| In this paper, the as-cast AZ91 magnesium alloys were extruded under different extrusion ratios. And then aging treatment was carried out on some hot-extruded AZ91 magnesium alloys. The evolution in the microstructure of AZ91 magnesium alloys due to hot extrusion and aging treatment were studied through investigation of the microstructures and tests of the mechanical properties and the ultimate tensile strength σb, the yield strength σ0.2 and elongation to failure 5 were determined. By means of SEM, observations on fracture surfaces were carried out and features of fractures of hot-extruded AZ91 magnesium alloy were studied.The results showed that the hot extrusion processing could significantly reduce the grain size of AZ91 magnesium alloy, and grain refining effect was closely related with the extrusion ratios. At different testing temperatures, the ultimate tensile strength and the yield strength of hot extruded AZ91 magnesium alloy enhanced with the increase of the extrusion ratio. The exhibition of the elongation to failure of the hot extruded AZ91 magnesium alloy was different. At room temperature, the elongation to failure of the hot extruded AZ91 magnesium alloy decreased with the increase of the extrusion ratio. While at high temperatures, the elongation to failure of the hot extruded AZ91 magnesium alloy rose first and then dropped with the increase of the extrusion ratio. The testing temperature had great influence on the mechanical properties of hot extruded AZ91. For all hot extruded AZ91 magnesium alloy under different extrusion ratios referred in this paper, the highest ultimate tensile strength and yield strength were obtained at temperature of 150℃, while the elongation to failure of the hot extruded AZ91 magnesium alloy increased with the elevated testing temperature. The aging treatment also had great influence on the hot extruded AZ91 magnesium alloy. For the hot extruded AZ91 magnesium alloy subjected to extrusion ratios of 9 and 18, the ultimate tensile strength and the yield strength of the alloys which were subjected to aging treatment were higher than the alloys which were not subjected to aging treatment at room temperature and high temperatures. For the hot extruded AZ91 magnesium alloy under extrusion ratio of 36, the aging treatment couldimprove the strength at room temperature, but it reduced the strength at higher temperatures. On the other hand, the aging treatment could result in the decrease of the elongation to failure at room temperature, but it could increase the elongation to failure at high temperatures.Fracture surface analysis revealed that the magnesium alloy subjected to different processing and treatment exhibited a characteristic of mixed ductile and brittle fracture during test at room temperature. At elevated temperatures, the more and deeper dimples could be observed on the fracture surfaces of the tension deformed magnesium alloys. It implied that the tensile fracture of the alloys occurred basically in a ductile mode.
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