Effects of Mg addition on the kinetics of low-temperature precipitation in Al–Li–Cu–Ag–Zr alloys

The thermal stability at around 343 K is one important problem of Al–Li alloys related to the microstructural changes during the low temperature exposure. The present work aims to investigate the effects of Mg addition on the age-hardening behavior and precipitation kinetics of an Al–Li–Cu–Mg–Ag–Zr alloy in the low temperature range from 278 to 373 K. The microstructures observed with transmission electron microscopy (TEM) and the hardness changes indicate that a small amount of Mg markedly accelerates the formation of GP(1) zones, not the δ′ (Al3Li) phase, resulting in an enhanced age- hardening. Quantitative analysis of the precipitation kinetics determined by the electrical resistivity changes also elucidates the more detailed effects of Mg addition on the characteristic precipitation phenomena in the Al–Li–Cu alloys, i.e. a decreased activation energy for the GP(1) zone nucleation and a suppressed growth of both GP(1) zones and the δ′ phase (and/or its precursory structures). The complicated effects of Mg addition are well explained in terms of both the enhanced nucleation rate of GP(1) zones with the aid of Mg/Cu/Vacancy complexes and the pronounced decrease in free-vacancies available for Cu and Li diffusion due to the preferential vacancy trapping by Mg atoms.