EFFECTS OF COLD WORK ON PRECIPITATION IN AL-CU-MG-(AG) AND AL-CU-LI-(MG-AG) ALLOYS

A study has been made of the effects of cold work prior to aging on precipitation hardening in selected Al-Cu-Mg-(Ag) and Al-Cu-Li-(Mg-Ag) alloys. General aging characteristics have been determined by differential scanning calorimetry, and response to hardening has been correlated with microstructure using transmission electron microscopy (TEM), selected area electron diffraction (SAED), and quantitative stereology. Particular attention has been given to the phases Omega and T-1 that form on the {111}(alpha) planes, although information on the precipitates theta', S' (or S), and delta' is also reported. Although Omega and T-1 have similar morphologies and habit planes, their response to cold work prior to aging is different. Deformation promotes T-1 formation at the expense of the delta' phase in Al-Cu-Li alloys and at the expense of delta', theta', and S' in Al-Cu-Li-Mg-Ag alloys. On the other hand, in Al-Cu-Mg-Ag alloys, deformation assists precipitation of theta' at the expense of Omega phase, and some decrease is recorded in the hardening response. Prior cold work is also found to reduce the response during natural aging in most alloys. These results are discussed in terms of the role of particular alloying additions.