Low-Cycle Fatigue Behavior Of Extruded AZ And AM Series Magnesium Alloys

As the light metallic materials with high specific strength and high specific rigidity, magnesium alloys have been widely used in automobile, aeronautical, computer and communication fields. Fatigue is a main failure form of various structural components during operation. For the magnesium alloy components, the same case is also true. Therefore, the investigation concerning fatigue behavior of magnesium alloys is of both academic and practical significance. In this investigation, the strain-controlled fatigue deformation and fracture behaviors of extruded AZ and AM series magnesium alloys with different treatment states have been studied in order to provide a reliable theoretical foundation for both fatigue resistant design and reasonable usage of these magnesium alloys.The results of low-cyclic fatigue tests reveal that the extruded AZ31 magnesium alloys with different treatment states at various strain amplitudes exhibit the cyclic strain hardening. The as-extruded and aged AZ61 magnesium alloys exhibit the stable cyclic stress response followed by cyclic strain softening at lower total strain amplitude, while show the cyclic strain hardening at other imposed total strain amplitudes. For the extruded AZ61 magnesium alloys subjected to solid solution and solution plus aging treatment, the cyclic strain hardening can be observed at all the total strain amplitudes used in this investigation. The extruded AM20 and AM30 magnesium alloys with different treatment states exhibit cyclic hardening, softening and stability, which depends on the imposed total strain amplitudes and heat treatment states. It is noted that the heat treatment can enhance the fatigue lives of the hot-extruded AZ31 magnesium alloys at higher total strain amplitudes, while leads to a reduction in the fatigue lives of the hot-extruded AZ31 magnesium alloys at lower total strain amplitudes. Aging treatment can effectively enhance the fatigue lives of the extruded AZ61 magnesium alloys at lower total strain amplitudes, while solution plus aging treatment leads to a reduction in the fatigue lives of the AZ61 magnesium alloys. Solution plus aging treatment can effectively enhance the fatigue lives of the extruded AM20 magnesium alloy at higher total strain amplitudes, while solution treatment can prolong the fatigue lives of the extruded AM30 magnesium alloys in both higher and lower total strain amplitude regions. For the AZ and AM series magnesium alloys with different treatment states, the relations between elastic strain amplitude, plastic strain amplitude and reversals to failure can be described by Basquin and Coffin- Manson equations, respectively. In addition, a linear relationship between cyclic stress amplitude and plastic strain amplitude is also noted for the extruded AZ and AM series magnesium alloys with different treatment states. It has also been observed that when the extruded AZ and AM series magnesium alloys with different treatment states are subjected to fatigue deformation at higher total strain amplitudes, the width of theσεhysteresis loop in the compressivedirection is greater than that in the tensile direction. It means that the AZ and AM series magnesium alloys exhibit the pronounced anisotropic deformation behavior in the direction of tension and compression during strain-controlled fatigue deformation. For the extruded AZ and AM series magnesium alloys with different treatment states, the fatigue cracks initiate in a transgranular mode at the surface of fatigue specimens, and propagate transgranularly. In addition, the cleavage fracture feature can be found in the fatigue crack propagation region.