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The properties of an age-hardening aluminum alloy can be affected greatly by the heterogeneous precipitation.The microstructures of the heterogeneous precipitation in an Al-Cu-Mg alloy after artificial ageing were investigated.Two kinds of S phase formed along dislocation were observed by HRTEM and the precipitation sequence,variant selection and the strengthening effect of the S precipitates were discussed.The three-dimentional morphology of the S phase formed along dislocation lines,on grain boundary were characterized by using the technique of STEM-HAADF 3D tomography.Quantitative parameters describing the shape and the distribution of the grain boundary precipitates (GBP) were calculated based on the 3D tomography analysis.