Fatigue behaviour and fractography of extruded AZ80 magnesium alloys in very high cycle regime

In order to investigate the fatigue properties of extruded magnesium alloy in very high-cycle regime, rotary bending fatigue test was performed in ambient atmosphere at room temperature using the hourglass shaped specimens of AZ80 alloys extruded (F-specimen) and treated by an artificial aging after extrusion (T5-specimen). From the experimental results, both specimens show a clear step-wise S-N curve on which two knees exists. Specific stress amplitude formed the knee corresponded to the 0.2% offset proof stress of 160MPa in compression. From the detailed observation of fracture surface, small facet-like smooth area appeared at crack initiation site on the specimen failed at low-stress amplitude level and in a high-cycle regime. On the other hand, lamella structure appeared at crack origin in case of the test at high-stress amplitude level. It was concluded that the step-wise S-N curve was induced by the crack initiation mechanism changing from the twin deformation in high-stress amplitude level to slip deformation in low-stress amplitudes, because of a mechanical anisotropy caused by the extrusion and the particular deformation characteristics of the hexagonal crystal structure in a magnesium alloy. (C) 2010 Published by Elsevier Ltd.