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This paper presents the fatigue crack propagation (FCP) characteristics and the fatigue behaviour of smooth specimens in an extruded AZ61 magnesium alloy. FCP tests and axial fatigue tests have been performed at the stress ratio of 0.05 and -1 respectively, in laboratory air at ambient temperature. Fatigue properties were evaluated and compared with those of a rolled AZ31 alloy in a previous report, and fracture mechanisms were discussed on the basis of crack initiation, small crack growth and fracture surface analysis. The relationship between FCP rate and stress intensity factor for large cracks consisted of two parts with different slopes, which became much more remarkable when FCP rates were characterized in terms of the effective stress intensity factor. This was attributed to a fracture mechanism transition because the fracture surface appearance was clearly different above and below the stress intensity factor where the slopes changed. The fatigue strength of the AZ61 alloy was higher than that of the AZ31 alloy and the fatigue strengths at 107 cycles were 60MPa and 50MPa respectively. However, the fatigue strengths characterized in terms of fatigue ratio were similar with a considerably low fatigue ratio of 0.22. Regardless of stress level, fatigue cracks initiated at the specimen surface in intergranular or transgranular mode due to cyclic slip deformation at a very early stage of fatigue cycling. The growth behaviour of small cracks was consistent with the FCP characteristic after allowing for crack closure for large cracks, but the operative micromechanisms were different between small and large cracks. Except for subsurface fracture occurred in the AZ31 alloy, the fatigue properties described above were basically the same as observed in the AZ31 alloy.

Fatigue Behaviour and Fracture Mechanism of an Extruded AZ61 Magnesium Alloy