Influence of grain size and precipitates on the fatigue lives and deformation mechanisms in the VHCF-regime

Abstract

It is meanwhile established that the “classical” fatigue limit in the Wöhler-SN-plot at approximately 107 cycles is not generally valid for the design of components subjected to loading in the VHCF-regime. Fatigue failure of metallic materials might occur even at stress amplitudes below the “classical” fatigue limit in the VHCF regime. However, the principal understanding of the relevant damage mechanisms and of the influence of the microstructure on the fatigue lives in this regime is rather poor. It is also established to distinguish between single-phase and inclusion free (type I) materials and multiphase (type II) materials when discussing the VHCF-fatigue behaviour. For type II materials the fatigue cracks in the VHCF regime mainly originate at nonmetallic inclusions located in the interior, the so-called fish-eye fracture, often reported for high strength steels and other alloys. However, crack initiation in the interior without forming typical fish eye type fracture and without any clear influence of inclusions were also reported for aluminium alloys. The reasons for the formation of these so-called “featureless crack initiation sites” are unclear. On the other hand, in single phase type I materials crack initiation in the VHCF-regime start from the surface of the specimen, triggered by repeated, not fully reversible plastic deformation in the grains adjacent to the surface. Due to the not fully reversible and localized plastic deformation surface roughening takes place and micro-persistent slip bands (PSB) form. In order to identify relevant deformation- and damage mechanisms commercial pure (CP) aluminium AA1050 and the aluminium alloy AA6082 were investigated 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. It is shown, that the grain size, ranging from the micrometer to the nanometer range significantly affects the VHCF-behaviour. For CP Al with a grain size of ca. 55 μm no late failure was obtained, whereas for the same material with an ultrafine grain size of ca. 600 nm a stepwise SN-curve was found. In the case of the precipitation hardened material AA6082 it is found that the local mechanical properties of the matrix significantly governs the crack initiation site and hence the VHCF-behaviour.