Influence of grain size and precipitation state on the fatigue lives and deformation mechanisms of CP aluminium and AA6082 in the VHCF-regime

Investigations of the fatigue behaviour of commercial pure (CP) aluminium and the aluminium alloy AA6082 have been carried out in the very high cycle fatigue (VHCF) regime up to 1 × 1010 cycles. The evolution of surface roughening was investigated with light, scanning electron and atomic force microscopy, from which the relevant deformation and damage mechanisms were derived. Commercially pure aluminium with an average grain size of 350 nm obtained by equal channel angular pressing (ECAP) revealed improved fatigue properties in comparison to conventional grained (CG) aluminium. Furthermore, the evolution of surface roughening differs strongly between the ultrafine-grained (UFG) and conventional grained (CG) for CP Al. In the case of the AA6082 alloys different precipitation states were introduced by appropriate heat treatments. It is shown that the heat treatment significantly influences the crack initiation sites in the VHCF-regime. The results for both materials investigated are discussed in the view of relevant microstructural details governing the VHCF-behaviour.