Fatigue behaviour of AZ91 magnesium alloy in as-cast and severe plastic deformed conditions

Fatigue strength, crack initiation and microstructure were experimentally investigated in an as-cast AZ91 alloy and in ultrafine-grained (UFG) AZ91 alloy processed by equal channel angular pressing (ECAP). The microstructure after ECAP is bimodal, consisting of fine-grained regions and clusters of larger grains with lower density of intermetallic particles. It has been found that the ECAP substantially increases the tensile strength (factor of two), improves ductility (factor of five) and improves the fatigue strength in low-cycle fatigue region. The improvement of the endurance limit based on 10(7) cycles is weak. The cyclic slip bands, as sites of the fatigue crack initiation on material surface, were investigated. Focussed ion beam technique (FIB) was applied to reveal the surface relief and the microstructure in the vicinity of early fatigue cracks. No grain coarsening was observed in the close vicinity of the initiated cracks. Fatigue cracks in ultrafine-grained structure develop both in the regions of larger grains and also in the fine grained areas. Two types of crack initiation were observed.