Secondary Phases In An Al-Zn-Mg-Cu Alloy:Atomic-scale Aberration-corrected Transmisson Electron Micoscopy Investigation

The present study,using advanced Cs-corrected high-angle annular dark field-scanning transmission electron microscopy（HAADF-STEM）,reports on a comprehensive investigation into the structures of secondary phases in an Al-Zn-Mg-Cu alloy,including precipitates and dispersoids.In the study of precipitates,the current work reports different nucleation mechanisms that occur at 150℃and 120℃.It is found that the main nucleation mechanism of ageing at 150℃is based on Mg atoms enriching as spherical clusters combining with the enrichment of Zn on{111}_All planes,while the main nucleation mechanism of ageing at 120℃is based on a structure of 7-layers of{111}_All planes.The whole precipitation process of ageing at 150℃is revealed including the structures of GP zones,η’precipitates andηprecipitates.In the nucleation stage,Zn atoms enrich on the{111}_Al-planes abutting spherical Mg clusters that are approximately3-6 nm in diameter.In the subsequent growth,the as-nucleated structures extend and grow with a constant width.The precipitation sequence is identified as:SSSS→GP zones→η’_P→η’→η,among which the formation ofη’_P indicates that the precipitates start to become semi-coherent with the Al matrix.The Al₄Mg₂Zn₃ is proved to be one of the structures ofη’phase and it will subsequently transform into a structure featured by several rhombohedral and orthorhombic subunits.Theηphase exist in various types based on the rhombohedral repetitive unit.In the study of the dispersoids,the current work identifies dispersoids in the alloy,which is proved to be Al₁₈Cr₂Mg₃.The particles attached to Al₁₈Cr₂Mg₃ is proved to be MgCu_xZn_2-x.Al₁₈Cr₂Mg₃ particle often contains twin defects while MgCu_xZn_2-x particle may contain stacking fault.It is found that the MgCu_xZn_2-x particles form on Al₁₈Cr₂Mg₃ particles by heterogeneous nucleation and the habit plane is{111}_Al18Cr2Mg3.The solute atoms would diffuse towards Al₁₈Cr₂Mg₃ particles during ageing,which results in the significant decrease in precipitate number density and hence decrease the mechanical properties.