Precipitation behavior in the early stage of aging in an Al-Li-Cu-Mg-Zr-Ag(Weldalite 049) alloy

The precipitation behavior of various phases during the aging process of an Ag-Li-Cu-Mg-Zr-Ag (Weldalite 049) alloy was investigated by high-resolutio n electron microscopy and in situ hot-stage microscopy. Two kinds of domain s with L1(2)-type ordered structures, which are considered to be delta' and beta' phases, are observed with different domain sizes in the alloy quench ed from 530 degrees C. In the early stage of aging at 190 degrees C, the de lta' phase is precipitated as surrounding the beta' phase, and the delta' d omains appear with in-phase and antiphase relationships to the beta' lattic es. In situ observations at 190 degrees C clearly show that the T1 phase pr ecipitates predominantly on dislocations at subgrain boundaries and then is homogeneously formed in the matrix with increasing aging time. The nucleat ion of the S' phase is associated with clustering of Cu and Mg in the matri x, and the S' domains are grown with {210} habit planes.