Al–1.0 mass%Mg2Si–0.34 mass%Si alloy specimens which had been aged at 423 K for various times were tensile tested at room temperature. Changes in local deformation near grain boundaries with aging time were investigated by scanning tunneling microscopy. When a little loss in elongation occurred at early stage of aging, the folds generated at the grain triple junction. But they were very ambiguous and did not develop with increasing amount of deformation. The fold formations were difficult to be explained by the maximum resolved shear stress on a grain boundary plane (Fmax). If the elongation appreciably decreased and intergranular fracture began to occur with the progress of aging, the folds developed and became more clear. The fold formations at this stage could be explained by the Fmax in many cases. Frequencies of fold formations increased with the aging time over a range of under-aged to peak-aged condition, and this correlated approximately to the decrease in elongation. Even if the elongation considerably recovered in the specimen which had been aged at 473 K up to over-aged condition, the frequency of fold formation was similar to that of the peak-aged condition. In this case, however, two types of folds which had developed largely or had developed scarcely with the progress of deformation were observed and the frequency of the former ones decreased to about a half of the all folds.
