Microstructure And Mechanical Properties Of Fine-grained Magnesium Alloys

In this paper, the mechanical properties of as-extruded AZ91 and AM50 magnesium alloys were investigated. The ultimate tensile strength σ_b, 0.2% offset yield strength σ₀.2 and elongation to failure 5 of two extruded magnesium alloys at different test temperatures and under various heat treatment conditions were obtained. The evolution in the microstructure of as-extruded AZ91 and AM50 magnesium alloys due to hot extrusion and heat treatment were studied. By means of SEM, observations on fracture surfaces were carried out. And the effect of test temperature and heat treatment on the mechanical properties and fracture mechanism of as-extruded magnesium alloys were determined. Moreover, the microstructure and mechanical properties of the as-cast and heat-treated AZ61 magnesium alloys with an addition of C2Cl6 in magnesium alloy melt were also studied.The results showed that the hot extrusion processing could significantly reduce the grain size of as-cast AZ91 and AM50 magnesium alloys, and thus enhance the mechanical properties of those alloys at room temperature. The influence of test temperature on mechanical properties of the magnesium alloys subjected to different processing and treatment was obvious. In general, with increasing test temperature, the ultimate tensile strength and yield strength decreased while elongation to failure increased obviously. This can be mainly attributed to an increase in number of slip systems of the matrix with test temperature. The considerable effect of heat treatment on mechanical properties of as-extruded AZ91 and AM50 magnesium alloys was also noted. After solid solution treatment (T4), the growth of grains and the partial solution of those strengthening phases in the extruded AZ91 and AM50 alloys occurred, which led to a decrease in mechanical properties of two alloys. When the aging was applied following the solid solution treatment (T6), the ultimate tensile and yield strengths of two alloys at room temperature would become higher because the fine secondary phase particles precipitated and the matrix got reinforced. The aging directly following hot extrusion (T5) could result in the uniform microstructure and the precipitation of those strengthening phases such as Mg₁7Al₁2, and thus the mechanical properties of the AZ91 alloy at room temperature as well as the tensileproperties of the AM50 alloy at room and elevated temperatures got enhanced.It was also found that adding a appropriate amount of C₂Cl₆ into the AZ61 alloy melt would improve the microstructure of as-cast AZ61 alloy and enhance the mechanical properties of the alloy at room temperature. The solid solution treatment performed along with the addition of C₂Cl₆ could also give an influence on the mechanical properties of as-cast AZ61 alloy at room temperature.Fracture surface analysis revealed that the magnesium alloys subjected to different processing and treatment exhibited a characteristic of mixed ductile and brittle fracture during tension 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.