Basic Research About Controlling Of Microstructures And Mechanical Properties For Mg-Gd-Y Based Magnesium Alloys With High-strength

Mg-Gd-Y based magnesium alloys which are thought as potential high-strengthmagnesium alloys, have received much global attention. However, at present the systemicand perfect researches about the effects of alloying and/or micro-alloying, extrusion andheat treatment et al. on the microstructure and mechanical properties of Mg-Gd-Y basedhigh-strength alloys have not been carried out. Therefore, it is very necessary to design anddevelop new Mg-Gd-Y based alloys according to the previous research results and thenfurther investigate the effects of alloying and/or micro-alloying, extrusion and/or heattreatment on the microstructures and mechanical properties of the designed-alloys, whichhas very important theoretical significance and practical value for successfully developingMg-Gd-Y-based high-strength magnesium alloys and promoting their application.Based on the casting, extrusion and heat treatment et al. and combined with usingoptical microscope(OM), scanning electron microscope(SEM) and EDS analysis, X-Raydiffraction(XRD), DSC and tensile properties testing, the microstructure and mechanicalproperties of the Mg-Gd-Y based high-strength alloys, such as Mg-Gd-Y-0.6Mn-0.6Ca,Mg-12Gd-4Y-1Zn-xZr and Mg-Gd-Y-1Zn-0.6Zr (wt.%) alloys, were investigated, and thefollowing main results were obtained in the paper:1) All the experimental magnesium alloys are mainly composed of α-Mg, Mg5Gd andMg24Y5phases, and under the conditions of this paper the change in Gd and Y contentand/or Zr addition have no obvious effect the alloying phase type in the experimentalalloys. At the same time, during the solidification of all the experimental alloys, theeutectic reaction (L→α-Mg+Mg5Gd) is observed at about530℃. In addition, the lamellarLPSO (Long-period Stacking Order) structure which is not observed in theMg-Gd-Y-0.6Mn-0.6Ca experimental alloys, is very clear in the microstructures of theMg-Mg-12Gd-4Y-1Zn-xZr and Mg-Gd-Y-1Zn-0.6Zr experimental alloys.2) For the Mg-Gd-Y-0.6Mn-0.6Ca experimental alloys, the alloy with13wt.%Gd+5wt.%Y has relatively finer grains than the alloy with10wt.%Gd+5wt.%Y, and the amountof the secondary phases in the former is also larger than that in the latter. For a given Gdcontent of13wt.%, an increase in Y content from3wt.%to5wt.%has no obvious effect onthe grain sizes of the alloys with Y addition. However, the amount of the alloying phases inthe alloys with Y addition is increased due to the above mentioned change in Y content. At the same time, the morphology of the secondary phase changes from fine particle-likeshape and/or disconnected distribution to coarse quasi-continuous distribution. Among theMg-Gd-Y-0.6Mn-0.6Ca experimental alloys, the Mg-13Gd-5Y-0.6Mn-0.6Ca alloy exhibitsthe relatively optimal tensile properties after extrusion+aging treatment.3) Adding0.6-1.2wt.%Zr to the Mg-12Gd-4Y-1Zn experimental alloy can refine thegrains of the alloy. Among the different Zr additions, the addition of0.9wt.%Zr obtainsthe relatively highest refining efficiency, and respectively followed by the additions of0.6wt.%Zr and1.2wt.%Zr. Among the Mg-12Gd-4Y-1Zn-xZr experimental alloys, theMg-12Gd-4Y-1Zn-0.9Zr alloys with extrusion and extrusion+aging states respectivelyexhibit the relatively optimal tensile properties, especially the tensile and yield strengths.4) For the Mg-Gd-Y-1Zn-0.6Zr experimental alloys, when14wt.%Gd content is kept,an increase in Y content from2wt.%to4wt.%causes the amount and size of the secondaryphases in the experimental alloys to increase simultaneously. Furthermore, themicrostructure of the alloy with12wt.%Gd+4wt.%Y is relatively finer than that of thealloy with14wt.%Gd+4wt.%Y. Compared with the Mg-14Gd-(2-4)Y-1Zn-0.6Zr alloys, theMg-12Gd-4Y-1Zn-0.6Zr alloys with extrusion and extrusion+aging states respectivelyexhibit the relatively optimal tensile properties, especially the tensile and yield strengths.5) Aging treatment can improve the tensile properties of all the as-extrudedexperimental magnesium alloys, thereinto treatment under the conditions of200°C agingtemperature the relatively optimal aging time is60-84h for the Mg-Gd-Y-0.6Mn-0.6Caexperimental alloys. As for the Mg-12Gd-4Y-1Zn-xZr and the Mg-14Gd-(2-4)Y-1Zn-0.6Zrexperimental alloys, under the conditions of200°C aging temperature their relativelyoptimal aging time is48-60h.