Fatigue of as-extruded AZ61A magnesium alloy

Samples prepared from as-extruded AZ61A bars (18 mm in diameter) were used in a rotating bending test. The relation between stress amplitude and cycles to failure has been constructed, as well as the cycles to failure at two specific stress amplitudes. The probability of failure at these two specific stress levels was also analyzed. This study finally provided the predicted fatigue strength at 10⁷ cycles with different probabilities (10 to 90%). A fractography study indicated that fatigue cracking initiated from subsurface or surface inclusions. These inclusions near the surface served as stress raisers and induced clusters of slip bands during the rotating bending test. After initiation, the cracks grew under the dominant shear stress and resulted in a cleavage fracture over a large area. Microscopic cracks occurred, resulting from the induced deformation twins that developed from the blunting process. Consequently, the propagation of cracks followed the existence of microscopic cracks and resulted in a transgranular fracture.