Tailoring Young's modulus by controlling Al bond length and strength using positive Li and negative Mg

For decades, it has been widely accepted that adding Mg to Al alloys decays Young's modulus of Al alloys. However, the fundamental mechanism for Young's modulus reduction is still pending. In this study, we experimentally produce the Al–Li alloys and Al–Li–Mg alloys using centrifugal casting and trace Young's modulus as a function of the evolution of their precipitation structures. Atomic-resolution high-angle-annular-dark-field (HAADF) imaging in scanning transmission electron microscopy (STEM) indicates that the precipitate sizes of Al–Li and Al–Li–Mg alloys are essentially the same in the early and late stages of aging. While atomic resolution HAADF suggests that although the lattice constants of δ′-Al₃Li in Al–Li and Al–Li–Mg alloys (0.404 nm) are consistent in the early aging stage, it changes to 0.422 nm in the Al–Li–Mg alloy at the end of peak aging, showing a significant expansion to the same level as α-Al (0.421 nm). The nanoindentation results confirm that Young's modulus decreases strongly from 89.04 to 76.39 GPa for Al–Li–Mg alloys due to the lattice relaxation. Combining atomic-resolution HAADF and first-principles calculations, it has been found that Mg atoms diffuse into Al₃(Li, Mg) crystals by replacing Li in the L1₂ δ′-Al₃Li and thus doping the crystal. Further, it has been found that Mg atoms expand the spacing between Al–Al and Al–Li pairs thereby lowering the energy between Al–Al and Al–Li bonds, which in turn stretches their potential energy curves reducing Young's modulus.