Study On The Microstructure, Mechanical Properties And Corrosion Resistance Of Hot Extruded Mg-Mn-RE Alloy

Mg-Mn alloys are kind of the metallic materials, which have the good corrosion resisitance properties. However, the further development of magnesium alloys has been limited due to its low mechanical performance. Therefore, the research on developing new-type magnesium alloys with high strength and perfect corrosion resistance is one of the hotspot in the research field of magnesium alloys. Basing on Rare earth (RE) alloying method, the six kinds of Mg-Mn-RE alloys were developed by adding Y and Nd element into extruded Mg-Mn alloy. The alloys were prepared by conventional casting method from high purity 99.9% Mg, Mg-9.9%Mn, and Mg-21.5%Y alloy under a shielding gas of COB2B-0.05%SFB6B. The melts were poured into a steel die with the diameter of 80mm at 750°C. The determination of the phases in the investigated alloys was carried out by energy-dispersive spectroscopy and X-ray diffractmeter. The MTS, SEM and TEM were used for the deformational behavior and mechanical properties. The corrosion properties and effect factors of the Mg-Mn-RE alloys in different corrosive mediums were studied by atlas electric devices, potentiodymatic polarization curve and electrochemical impedance spectra. The innovative results have been obtained as follows:First, the effect of Y element on the microstructure and mechanical properties of extruded Mg-1Mn alloy are studied. The results show that Y element is beneficial for the grain refinement of the Mg-1Mn-5Y alloys. The average grain size of the Mg-1Mn-5Y alloys is finer than that of Mg-1Mn-1Y alloy. As a result, Mg-1Mn-5Y alloy exhibits higher tensile strengths, but lower elongation rate by comparing with Mg-1Mn-1Y alloy in a temperature range from room temperature to 300°C. Furthermore, the fracture surface analyses show that the Mg-1Mn-5Y alloy fractures by the cleavage mode, while that of the Mg-1Mn-1Y alloy by the dimple mode. The results demonstrate that the texture type is an important factor influencing the fracture mode and the ductility of Mg-1Mn-1Y and Mg-1Mn-5Y alloys. The experimental results show that the most T(T1010T)T crystal planes in the Mg-1Mn-1Y alloy are parallel to the normal direction, while the most (T1011T)T crystal planes in the Mg-1Mn-1Y alloy are parallel to the normal direction.Secondly, the effect of earth element Y and Nd on the microstructure and mechanical properties of extruded Mg-Mn alloy are studied. Y element can improve the strength of Mg-Mn alloy, and Nd element can enhance the toughness as well as the strength in high temperature condition of Mg-Mn alloy. The collective effects of Y and e Nd elements on the overall performances of Mg-Mn alloy is more singnificant than effect of signel Y lement. The Mg-1Mn-3.5Y and Mg-1Mn-1Y-2.5Nd alloys, which contain the same weight percentage of rare earth elements. The tensile tests at room temperature, 100°C and 200°C show that the extruded Mg-1Mn-3.5Y alloy exhibits lower yield strength andr elongation rate to failure as compared with the extruded Mg-1Mn-1Y-2.5Nd alloy. These differences in tensile mechanical properties between two alloys are mainly attributed to their different slip planes and amount and distribution of Mg24Y5 precipitation phases. The Mg-1Mn-3.5Y alloy shows the cleavage fracture mode, while the fracture surface of Mg-1Mn-1Y-2.5Nd alloy exhibits the some toughnessive dimples.Fourthly, the effect of Nd element on the microstructure and mechanical properties of extruded Mg-Mn-2Y alloy are studied. The obvious recrystallization occurred in the Mg-2Y-1Mn-2Nd alloy leads to the large average grain size as well as a number of Mg12Nd phase particles and fine Mg24Y5 phase particles.Tensile tests showed that the Mg-2Y-1Mn-1Nd alloy exhibits higher yield and ultimate tensile strengths, but lower elongation to failure at room temperature and, in contrast, the Mg-2Y-1Mn-2Nd alloy shows higher strengths and ductility at 100°C and 200°C, respectively. These differences in the tensile mechanical properties between the two alloys arise from the different contributions of the strengthening due to the grain refinement and the precipitation and dispersion strengthening effects generated by the coarse Mg12Nd phase particles and the fine Mg24Y5 phase particles.Finally, the effects of Y and Nd elements on the corrosion behaviors and mechanisms of Mg-Mn-Y alloys are studied by observing and analyzing the corrosion behaviors. The results show that the corrosion rate of Mg-Mn-Y is decreasing with increasing Y content. but the Nd elements make Mg-Mn alloy suffer corrosion easily.