Changes in localized deformation behavior near grain boundaries with aging temperature in an Al-1 mass%Mg₂Si alloy with excess Si

Specimens of an Al-1.0 mass%Mg₂Si-0.34 mass%Si alloy which had been aged at temperatures ranging between 423 and 573 K up to peak-aged condition were tensile-tested at a room temperature. Localized deformation behavior near grain boundaries on the specimen surface was observed with a scanning tunneling microscope. With increasing aging temperature from 423 to 573 K, elongation increased from 0.4% to about 10% and the fracture mode changed from intergranular to transgranular fracture. For aging temperatures of 473 K and below, fold formations was explicable in terms of the magnitude and the direction of the maximum resolved shear stress (F_max) on a grain boundary plane. In contrast, in the specimen aged at 573 K where elongation appreciably increased and transgranular fracture occurred, the displacement of scratch line and the step at the grain boundary were not always generated at all the grain boundaries that had a large magnitude and suitable direction of F_max. In this specimen, two types of folds were observed: one was developed with increasing amount of deformation and the other was not developed. The increased elongation of this specimen was attributed to relatively low frequency of markedly developed folds.