Complex precipitation sequences of Al-Cu-Li-(Mg) alloys characterized in relation to thermal ageing processes

Abstract

The Al-Cu-Li-(Mg) alloy is a high-performance lightweight material strengthened by complex coexisting precipitates that form in the alloy upon thermal ageing. Using high-resolution (scanning) transmission electron microscopy in association with first-principles energy calculations, we systematically studied the complex coexisting precipitates in the alloys and correlated their precipitation sequences with thermal ageing processes applied. The principal results are the following: (1) eight types of precipitates can be observed in the alloy; (2) of these precipitates, the T1-phase is most stable. The S-phase precipitates with segregated Li atoms at their interfacial edges are unexpectedly more stable than the σ-phase; (3) the T1-phase has a characteristic precursor that plays the key role in its nucleation and growth.