Achieving highest Young's modulus in Al-Li by tracing the size and bonding evolution of Li-rich precipitates

For decades, it has been well accepted that every 1 wt.% Li addition to Al will reduce Al alloy's density by 3% and increase its Young's modulus by 6%. However, the fundamental mechanism of modulus improvements stays controversial though all studies agreed that the contribution of such a substantial boosting comes from Li-rich clusters either in solid solution or precipitations. In this study, we experimentally produce nano-sized Li-rich clusters by non-equilibrium solidification using centrifugal casting and trace their evolutions as a function of subsequent heat treatments. High-resolution transmission electron microscopy (HRTEM) reveals a further decrease in the lattice constants of Li-rich regions from the as-cast (0.406 nm), solid solution (0.405 nm) to the aged state (0.401 nm), while Young's modulus of the Al-Li alloy reaches 89.16 GPa. Small-angle neutron scattering (SANS) experiments and first-principle calculations based on density functional theory have shown both the bond strength around precipitates and the size of those Li-rich region dominate Young's modulus. At the beginning, it is volumetric compression due to Li addition that increases modulus, tightening the Al-Al potential curves. In the end, it is the Al-Al and Al-Li valence bonds in Al₃Li at large size and high-volume fraction which increase its second derivative of internal energy and thus Young's modulus.