NUCLEATION OF PRECIPITATES IN AGED AL-CU-MG-(AG) ALLOYS WITH HIGH CU MG RATIOS/

Clustering and precipitation reactions during the early stages of agei ng at 180 degrees C in an Al-1.7Cu-0.3Mg (at.%) alloy and equivalent a lloys containing 0.1 and 0.2 at.% Ag have been studied. Transmission e lectron microscopy, electron diffraction and atom probe field ion micr oscopy have been used to clarify the precipitation processes in these alloys following ageing for 15, 30, 120, 720 and 9000 s. Reactions inv olving the formation of Mg and Mg-Cu co-clusters were observed in the ternary Al-Cu-Mg alloy whereas Mg, Ag and Mg-Ag co-clusters were found to co-exist with Cu-clusters in the quaternary Al-Cu-Mg-Ag alloy. Res ults indicate that the Mg-Ag interaction is strong and it is proposed that the role of Ag in quaternary alloys is to effectively trap Mg ato ms, resulting in Mg-Ag co-clusters, which provide favorable sites for nucleation of the Omega phase. Various models for precursor phases hav e also been examined. Apart from the presence of Mg-Ag co-clusters, no evidence of a distinct precursor phase was observed.