Basic Research About Controlling Of Microstructure And Mechanical Properties For Mg-Gd-Sr/Ca Magnesium Alloys

Magnesium alloys which are thought as the lightest engineering structural materialshave great potential for the applications in automotive, aerospace and other areas. However,the mechanical properties at high temperatures for the available magnesium alloys do notmeet the needs of industrial production. Therefore, the reseach and development of the newheat-resistant magnesium alloys with high performance and low cost are very necessary.Mg-Gd based rare-earth magnesium alloys have received widespread attention at home andoverseas due to their high strength and creep properties at high temperatures. At present,the research and development of Mg-Gd based heat-resistant magnesium alloys with highstrength mainly focus on the Mg-Gd-Y based alloys with high cost, and the investigationof the Mg-Gd-Sr/Ca based alloys with low-cost has not been carried out basically.Therefore, the basic research about the controlling of the microstructure and mechanicalproperties for the Mg-Gd-Sr/Ca based alloys with low-cost, which is predicted to has veryimportant theoretical significance and practicable value in developing and widening theapplication for the Mg-Gd based heat-resistant magnesium alloys with high strength andlow cost.Based on the designed Mg-5Gd-2Sr/1.5Ca-0.25Mn (wt%) magnesium alloys andcombined with using optical microscope(OM), differential scanning calorimetry(DSC)，X-Ray diffraction(XRD), scanning electron microscope(SEM) and tensile propertiestesting, the effects of Zr, Ca and/or Sr micro-alloying and heat treatment on themicrostructures and tensile properties at room temperature (RT) for theMg-5Gd-2Sr/1.5Ca-0.25Mn alloys, were investigated, and the following main results wereobtained in the paper:1) The as-cast Mg-5Gd-2Sr-0.25Mn experimental alloy is mainly composed of α-Mg,Mg5Gd and Mg17Sr2phases. Adding0.6wt%Zr,0.3wt%Ca or0.6wt%Zr+0.3wt%Ca to theMg-5Gd-2Sr-0.25Mn alloy have no obvious effect on the type of the secondary phases inthe alloy, however, the grains of the alloys with the additions of0.6wt%Zr,0.3wt%Ca and0.6wt%Zr+0.3wt%Ca are refined, thereinto the refining efficiency of0.6wt%Zr+0.3wt%Ca additions is the best, and followed by0.3wt%Ca and0.6wt%Zr additions,respectively.2) Adding0.6wt%Zr,0.3wt%Ca or0.6wt%Zr+0.3wt%Ca to the Mg-5Gd-2Sr-0.25Mnexperimental alloy have effects on the tensile properties at RT for the alloy. After adding0.6wt%Zr,0.3wt%Ca and0.6wt%Zr+0.3wt%Ca to the Mg-5Gd-2Sr-0.25Mn alloy, the as-cast tensile strength and elongation of the alloys gradually decrease but the as-cast yieldstrength of the alloys gradually increases. At the same time, after being treated by T5(225℃×45h+air-cooled), the tensile and yield strengthes of the Mg-5Gd-2Sr-0.25Mn alloyare improved but the elongation is slightly decreased. Furthermore, for a given T5treatedtime of45h, with the T5treated temperature increasing from225℃to275℃, the tensileand yield strengthes of the Mg-5Gd-2Sr-0.25Mn alloy gradually increase but theelongation of the alloy gradually decreases. In addition, the effects of0.6wt%Zr,0.3wt%Ca and0.6wt%Zr+0.3wt%Ca additions on the tensile properties of the T5treatedMg-5Gd-2Sr-0.25Mn alloy are similar to these of three additions on tensile properties ofthe as-cast alloy.3) The as-cast Mg-5Gd-1.5Ca-0.25Mn experimental alloy is mainly composed ofα-Mg, Mg5Gd and Mg2Ca phases, and adding0.6wt%Zr+0.15wt%Sr,0.6wt%Zr or0.15wt%Sr to the Mg-5Gd-1.5Ca-0.25Mn alloy have no obvious effect on the type of thesecondary phases in the alloy. In addition, the effects of0.6wt%Zr,0.15wt%Sr and0.6wt%Zr+0.15wt%Sr additions on the grain refinement of the Mg-5Gd-1.5Ca-0.25Mnalloy are different. After adding0.6wt%Zr and0.15wt%Sr to the Mg-5Gd-1.5Ca-0.25Mnalloy, the grains of the alloy are refined and coarsened, respectively. However, the grainrefinement of the alloy with the additions of0.6wt%Zr+0.15wt%Sr is not very obvious.4) The effects of0.6wt%Zr,0.15wt%Sr and0.6wt%Zr+0.15wt%Sr additions on thetensile properties at RT for the Mg-5Gd-1.5Ca-0.25Mn experimental alloy are different.Adding0.6wt%Zr or0.6wt%Zr+0.15wt%Sr to the Mg-5Gd-1.5Ca-0.25Mn alloy mightimprove the tensile and yield strengthes of the alloy but are not beneficial to the elongationof the alloy. Oppositely, adding0.15wt%Sr to the Mg-5Gd-1.5Ca-0.25Mn alloy mightimprove the elongation of the alloy but is not beneficial to the tensile and yield strengthesof the alloy. At the same time, after being treated by T5(225℃×45h+air-cooled), thetensile and yield strengthes of the Mg-5Gd-1.5Ca-0.25Mn alloy are improved but theelongation is slightly decreased. Furthermore, for a given T5treated time of45h, with theT5treated temperature increasing from225℃to275℃, the tensile and yield strengthes ofthe Mg-5Gd-1.5Ca-0.25Mn alloy gradually increase but the elongation of the alloygradually decreases. In addition, the effects of0.6wt%Zr,0.15wt%Sr and0.6wt%Zr+0.15wt%Sr additions on the tensile properties of the T5treatedMg-5Gd-1.5Ca-0.25Mn alloy are similar to these of three additions on tensile properties ofthe as-cast alloy.