of semi-coherent D0 3 -Mg 3 Al nanoparticles during rapid cooling followed by gradual coarsening and subsequent loss of coherency. Physical explanation of these phenomena allowed the creation of an exceptionally low-density alloy that is also structurally stable by controlling the lattice mismatch and volume fraction of the Mg 3 Al nanoparticles. The outcome is one of highest specific-strength engineering alloys ever developed.

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