Anomalous mechanical hysteresis loops in aluminium alloys

Abstract

The initial stages of high strain fatigue behaviour of two commercial aluminium alloys (Al–Cu–Mg and Al–Zn–Mg) have been studied metallographically and by monitoring their mechanical hysteresis loops. An unusual convex curvature in the latter during the first few cycles is shown to be associated with the presence of shearable precipitates, the intensification of deformation into narrow high dislocation density bands, and stability in strength. When the anomaly, or kink, has been cycled out it can be made to reappear by low temperature annealing. Specimens given heat treatments producing fine Guinier–Preston zones or non-shearable precipitates do not exhibit the anomaly, and the fatigue damage, which is homogeneous, produces significant hardening. In all cases the grain boundary precipitate free zone appears to play no part in the fatigue process at these high strain levels.