Low Cyclic Fatigue Behavior Of As-extruded Magnesium Alloys

As a lightweight structural material, magnesium alloys will get widely applied in automotive industry. It has received much more concerning how to utilize magnesium alloys in a safe and effective way. Fatigue is a main failure form of various structural components during operation. For those magnesium alloy components, the same case is also true. Obviously, the investigation concerning fatigue behavior of magnesium alloys is of both academic and practical significance. In this investigation, the low-cycle fatigue behavior of two extruded magnesium alloys with different processing status has been investigated 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 cyclic stress response behavior of as-extruded AM50 and AZ91 magnesium alloys is closely related to both imposed total strain amplitude and processing status. At the higher total strain amplitudes, the alloys subjected to different processing and treatment exhibit cyclic strain hardening or stable cyclic stress response. At the lower total strain amplitudes, the alloys show cyclic stability or even cyclic softening at the beginning of fatigue deformation, while the considerable strain hardening occurs at the latter stage. It has been found that both solid solution treatment and aging following hot extrusion can influence the cyclic stress response behavior of as-extruded AM50 and AZ91 magnesium alloys. Especially at the lower total strain amplitudes, the effect of heat treatment on the cyclic stress response behavior becomes more significant.For the AM50 and AZ91 magnesium alloys subjected to various processing, the relation between elastic and plastic strain amplitudes with reversal cycles to failure shows a monotonic linear behavior, and can be well described by the Basquin and Coffin-Manson equations, respectively. The heat treatment after hot extrusion can give a significant effect on the fatigue life behavior of two extruded magnesium alloys. It has been observed that at the higher total strain amplitudes, the width of the a-s hysteresis loop of two extruded magnesium alloys in the tensile direction is greater than that in compressive direction. The alloys exhibit the pronouncedanisotropic deformation behavior in the direction of tension and compression. At the total strain amplitude of 1.5%, a serrated flow can be observed in the compressive direction of the σ-ε hysteresis loop for the extruded magnesium alloys with different processing status. It means that the so-called dynamic strain aging takes place during fatigue deformation. In addition, the tensile hysteresis energy of two extruded magnesium alloys has been calculated and compared with the tensile hysteresis energy measured from the hysteresis loop at half life. It can be noted that the calculated and measured tensile hysteresis energy are close with each other. There exists a linear relation between the tensile hysteresis energy and fatigue life for two extruded magnesium alloys. Thus, the low-cycle fatigue life of the extruded magnesium alloys can be predicted using the calculated tensile hysteresis energy as a material parameter.The SEM observations on fracture surfaces of the fatigued AM50 and AZ91 magnesium alloys with various processing status have been carried out. It is found that under all experimental conditions used in this investigation, fatigue cracks initiate transgranularly at the surface of samples and propagate in a transgranular mode.