AbstractAn Al–Mg–Si alloy 6063 was heat treated in the underaged, peak aged, and overaged conditions. The microstructures of the alloy in the different aging conditions were examined using transmission electron microscopy. Tensile and fatigue tests were carried out and the resulting fractures were studied using scanning electron microscopy. It has been found that the alloy shows different tensile and fatigue properties and different modes of fracture under different aging conditions. The results have been explained in terms of slip distribution and grain boundary strength.MST/1188