Effect of copper addition on localized deformation near grain boundaries in an Al-1.0 mass%Mg₂Si alloy

Age-hardened Al-1.0 mass%Mg₂Si alloy (base alloy) and Al-1.0 mass%Mg₂Si-0.46 mass%Cu alloy (0.5%Cu alloy) specimens were tensile-tested at room temperature. Localized deformation near grain boundaries in these alloy specimens were investigated by scanning tunneling microscopy (STM). The 0.5%Cu alloy specimen had intergranular fracture surface as the base alloy, although uniform elongation of the alloy was increased up to about 4% strain by copper addition. The surface topography of the 'fold' in both the base and 0.5%Cu alloys were consistent with those of the Al-1.0 mass%Mg₂Si alloy with 0.36 mass% excess silicon. Direction of the grain boundary motion in the base alloy was explained by the magnitude and direction of maximum resolved shear stress on the grain boundary plane (corresponding to F_max), but in the 0.5%Cu alloy, it was not. Frequency of fold formation in the base alloy reached to about 40% of all triple points in the gauge area, while it was about 30% in the 0.5%Cu alloy. For about 80% of the steps at grain boundaries, the height (at ε as an image of 4%) was less than 200 nm in the 0.5%Cu alloy.