Interrupted tensile tests were performed in alloy 6061 extrusion after
solutionizing and artificial aging treatments. Samples in the peak-ag
ed, under-aged, over-aged and solutionized conditions were studied. Tw
o phenomena were apparent in the stress-strain curves of interrupted t
ests. They are unload-reload hysteresis loops, and the yield effect on
reloading which is referred to as the unloading yield effect. The wid
th 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 d
islocation pile-up is used to explain the hysteresis loop phenomenon a
nd its relationship to prestrain and aging. The unloading yield effect
also increases as the prestrain is increased. (C) 1997 Acta Metallurg
ica Inc.