Phenomena in interrupted tensile tests of heat treated aluminium alloy 6061

Interrupted tensile tests were performed in alloy 6061 extrusion after solutionizing and artificial aging treatments. Samples in the peak-aged, under-aged, over-aged and solutionized conditions were studied. Two phenomena were apparent in the stress-strain curves of interrupted tests. They are unload-reload hysteresis loops, and the yield effect on reloading which is referred to as the unloading yield effect. The width of a hysteresis loop increased with prestrain and aging time until the peak-aged strength was attained. Over-aging caused the loop width to decrease. A mechanism based on relaxation of the back stress in a dislocation pile-up is used to explain the hysteresis loop phenomenon and its relationship to prestrain and aging. The unloading yield effect also increases as the prestrain is increased. (C) 1997 Acta Metallurgica Inc.