| At present, it is a hot topic of research on magnesium alloys due to the possible weight saving. However, limited fatigue durability restricts the range of their applications. Shot peening is a powerful method of enhancing the fatigue performance of structural metallic materials. The present study was aimed firstly at investigating the high cycle fatigue properties and fatigue fracture behavior of the ZK60 and GW103K magnesium alloys in different conditions. The key equipments such as shot peening and rotation bending fatigue machine for magnesium alloys were machined. Further study was put on the high cycle fatigue properties and fatigue fracture behavior of peened ZK60 and GW103K alloys. Make a final attempt to explain the shot peening strengthening mechanism of magnesium alloy. ZK60 and GW103K alloys in different conditions as well as different types of amples possessed different high cycle fatigue properties. The fatigue strengths at 107 cycles of smooth and notched specimens (Kt=2.3 and 2.7) of the as-extruded (or extruded-aged) ZK60 alloy were 140, 150 and 160 MPa (or 150, 155 and 165 MPa), respectively. The fatigue strengths of smooth specimens for the as-cast, cast-T6, extruded-F, extruded-UA, extruded-PA, extruded-OA, extruded-T4 and extruded-T6 GW103K alloys were 85, 105, 150, 160, 165, 160, 110 and 110MPa, respectively.The study on crack initiation, propagation and fracture behaviors by using the replica method and SEM show that the fatigue cracks of smooth specimens for the as-extruded and extruded-aged ZK60 alloys initiated at sample surface with only one initiation site. Its fatigue crack nucleation life was great longer than the crack propagation life. Comparatively, the fatigue cracks of the notched specimens for the as-extruded and extruded-aged ZK60 alloys initiated at the notch root surface with multiple initiation sites, and its fatigue crack nucleation life was great shorter than the crack propagation life.The fatigue cracks of the as-cast, cast-T6, extruded-T4 and extruded-T6 GW103K alloys initiated at porosities or inclusions that lie subsurface, and their fatigue strengths were determined by non-propagation cracks. Comparatively, the fatigue cracks of the as-extruded and extruded-aged alloys initiated at the regions with coarse grains that lie surface, and their fatigue strengths were determined by crack initiation.The research on the surface characteristics of the peened ZK60 and GW103K alloys show that the deformation depth, grains refinement degree, microhardness, compressive residual stress and surface roughness increased with the increasing of Almen intensity. Comparatively, at the same Almen intensity, the microhardness improvement and maximum compressive residual stress in the GW103K alloy were higher than those in the ZK60 alloy with similar condition. Furthermore, the relaxation degree of compressive residual stress of the GW103K alloy during fatigue cycling is significantly less than that of the ZK60 alloy.A pronounced overpeening effect was observed in the ZK60 and GW103K alloys. The fatigue life depends on the Almen intensity, with the fatigue life first dramatically increased with the Almen intensity, and then similarly dramatically decreased. The optimum shot peening process window of the as-extruded and extruded-aged ZK60 alloys is 0.02~0.15mmN for smooth specimens and 0.20~0.50mmN for notched specimens. For the as-extruded and extruded-aged GW103K alloys, the optimum shot peening process window is 0.05~0.30mmN.The optimum Almen intensities of the as-extruded and extruded-aged ZK60 alloys are 0.05 mmN for the smooth specimens, and 0.30 and 0.40mmN for the notched specimens, respectively. Compared to the unpeened smooth specimens, the increase of fatigue strengths for 107 cycles was 40 MPa for the as-extruded ZK60 and 45 MPa for the extruded-aged ZK60; while compared to the unpeened notched specimens, the increase of fatigue strength ( ? a ?Kt, Kt=2.3) was 70 MPa for the as-extruded ZK60 and 85 MPa for the extruded-aged ZK60. The optimum Almen intensities of the as-cast, cast-T6, extruded-F, extruded-UA, extruded-PA, extruded-OA, extruded-T4 and extruded-T6 alloys were 0.40, 0.50, 0.10, 0.10, 0.10, 0.10, 0.30 and 0.40mmN, respectively. The fatigue strengths of the peened GW103K alloys in eight conditions were 30, 20, 65, 70, 75, 40, 30 and 30MPa higher than those of the unpeened specimens, respectively.The fractography observation by SEM shows that the fatigue crack nucleation site of smooth specimen of ZK60 alloy shifted from surface to the subsurface, and then shifted to the surface again, with the increase in Almen intensity. Meanwhile, a significantly higher number of fatigue crack initiation sites resulted from overpeening can be seen. Compared to smooth specimen, shot peening exhibits no significant effect on the fatigue crack initiation of notched specimen. The fatigue cracks of the peened GW103K alloys in different conditions initiated in subsurface at all Almen intensities, and they still have one crack initiation site with the increase in Almen intensity. The small fatigue crack in the peened as-extruded GW103K alloy grew alont the cleavage planes. In contrast, the small crack in the extruded-aged GW103K alloy propagated by the coalescence of the sheared dimples. The measurement results on the surface and subsurface characteristics in the peened magnesium alloys show that the improvements of grain refinement, compressive residual stress and microhardness were efficient to improve high cycle fatigue properties, while surface roughening accelerates the nucleation and early propagation of cracks.For the as-extruded and extruded-aged ZK60 alloys, during the compression and tension fatigue cycling, mechanical twinning dominates deformation in compression, while dislocation slip dominates deformation in tension, i.e., the as-extruded and extruded-aged ZK60 alloys exhibit twinning and detwinning phenomenon. Comparatively, dislocation slip dominates deformation in both compression and tension in the as-extruded and extruded-aged GW103K alloys. Similarly, the twinning deformation for the as-extruded and extruded-aged ZK60 alloys and dislocation slip for the as-extruded and extruded-aged GW103K alloys are the grains refinement mechanism by shot peening. The twinning induced by shot peening and detwinning during fatigue cycling leads to the residual stress relaxation. Meanwhile, the possibility of these twins acting as crack initiation sites decreases the positive effect of shot peening on fatigue resistance.
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