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In order to make clear the grain-size and precipitation dependence on the fatigue
properties of wrought Mg-Al-Zn alloys in terms of dislocation theories, pure
magnesium, AZ31 and AZ61 alloys with the average grain sizes ranging from 18
to 101μm were produced by the strain-annealing method. In AZ61 alloys, two
kinds of specimens were prepared by solution treating at 683K and followed by
T6 aging at 423K. Fatigue tests at room temperature were performed under the
constant stress amplitude over a life range of 104 to 107 cycles. The fatigue crack
propagation and fracture were also examined in each specimen. The grain-size
dependence on the fatigue strength for pure magnesium and AZ31 alloys is
uniformly confirmed at the whole life, whereas that of as-quenched AZ61 alloys
decreases with an increase of the number of cycles and disappears beyond about
105 cycles.