Quantifying the kinetics of 8' precipitates in a novel Al-Li-Cu-Mg alloy during two-step aging by small-angle neutron scattering

The kinetics of Li-rich clustering in multicomponent Al-Li-Cu-Mg is not only critical for manufacturing highstrength and modulus products but also important for designing better alloying chemistry. In this study, the ordering of 8' (Al3Li) has been quantified as a function of temperature and time in a multicomponent Al-10.3Li0.6Cu-1.4 Mg (at.%) using Transmission Electron Microscope (TEM) and Small-Angle Neutron Scattering (SANS). By measuring the 8' radius and volume fraction using SANS, it has been found that the precipitation kinetics of 8' phase at 175 degrees C can be well described by Lifshitz-Slyozov-Wagner (LSW) theory due to the size of 8' phase stabilized by the pre-aging at 120 degrees C-4h. The 1.5% pre-strain before solution heat treatment accelerates the 8' precipitation kinetics and increases its volume fraction by providing more decomposition sites for the 8' phase and fast diffusing paths for Li atoms. Interestingly, a dissolution behavior of 8' has been found at the beginning of the second step of aging. This is further confirmed by HRTEM and Monte Carlo simulations, which are associated with the transition from ordering to disordering between the 8' and the surrounding solid solution. Finally, the strengthening effects of 8' are quantified and the strength of aged alloys has been found to largely correlate with the 8' radius while the elastic modulus is more dependent on 8' volume fraction.

Automated summary

The kinetics of Li-rich clustering in multicomponent Al-Li-Cu-Mg has been studied using Transmission Electron Microscope (TEM) and Small-Angle Neutron Scattering (SANS). The precipitation kinetics of 8' phase at 175 degrees C can be well described by Lifshitz-Slyozov-Wagner (LSW) theory and accelerated by pre-strain. Additionally, a dissolution behavior of 8' has been observed during the second step of aging, indicating a transition from ordering to disordering.