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

Abstract

The kinetics of Li-rich clustering in multicomponent Al–Li–Cu–Mg is not only critical for manufacturing high-strength and modulus products but also important for designing better alloying chemistry. In this study, the ordering of δʹ (Al3Li) has been quantified as a function of temperature and time in a multicomponent Al-10.3Li-0.6Cu-1.4 Mg (at.%) using Transmission Electron Microscope (TEM) and Small-Angle Neutron Scattering (SANS). By measuring the δʹ radius and volume fraction using SANS, it has been found that the precipitation kinetics of δʹ phase at 175 °C can be well described by Lifshitz-Slyozov-Wagner (LSW) theory due to the size of δʹ phase stabilized by the pre-aging at 120°C-4h. The 1.5% pre-strain before solution heat treatment accelerates the δʹ precipitation kinetics and increases its volume fraction by providing more decomposition sites for the δʹ phase and fast diffusing paths for Li atoms. Interestingly, a dissolution behavior of δʹ 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 δʹ and the surrounding solid solution. Finally, the strengthening effects of δʹ are quantified and the strength of aged alloys has been found to largely correlate with the δʹ radius while the elastic modulus is more dependent on δʹ volume fraction.