Mechanisms of near-threshold fatigue crack growth in magnesium AZ31 under different cyclic loading conditions

This study investigates the mechanism of near-threshold crack growth in the highly textured magnesium alloy AZ31. The experimental program consisted of tests at R = 0.1 and at fully reversed R = -1 loading to investigate the influence of the stress ratio. The findings reveal comparable crack growth behavior in all three primary directions of the material, even though there is a clear microstructural anisotropy. This can be explained by the negligible impact of microstructural anisotropy resulting from the small cyclic plastic zone size. Schmid factor calculation and EBSD analysis revealed non-basal slip as the predominant deformation mechanism in all three directions, with basal and pyramidal slip being more prevalent in the S-direction. It was also shown by the fractographic observations that cyclic cleavage fracture plays a decisive role in near-threshold crack growth in magnesium. Twin formation, as part of the plastic deformation process, was observed to be insignificant at R = 0.1, while at R = -1, tensile twins were formed, which had a visible impact on crack growth by reducing the threshold value.