Research About Microstructures And Mechanical Properties Of Mg-Gd-Y-Zn-Zr High-strength Casting Magnesium Alloys

Rare earth magnesium alloys whose applications in aerospace and automotive i ndustries are very wide,are thought to be the most potential and have excellent perf ormance such as high strength and toughness.Among the developed rare earth magn esium alloys,Mg-Gd-Y-Zn-Zr casting magnesium alloys are thought to have great de velopment potential.In the recent 10 years,the research and development of Mg-GdY-Zn-Zr casting magnesium alloys has been paid great attention at home and abroa d,and many positive researches about the design of alloy compositions and heat trea tment strengthening have been carried out.It is well known that Gd and Y are the main alloying elements of Mg-Gd-Y-Zn-Zr casting magnesium alloys,therefore,their o ptimization in the compositions has great significance for the development of Mg-G d-Y-Zn-Zr magnesium alloys with high-performance.At present,the range of the chan ge in Gd and Y compositions related to the microstructures and mechanical properti es of Mg-Gd-Y-Zn-Zr magnesium alloys,is relatively wide,and the related research is not very systematic and complete.Therefore,in order to provide a theoretical foundati on for the development and application of high-strength Mg-Gd-Y-Zn-Zr casting ma gnesium alloys,it is necessary to further investigate the effects of the optimization o f Gd and Y compositions and the following heat treatment on the microstructures a nd mechanical properties of Mg-Gd-Y-Zn-Zr casting magnesium alloys based on the small range of Gd and Y compositions.Based on the previous investigations about the high-strength Mg-Gd-Y-Zn-Zr ca sting Mg alloys and the small change in the Gd and Y compositions,the Mg-xGd-1Y-1Zn-0.5Zr and Mg-xGd-2Y-1Zn-0.5Zr(x=9-11 wt.%)casting magnesium alloys wer e first designed and prepared in this paper,and then by using optical microscope(O M),X-Ray diffraction(XRD),scanning electron microscope(SEM),energy spectrum an alyzer(EDS),differential scanning calorimetry(DSC),micro-hardness and tensile proper ties testing at room temperature and so on,the influence of the change in Gd and Y amounts on the microstructure and mechanical properties of the Mg-Gd-Y-Zn-Zr casting magnesium alloys were investigated.At the same time,the heat-treated microst ructures and properties of the Mg-xGd-1Y-1Zn-0.5Zr and Mg-x Gd-2Y-1Zn-0.5Zr(x=9-11 wt.%)casting magnesium alloys with different compositions were compared.Finall y,the following main results are given:1)The as-cast microstructures of the Mg-xGd-1Y-1Zn-0.5Zr(x=9-11 wt.%)experi mental magnesium alloys exhibit the typical dendrite morphology,which mainly cont ain the ?-Mg matrix and lots of the secondary phases distributed along the grain b oundaries and/or the grains.At the same time,the change in Gd amount has certain i nfluence on the size of the grains in the experimental alloys.An increase in Gd am ount from 9 to 10.5wt.% causes the grains in the alloys to gradually refine,and the n with the increase of Gd amount from 10.5 to 11 wt.%,the grains in the alloys exh ibit an coarsening tendence.Accordingly,the experimental alloy with 10.5wt.%Gd obta ins the relatively optimum as-cast tensile properties,its tensile strength,yield strength and elongation at room temperature(RT)reach 210 MPa,158MPa and 7.3%,respectiv ely.2)The effects of the heat treatment on the microstructures and mechanical properties of the Mg-xGd-1Y-1Zn-0.5Zr(x=9-11 wt.%)are very obvious.After being solutionized at 460-520? for different holding times,the dendrite morphology in the microstructures of the alloys disappears and the amount of the secondary phases gradually decreases.At the same time,for a given solutionized process,the secondary phases in the microstructures of the alloys gradually change to the short rod shapes and their volume fraction exhibits an increase tendence.In addition,under all the different solutionized processes,the alloy with 10.5wt.%Gd exhibits relatively higher tensile properties than other alloys,after being solutionized at 480? for 16 h,the tensile strength,yield strength and elongation at RT for the alloy reach 234 MPa,172MPa and 14.1 %,respectively.Finally,after being further aged at 200? for 24 h,the alloy with 10.5wt.%Gd obtains the relatively optimum comprehensive tensile properties than the alloys with other Gd contents,its tensile strength,yield strength and elongation at RT reach 292 MPa,243 MPa and 3.2 %,respectively.3)The as-cast microstructures of the Mg-xGd-2Y-1Zn-0.5Zr(x=9-11 wt.%)experi mental magnesium alloys exhibit the obvious dendritic structure characteristics,which are mainly composed of the ?-Mg matrix and the secondary phases distributed alon g the grain boundaries.However,the change in Gd content has certain effects on the grain size,the amount and size of the secondary phases for the experimental alloys.With the increase of Gd content from 9 to 11 wt.%,although the grains of the alloys are gradually refined,the amount and size of the secondary phases in the microstruc tures of the alloys exhibit the gradual increase tendence.Amongst the experimental magnesium alloys with different Gd contents,the as-cast tensile properties of the exp erimental alloy with 10 wt.%Gd are relatively high,its tensile strength,yield strength and elongation at RT reach 223 MPa,180MPa and 7.3%,respectively.4)Some differences exist for the microstructures and mechanical properties of t he Mg-xGd-2Y-1Zn-0.5Zr(x=9-11 wt.%)experimental magnesium alloys followed by the heat treatment.After being solutionized at 460-520? for the different holding ti mes,with the increase in Gd content from 9 to 11 wt.%,the dendrites disappeared an d the amount of the lamellar LPSO phases obviously increases for the alloys.At the same time,under the different solutionized processes,the experimental alloys with diff erent Gd contents exhibit certain difference in the tensile properties,thereinto after b eing solutionized at 480? for 16 h for the experimental alloy with 10 wt.%Gd,the al loy obtains the relatively optimum tensile properties at RT,its tensile strength,yield s trength and elongation reach 221 MPa,172MPa and 11.1%,respectively.Furthermore,aft er being further aged at 200? for 24 h,the experimental alloy with 10 wt.%Gd also obtains the relatively optimum comprehensive tensile properties than the alloys with other Gd contents,its tensile strength,yield strength and elongation at RT reach 292 MPa,251MPa and 3.2%,respectively.5)For the given Gd contents,the change in Y content has certain effects on the tensile properties of the Mg-(9-11 wt.%)Gd-Y-1Zn-0.5Zr experimental magnesium allo ys.In general,the as-cast,solutionized and aged experimental alloys with 2wt.%Y resp ectively exhibit the relatively higher tensile properties at RT than the corresponding alloys with 1wt.%Y.Obviously,under the experimental conditions of this paper,the rel atively optimum contents of the two main alloying elements,Gd and Y,are 10 and 2wt.%,respectively,and the relatively optimum heat-treated processes for the correspon ding Mg-10Gd-2Y-1Zn-0.5Zr experimental magnesium alloy is 480?×16h(solution t reatment)+200?×24h(aging treatment).