Study On Mechanical Behavior Of AZ91 Magnesium Alloy Subjected To Hot Extrusion

The AZ91 magnesium alloy widely used in engineering was chosen as an experimental material in this investigation. The microstructural evolution resulted from hot extrusion and subsequent heat treatment was investigated for the AZ91 alloy. The ultimate tensile strength σ_b, 0.2% offset yield strength σ_0.2 and elongation to failure 5 of the AZ91 magnesium alloys with different processing statuses at various test temperatures were determined. The low-cycle fatigue behavior of the hot-extruded AZ91 magnesium alloys subjected to due different heat treatments was studied. By means of scanning electron microscope, the observations on the fracture surfaces of tensile and fatigued samples were carried out.The results showed that the hot extrusion could significantly reduce the grain size of as-cast AZ91 magnesium alloy, and thus enhance the mechanical properties of the alloy at room temperature. Generally, for the extruded AZ91 alloy with different treatment statuses, the ultimate tensile strength and yield strength decreased while elongation to failure increased obviously with increasing test temperature. Direct aging and solution plus aging after hot extrusion could increase the tensile properties of the AZ91 alloy. Especially, the AZ91 alloy subjected to aging directly following hot extrusion could give the excellent tensile properties at room temperature.The cyclic stress response behavior of the extruded AZ91 magnesium alloys was closely related to both imposed total strain amplitude and treatment statuses. At the higher total strain amplitudes, the alloys exhibited cyclic strain hardening or stable cyclic stress response. At the lower total strain amplitudes, the alloys showed cyclic stability or even cyclic softening at the beginning of fatigue deformation, while the considerable strain hardening occurred at the latter stage. It was also found that both solid solution treatment and aging following hot extrusion could influence the cyclic stress response behavior of the extruded AZ91 magnesium alloy. For the extruded AZ91 magnesium alloys with different treatment statuses, the relation between elastic and plastic strain amplitudes with reversal cycles to failure showed a monotonic linear behavior, and could be well described by the Coffin-Manson equations, respectively.Fracture surface analysis revealed that the AZ91 alloy magnesium alloys subjected to different processing and treatment exhibited a characteristic of mixed ductile and brittle fracture during tension at room temperature while the tensile fracture of the alloys occurred basically in a ductile mode. For the extruded AZ91 magnesium alloy, the fatigue cracks initiated transgranularly at the surface of samples and propagated in a transgranular mode.