Effect of grain size on localized deformation near grain boundaries in Al-1 mass%Mg₂Si base alloys

Localized deformation near grain boundaries in Al-1 mass%Mg₂Si-0.4 mass%Si alloys with and without 0.14 mass% Mn were investigated by scanning tunneling microscopy in relation to the grain size. Test pieces with gage length of 18 mm and width of 6 mm were cut from cold-rolled sheets, solutionized at 848 K, water-quenched peak-aged at 423 K and finally deformed in tension at room temperature at a strain rate of 1.86 × 10^-4 s^-1. The average grain size of the Mn-bearing and Mn-free alloys was 60 (small-grained) and 400μm (large-grained) after heat treatment, respectively. Although elongation was increased upto about 8% strain by the manganese addition, the small-grained specimen had an intergranular fracture surface as well as the large-grained one. Fold formation mechanism in the small-grained specimen seemed to be the same as that in the large-grained specimen, and small surface cracks were observed occasionally at the grain boundaries having folds. In the small-grained specimen, step height at some grain boundaries attained a large value of about 1.5 μm. Frequencies of fold formation in the large-grained specimen reached to about 40% of all triple points in the gage area, while it was only about 10% in the small-grained specimen.