The effect of heating temperature and holding time on the phase transformation from (Fe, Mn)Al 6 to α-Al 12 (Fe, Mn) 3 Si in AA3104 aluminum alloy during homogenization was studied by BSE and EDS in SEM. The 3D morphology of the eutectic phases heated at 600 ª for different holding times was reconstructed by a novel serial sectioning method. Evolution of the microstructure could be revealed in 3D for the first time. Al spots were observed within the α-Al 12 (Fe, Mn) 3 Si phase and counted during different transformation stages. Large number of Al spots disappeared by diffusing into the neighboring matrix after extended heating times, while the volume of the α phase was also decreased at the same time. The microstructure of the α phase after transformation was analyzed, showing ¢duplex interfaces¤created by partly transformed particles. The dense holes within α phase particles are believed to promote particle break up during hot rolling of the homogenized material.