Anisotropic degradation behavior of moduli of extruded pure magnesium during low cyclic-tension fatigue

Abstract

The mechanical properties of extruded pure magnesium during low cyclic-tension fatigue at room temperature were investigated in the extrusion and transverse loading directions (LDs) using ultrasonic reflection methods with longitudinal and shear waves (SWs), to clarify if there was an anisotropic degradation behavior between the extruded directions (EDs), which had not yet revealed. Regardless of the LD, the acoustic velocities and calculated Young’s and shear moduli decreased significantly with an increasing number of cycles because of the growth of voids at the grain and twin boundaries. An anisotropic behavior was revealed: when the deflection surface of the SWs was aligned in the LD, the amount of decrease in the moduli was greater than when the alignment was in the transverse direction (TD). Additionally, when the stress amplitude was adjusted to provide the same number of cycles to failure, the decrease in the moduli was somewhat greater when the LD was parallel to the extruded direction than to the TD. Longitudinal and SW propagation characteristics, and investigations of grain and twin boundaries after fatigue using electron backscatter diffraction (EBSD) based on field-emission scanning electron microscopy (FE-SEM), revealed that the most of the void-gap width was less than several nanometers (almost closed), which corresponded to the longitudinal wave amplitude. Other damaged-phase data were obtained using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and Vickers-hardness test; the anisotropic degradation behaviors were attributed to the void morphology and the slight difference in orientation.