High cycle fatigue behaviour of magnesium alloys

The influence of crystallographic texture on fatigue failure mechanisms was studied by comparing the fatigue behaviour of two extruded magnesium alloys, AZ31 and ZN11. The microstructures and fracture surfaces of specimens were examined using scanning electron microscopy (SEM) to reveal the micromechanisms of fatigue crack initiation. The AZ31 alloy has an inhomogeneous grain structure and strong fibre texture, which cause strong asymmetry in the tensile and compressive yield strengths. This yield stress asymmetry is related to the high twinning activity under compressive loading. The metallographic investigation reveals that the cracks are mainly initiated at twin boundaries. On the other hand, a weak texture and fully recrystallised, homogeneous grain structure are found in the experimental alloy ZN11 after extrusion. As a consequence, twinning is suppressed and no yield stress asymmetry is observed. The fatigue failure of ZN11 is initiated by cyclic slip deformation.