Fatigue Deformation And Fracture Behavior Of Extruded AZ81 Mg Alloy

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 strain-controlled fatigue deformation and fracture behavior of extruded AZ81 magnesium alloys with different processing statuses has been investigated in order to provide a reliable theoretical foundation for both fatigue resistant design and reasonable usage of this wrought magnesium alloy.The results of fatigue tests reveal that under all total strain amplitudes used in this investigation, the alloys subjected to different processing and treatment exhibit cyclic strain hardening till final fracture. Both solid solution treatment and aging treatment will result in a decrease in cyclic stress amplitude for extruded AZ81 magnesium alloy. At those total strain amplitudes ranging from 0.65% to 1.5%, both solid solution treatment and solution plus aging treatment can effectively increase the strain fatigue life of extruded AZ81 alloy. Aging treatment can also enhance the strain fatigue life of extruded AZ81 alloy at either high or lower total strain amplitudes, but decrease the strain fatigue life of the alloy. For extruded AZ81 alloys with different processing statuses, the relations between elastic strain amplitude, plastic strain amplitude and reversals to failure can be respectively described by Basquin and Coffin-Manson equations. In addition, a linear relation between calculated tensile hysteresis energy and strain fatigue life has been noted and can be used to predict the strain fatigue lives of extruded AZ81 alloys with different processing statuses. Between cyclic stress amplitude and plastic strain amplitude, a single-slope behavior can be noted for as-extruded AZ81 alloy and extruded AZ81 alloys subjected to aging and solution plus aging treatments, while a two-slope behavior can be observed for extruded AZ81 alloy subjected to solid solution treatment. The observations on fracture surface of fatigued specimens reveal that for extruded AZ81 alloys with different processing statuses, the fatigue cracks initiate in a transgranular mode at the surface of fatigue specimens, and propagate transgranularly. In addition, so-called cleavage feature can be found in the fatigue crack growth region.