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 AI-L i alloys related to the microstructural changes during the low tempera ture exposure. The present work aims to investigate the effects of Mg addition on the age-hardening behavior and precipitation kinetics of a n 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 mar kedly accelerates the formation of GP(I) zones, not the delta' (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 t he characteristic precipitation phenomena in the Al-Li-Cu alloys, i.e. a decreased activation energy for the GP(I) zone nucleation and a sup pressed growth of both GP(I) zones and the delta' phase (and/or its pr ecursory structures). The complicated effects of Mg addition are well explained in terms of both the enhanced nucleation rate of GP(I) zones with the aid of Mg/Cu/Vacancy complexes and the pronounced decrease i n free-vacancies available for Cu and Li diffusion due to the preferen tial vacancy trapping by Mg atoms. (C) 1998 Elsevier Science S.A.