The fatigue crack growth in magnesium single crystals and polycrystals has been investigated in laboratory air at room temperature. Four types of CT specimens A, B, D and F with four different notch orientations were prepared. The notch plane and direction in the A-specimen were (1120) and [1100], those in the B-specimen were (1100) and [1120], those in the D-specimen were (1120) and [0001], and those in the F-specimen were (0001) and [1120], respectively. Dependences of crystallographic orienting on ΔK_(th) and da/dN are obtained. The ΔK_(th) value is higher for the B-specimen and the value is lower for the F-specimen. The cracks in both the A- and B-specimens propagate parallel to (1120)[1100] and parallel markings along to (0001) are observed on fatigue surfaces, (c+a) dislocations are formed near crack surfaces. Therefore, the crack is deduced to extend by alternating shear on two intersecting second order pyramidal slip systems ((1122)[1123] and (1122)[1123]) at the crack tip. First order pyramidal slip is activated at higher ΔK range. The behavior of crack propagation in the D-specimen is sensitive to ΔK. No fatigue crack grows at ΔK < 0.7 MPam~(1/2) and a crack parallel to [0001] propagates at ΔK ～ 0.8 MPam~(1/2). At ΔK> 0.8 MPam~(1/2), a fatigue crack propagates along (0001) against the initial notch plane. The fatigue crack propagates parallel to (0001) in the F-specimen and {1012} twin traces are found near the crack. Large twin bands are observed on the fatigue surfaces and striations are found in the twin. These indicates that the fatigue crack grows across the twin bands which have been formed at the crack tip in the F-specimen. Two types of polycrystal specimens Polyl and Poly2 were prepared. The notch directions in the polyl and in the Poly2 were parallel and perpendicular to grain growth direction of each specimen, respectively. A crack propagation behavior in the Poly 1 is similar to that in the F-specimen, and the crack propagates parallel to notch plane . A crack in the Poly2 propagates through curved route similar to that in the D-specimen. The ΔK_(th) value is higher for the Poly2 than that for the polyl, however, da/dN is low for the Polyl than that for the Poly2 at same ΔK range.
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