The specimens of polycrystalline pure aluminum were irradiated with high-current pulsed electron beam （HCPEB）. The microstructure of vacancy defect clusters has been investigated in detail by using transmission electron microscopy （TEM）. The results reveal that large numbers of vacancy cells including dislocation loop and even stacking fault tetrahedra （SFT） can be formed in the specimens of polycrystalline pure aluminum irradiated with HCPEB. For the specimen irradiated with one pulse, vacancy dislocation loops were formed in the vacancy cells. SFTs became the dominating structures after five pulses. For the specimen irradiated with ten pulses, dislocation loops were frequently present and SFTs were only formed in some local zones of vacancy cells. In the vicinity of SFT formation, dislocation-free or very low dislocation densities were observed. It is suggested that high stress and strain rate induced by rapid heating and cooling due to HCPEB irradiation could cause the shifting of whole atomic planes synchronously. This is the more probable mechanism of the formation of SFTs.
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