Influence of ageing on the low cycle fatigue behaviour of an Al–Mg–Si alloy

Abstract

The low cycle fatigue (LCF) performance of AA6063 Al–Mg–Si alloy at under-aged (UA), peak-aged (PA) and over-aged (OA) conditions has been examined to understand the micromechanism of fatigue and the associated dynamic structural changes in this alloy. The LCF behaviour of the differently aged AA6063 alloys has been studied at strain amplitudes ranging between 0.2 and 1.0% under strain control mode. The UA state exhibits pronounced cyclic hardening unlike the PA and the OA states at strain amplitudes greater than 0.4%. The PA and the OA states show hardening only for a few cycles followed by prolonged softening. Characterisations of the micro- and the sub-structural alterations due to LCF establish that the phenomenon of dynamic precipitation results in cyclic hardening the UA alloy. The softening of PA alloy occurs due to shearing of precipitates and that in the OA alloy takes place owing to reversibility of slip by the formation and annihilation of the Orowan loops around the β (Mg₂Si) precipitates. Analyses of the hysteresis loops reveal Masing, nearly-Masing and non-Masing behaviour in the UA, OA and PA states, respectively. Analyses of the asymmetry factor of the hysteresis loops assist to infer that the Masing behaviour in the UA alloy is due to dislocation–dislocation interactions, whereas the nearly-Masing behaviour in the OA alloy and the non-Masing behaviour in the PA alloy are the consequence of varying degrees of dislocation–precipitate interactions associated with inhomogeneous deformation.