Research About Microstructures And Mechanical Properties Of Mg-3.5Sr-1Mn Based Magnesium Alloys

Magnesium alloys which are thought to be a kind of light engineering materials,have received more attentions all over the world. At present, the mechanical properties especially the properties at elevated temperatures for the developed magnesium alloys with low cost are relatively poor, which seriously hinders the application of magnesium alloys in the automotive and other industries. At the same time, some magnesium alloys with relatively high properties at elevated temperatures mainly focus on Mg-RE alloys, which also restrict the application of these alloys due to the high price of rare earth elements. Due to the above mentioned reasons, more attentions are paid to alkaline-earth magnesium alloys with low cost and high properties at elevated temperatures. However, the current research about alkaline-earth magnesium alloys mainly focuses on the Ca-containing alloys. The research for the Sr-containing alkaline-earth alloys is very limited. Therefore, further investigations about the design of the new type Sr-containing alkaline-earth magnesium alloys and their microstructures and mechanical properties, have very important theoretical and practical significance to develop alkaline-earth magnesium alloys and promote their application.Based on the designed Mg-3.5Sr-1Mn(wt %) Sr-contining new type alkaline-earth magnesium alloy in this paper, the influence of Ce, Sc and Nd alloying and/or micro-alloying and heat treatment on the microstructure and mechanical properties of the Mg-3.5Sr-1Mn magnesium alloy was investigated by using optical microscope, DSC differential thermal calorimetry, X-ray diffraction, scanning electron microscope, tensile testing machine and micro hardness tester, and the following main research results were obtained:1) The as-cast microstructure of the Mg-3.5Sr-1Mn experimental magnesium alloy is mainly composed of the α-Mg, α-Mn and Mg17Sr2 phases, and after being T4-treated(520℃×24 h + quenching water), the α-Mn and Mg17Sr2 phases in the microstructure of the alloy still exist. In addition, after respectively adding 0.5-1.5 wt%Ce, 0.15-0.45 wt%Sc and 0.5-1.5 wt%Nd to the Mg-3.5Sr-1Mn alloy, the type of the secondary phases in the microstructures of the as-cast and T4-treated(520℃×24 h + quenching water) alloys does not change, but the grains of the as-cast alloys are refined.2) After adding 0.5-1.5 wt%Ce to the Mg-3.5Sr-1Mn experimental magnesium alloy, the tensile properties at room temperature(RT) for the alloy are improved, and an increase in Ce amount from 0.5 wt% to 1.5wt% causes the tensile strength, yield strength and elongation of the Ce-containing alloys to first decrease and then increase, respectively. Among the experimental alloys with 0.5, 1.0 and 1.5 wt%Ce, the alloy with 1.5 wt%Ce exhibits relatively higher tensile properties at RT than other alloys. In addition, adding 0.5-1.5 wt%Ce to the Mg-3.5Sr-1Mn alloy do not significantly change the fracture mode of the alloy, and all the tensile fracture surfaces of the alloys with and without Ce addition have typical mixed characteristics of cleavage and/or quasi cleavage fracture.3)After adding 0.15-0.45 wt%Sc to the Mg-3.5Sr-1Mn experimental magnesium alloy, the distribution of the secondary phases in the microstructure of the alloy becomes more uniform. At the same time, the tensile strength, yield strength and elongation at RT for the alloy are also improved by adding 0.15-0.45wt%Sc. However, for the Sc-containing alloys, an increase in Sc amount from 0.15 wt% to 0.45 wt% causes the tensile strength and elongation to gradually decrease but to slightly increase for the yield strength. In general, among the experimental alloys with 0.15, 0.30 and 0.45 wt%Sc, the alloy with 0.15wt% Sc has the relatively optimum tensile properties at RT. In addition, after adding 0.15-0.45wt% Sc to the Mg-3.5Sr-1Mn alloy, the fracture mode of the alloy does not significantly change, and all the tensile fracture surfaces of the alloys with and without Sc addition exhibit typical mixed characteristics of cleavage and/or quasi cleavage fracture.4) After adding 0.5-1.5wt% Nd to the Mg-3.5Sr-1Mn experimental magnesium alloy, the distribution of the secondary phases in the microstructure of the alloy becomes more dispersive. At the same time, adding 0.5, 1.0 and 1.5 wt%Nd to the Mg-3.5Sr-1Mn alloy result in different influence on the tensile properties at RT for the alloy, thereinto adding 0.5 and 1.0 wt%Nd can improve the tensile strength, yield strength and elongation, however, after adding 1.5 wt%Nd, the elongation increases but the tensile strength and yield strength decrease. Among the experimental alloys with 0.5, 1.0 and 1.5 wt%Nd, the alloy with 1.0 wt%Nd has the relatively optimum tensile properties at RT. In addition, all the tensile fracture surfaces of the alloys with and without Nd addition have typical mixed characteristics of cleavage and/or quasi cleavage fracture, indicating that adding 0.5-1.5 wt% Nd to the Mg-3.5Sr-1Mn alloy also do not change the fracture mode of the alloy.