Fold formation near grain boundaries in an age-hardened aluminum alloy deformed at room temperature

Localized deformation near grain boundaries in an Al-0.996 mass%Mg₂Si-0.395 mass%Si alloy was investigated by interference microscopy and scanning tunneling microscopy. Both the displacement of scratch lines and the steps at the grain boundaries were observed. The number of the grain boundaries with displacement of scratch lines had a maximum at the grain boundaries making angles of about 45 degrees to the tensile axis. While a large numbers of the steps were observed at the grain boundaries making angles of near 90 degrees to the tensile axis. In both types of grain boundaries, the direction of the maximum resolved shear stress (corresponding to F_max) on a grain boundary plane is close to the moving direction of the grain boundary. When the step was formed simultaneously at two neighboring grain boundaries, fold was not observed. In such a case, the direction of F_max on the grain boundary plane was slightly deviated from the moving direction. The values of F_obs (corresponding to the resolved shear stress on the grain boundary plane in the moving direction) tended to increase with the amounts of steps and displacements of scratch lines at the grain boundaries.