STM observations near grain boundaries in deformed Al-Zn-Mg alloy

Age-hardened Al-5.4 mass%Zn-1.8 mass%Mg alloy specimens were deformed to 0.3% strain at room temperature. The accurate surface topography in the vicinity of the grain boundaries including folds which was generated in the grain interior from the triple point was observed by scanning tunneling microscopy. At a triple point, a fold was generated along the extension of the grain boundary having a larger step as well as a higher maximum resolved shear stress on the boundary plane than the other two. At another triple point, when a fold was generated ahead of a grain boundary by a shear displacement parallel to the specimen surface, one of the adjacent grain boundaries had a step. In this case, the surface topography had a sunk shape between the fold and the step. When two steps were formed simultaneously at two neighboring grain boundaries, fold was not generated. Among the grain boundaries surrounding a single grain, displacement was observed only in part. When a large step occurred at one of these boundaries, it caused a deformation at the grain interior up to a distance about 200 μm.