Fatigue characteristics of an extruded AZ61 magnesium alloy.

The objective of this research is to obtain basic fatigue characteristics of extruded AZ61 magnesium alloy in ambient air (20~50°C and 55~80%RH) . Fatigue tests were performed under tension-compression loading with stress ratio R = −1, frequencies of 1 and 10 Hz. At 55%RH, S–N curve clearly showed the fatigue limit, which was 145–150 MPa for both 20°C and 50°C. At a higher humidity of 80%RH, fatigue fracture occurred along the extrapolated S–N line even in the lower stress regime than the fatigue limit, which is known as corrosion fatigue. Time-dependent fatigue behavior was observed in the stress region lower than the fatigue limit, while cyclic-dependent fatigue behavior was observed in the stress region higher than the fatigue limit. In the corrosion fatigue regime, corrosion pits were observed on the specimen surfaces and also at the fatigue initiation point on the fracture surfaces. The corrosion pits were initiated at very early stage of fatigue and grew to make a fatigue crack started at about half of the fatigue life. Slip deformation on the specimen surface as well as humidity will be the essential factor to form corrosion pits.