| Mg-Y-Nd-Zr and Mg-Nd-Zn-Zr based casting magnesium alloys are thought to have wide applications in automotive,aerospace and other fields because of their high mechanical properties and excellent casting performance.However,the mechanical properties of the existing Mg-Y-Nd-Zr and Mg-Nd-Zn-Zr based casting magnesium alloys are not satisfactory in the requirement of high strength for magnesium alloys.Therefore,further research and development about Mg-Y-Nd-Zr and Mg-Nd-Zn-Zr based casting magnesium alloys are necessary.It is well known that alloying and/or micro-alloying,which have been actively applied in the development of magnesium alloys with high performance,are important methods to improve the mechanical properties at room and high temperatures for magnesium alloys.At present,amongst the alloying and/or micro-alloying elements for the modified research of the high performance magnesium alloys,Gd,Y and Zn are very attractive due to their strong precipitation strengthening effect and/or the low cost.However,up to now the investigations about the modified Mg-Y-Nd-Zr and/or Mg-Nd-Zn-Zr based casting magnesium alloys by adding Gd,Y and Zn especially adding Gd,Y and Zn with small amount,are relatively limited.Therefore,the research about the alloying and/or micro-alloying of Gd,Y and/or Zn elements for Mg-Y-Nd-Zr and Mg-Nd-Zn-Zr based casting magnesium alloys and the corresponding microstructure and mechanical properties,would have very important theoretical guiding significance and practical applied value for the development and application of Mg-Y-Nd-Zr and Mg-Nd-Zn-Zr based casting magnesium alloys with high strength.Based on the designed and prepared Mg-4Y-2.8Nd-0.4Zr-Gd/Zn and Mg-3Nd-0.2Zn-0.45Zr-Gd/Y(wt.%)experimental magnesium alloys,in this paper the microstructure and mechanical properties of the as-cast and heat-treated Mg-4Y-2.8Nd-0.4Zr-Gd/Zn alloys and the effects of Gd and Y additions on the as-cast and heat-treated microstructure and mechanical properties of the Mg-3Nd-0.2Zn-0.45Zr-Gd/Y alloys,were investigated by using optical microstructure(OM),X-Ray diffraction(XRD),scanning electron microstructure(SEM),energy disperse spectroscopy(EDS),differential scanning calorimeter(DSC),micro-hardness and tensile properties testing at room temperature,and the following main results were obtained:1)All the as-cast Mg-4Y-2.8Nd-0.4Zr experimental magnesium alloys with 0.2wt.% Zn,1wt.% Gd or 0.2wt.% Zn+1wt.% Gd additions have the same phases which are mainly composed of ?-Mg,Mg24Y5 and Mg12 Nd.At the same time,although the distribution of the eutectic structure at the grain boundaries for all the experimental alloys are not uniform,the difference in the amount of the eutectic structure at the grain boundaries for the different alloys is observed,thereinto the amount of the eutectic structure at the grain boundaries for the two alloys with 1wt.% Gd and 0.2wt.% Zn+1wt.% Gd additions is relatively larger than that for the alloy with 0.2wt.%Zn addition.In addition,under the experimental conditions in the paper,the as-cast tensile properties at room temperature for the two alloys with 1wt.% Gd and 0.2wt.% Zn+1wt.% Gd additions is slightly higher than those for the alloy with 0.2wt.% Zn addition.2)The tensile properties at room temperature for the Mg-4Y-2.8Nd-0.4Zr experimental alloys with 0.2wt.% Zn,1wt.% Gd or 0.2wt.% Zn+1wt.% Gd additions are obviously improved by combining solid solution with aging treatment.Under the experimental conditions in the paper,the optimal heat treatment technologies are the solid solution at 500? for 10 h and followed by aging treatment at 200? for 36 h.Based on the optimal heat treatment technologies,the Mg-4Y-2.8Nd-0.4Zr experimental alloy with 0.2wt.% Zn+1wt.% Gd additions obtains high tensile properties at room temperature,namely,the ultimate tensile strength(UTS),yield strength(YS)and elongation respectively reach 343 MPa,196 MPa and 4.3%,which might meet the requirement of high strength and toughness for industrial application.3)All the as-cast Mg-3Nd-0.2Zn-0.45 Zr experimental magnesium alloys without Gd and Y additions and with 1wt.% Y or 1wt.% Gd additions have the same phases which are mainly composed of ?-Mg and Mg12 Nd,and their grain size is similar.However,after Y or Gd additions,the amount of the eutectic structure at grain boundaries for the Mg-3Nd-0.2Zn-0.45 Zr alloy increases.In addition,under the experimental conditions in the paper,the experimental alloy with 1wt.% Y addition exhibits the relatively high as-cast tensile properties at room temperature: 192 MPa for UTS,99 MPa for YS and 13.3 % for elongation.4)The tensile properties at room temperature for the Mg-3Nd-0.2Zn-0.45 Zr experimental magnesium alloys without Gd and Y additions and with 1wt.% Y or 1wt.% Gd additions are obviously improved by combining solid solution with aging treatment.Under the experimental conditions in the paper,the optimal heat treatment technologies are the solid solution at 500? for 10 h and followed by aging treatment at 175? for 36 h.Based on the optimal heat treatment technologies,the Mg-3Nd-0.2Zn-0.45 Zr experimental alloy with 1wt.% Y addition obtains high tensile properties at room temperature,namely,the UTS,YS and elongation respectively reach 290 MPa,144 MPa and 8.4 %,which might basically meet the requirement of high strength and toughness for industrial application. |
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