ATOM-PROBE STUDIES ON THE EARLY STAGES OF PRECIPITATION IN AL-MG-SI ALLOYS

Pre-precipitation stages of Al-0.70Mg-0.33Si and Al-0.65Mg-0.70Si (at. %) alloys have been investigated by atom probe field ion microscopy ( APFIM) and high resolution transmission electron microscopy (HREM). At om probe results show that clusters of Mg atoms are present in the as- quenched state. After a prolonged aging at room temperature, clusters of Mg, Si and their co-clusters are detected, although no contrasts su ggesting the presence of precipitates are observed in HREM images. In the specimens aged at 175 degrees C for 30 min, small equiaxed Mg-Si p recipitates are observed by TEM. APFIM results show that the ratio of Mg to Si atoms in the precipitates is close to 1, rather than 2, which is expected from the equilibrium concentration of Mg,Si. (C) 1998 Els evier Science S.A. All rights reserved.