Study On The Microstructural Evolution And Mechanical Properties Of Mg-Nd-Sm-Zn-Zr Alloys

Mg-RE alloys have received considerable attention due to their high specific strengths,good corrosion resistance and outstanding biocompatibility.However,high performance Mg-RE alloys usually use the heavy rare earth elements as the key strengthening elements.The high price of heavy rare earth elements lead to a high price of the Mg-RE alloys.A low price and high performance Mg-RE alloy should be developed as early as possible to expand applications of the Mg-RE alloy.Therefore,we propose that the low price of light rare earth elements Nd and Sm are used as a new route to strengthen the Mg alloys.Furthermore,the Zn and Zr elements are also used to improve microstructure of the alloy.Up to now,the effect of the mixed light rare earth elements on Mg-RE alloy are rarely studied.The microstructure and mechanical properties of the Mg-Nd-Sm-Zn-Zr alloys are not clear.In this paper,the composition of Mg-Nd-Sm-Zn-Zr alloys is optimized.The microstructural evolution of solution treatment and aging treatment is comprehensively studied.In addition,a high performance Mg-NdSm-Zn-Zr alloys is prepared by integrated extrusion and equal channel angular pressing(IEECAP).The effect IEECAP and subsequent aging treatment on microstructure and mechanical properties of the Mg-Nd-Sm-Zn-Zr alloys is investigated.In order to optimize the composition of Mg-Nd-Sm-Zn-Zr alloys,numerous studies has been carried out by our team in the past years.The amount of Sm,Zn and Zr has been optimized.Thus,the amount of Nd is further optimized in this paper.The effect of Nd on the microstructure and mechanical properties of ascast solution-treated and aged Mg-x Nd-2.0Sm-0.4Zn-0.4Zr(0?x?2.5)alloy are systematically studied.The fracture behavior of the alloys is revealed.The results indicate that the microstructure of as-cast is composed of ?-Mg grains and eutectic ? phase.The ? phase continuously distributes in the grain boundaries.After the solution treatment,the ? phase dissolves in the ?-Mg matrix,and the supersaturated solid solution(SSSS)is obtained.Then the solution-treated alloys is aged.The GP zones,?? phase and ?1 phase precipitate from the matrix.The ?? phase and ?1 phase precipitate on prismatic plane.The Nd enhances the the age strengthening response.The aged alloy can be obtained a higher strength,when the amount of Nd is 2.0 wt.%.The yield strength(YS),ultimate tensile strength(UTS)and elongation of the aged alloy are 154 MPa,261 MPa and 5.8%,respectively.The fracture behavior of the as-cast alloys after the solution treatment and subsequent aging treatment transforms from intergranular fracture to transgranular fracture.The temperature and time of solution treatment for the optimized Mg-2.0Nd-2.0Sm-0.4Zn-0.4Zr alloys is adjusted.The influence of temperature and time of solution treatment on the microstructure and mechanical properties of the alloy is investigated.The results indicate that the grain size gradually increases,the amount of ? phase gradually decreases,and the strength increases first and then decreases with the increasing in temperature and time of solution treatment.The ? phase fully dissolves in the matrix when the temperature of solution treatment is higher than 515 ?,meanwhile,the time of solution treatment is more than 8 h.The optimized solution treatment process is 515 ?×8 h.The aging treatment process further investigates on basis of the optimized solution treatment process.The temperature and time of solution treatment for the Mg-2.0Nd-2.0Sm-0.4Zn-0.4Zr alloys is adjusted to study the their effect on the microstructure and mechanical properties of the alloy.The results indicate that the size of ?? phase and ?1 phase gradually increase with the increasing in temperature of aging treatment.The amount of the precipitates obviously decreases when temperature of aging treatment further increases.The strength of the alloy increases first and then decreases with the increasing in temperature aging treatment.The elongation of the alloy exhibits an opposite trendency of the strength.The hardness of the alloy undergoes a four-stage evolution: under-aged stage,half-peak-aged stage,peak-aged stage and over-aged stage.The amount and size of the ?? phase and ?1 phase increase during the under-aged stage.A largest amount of the ?? phase and ?1 phase can be obtained in peak-aged stage.The size of ?1 phase increases,meawhile,the amount of the precipitates decreases obviously in the over-aged stage.It should be noted that a checkerboard-shaped dual-phase structure is first observed.The dual-phase structure consists of ordered nano-particles and coarse ?1 phase.A higher strength can be obtained when the alloy aged at 190 ?×18 h.The YS,UTS and elongation of the aged alloy are 152 MPa,266 MPa and 6.1%,respectively.The Mg-Nd-Sm-Zn-Zr alloy are subjected plastic compression at room temperature before aging treatment to shorten the time of aging treatment.The results indicate that the peak-aged time of the alloy is shorten to 10 h,which is only a half of the aging time for the alloy without pre-deformation.Moreover,the hardness of the peak-aged alloy is higher than that of the alloy without predeformation.A new phase precipitates is designated as ?B?,which precipitates on basal plane of the ?-Mg matrix.A enclosed microstructure is composed by ?B? precipitating on basal plane and ?? precipitating on prismatic plane.The age strengthening is enhanced by formation of the enclosed microstructure.The sequence of precipitation for the Mg-Nd-Sm-Zn-Zr alloy is identified as: SSSS?GP zones?????1?? by the HAADF-STEM and other analysis and testing techniques.The match of crystal structure between the precipitates and the ?-Mg matrix gradually weaken with the transformation of the precipitates.The coherent and orientation relationship between the precipitates and the ?-Mg matrix also gradually weaken with the transformation of the precipitates.The high performance Mg-Nd-Sm-Zn-Zr alloy is prepared by IEECAP.The results indicate that the grain size and elongation of the IEECAPed alloys gradually decrease with the increasing of the Nd.On the contrary,the strength of the IEECAPed alloys gradually increases with the increasing of the Nd.Numerous of precipitates present in the matrix after the IEECAPed alloys aging at 190 ? for 18 h.Moreover,the amount of precipitates and strength of the alloys increase with the increasing of the Nd.On the contrary,the elongation of the IEECAPed alloys gradually decreases with the increasing of the Nd.A higher strength can be obtained when the amount of Nd is 2.0%.The YS,UTS and elongation are 187 MPa,315 MPa and 8.5%,respectively.In conclusion,a low price and high performance Mg-Nd-Sm-Zn-Zr alloy has been designed and prepared in this paper.The microstructural evolution and mechanical properties of Mg-Nd-Sm-Zn-Zr alloys are systematacially studied.The as-cast alloys and solution-treatment alloys have a potential for automotive and 3C applications,and so on,due to their moderate strength and superior ductility.The aged alloys have a wide prospect for the light weight applications,such as aviation industry due to their high specific strength.The high strength and high ductility alloys prepared by IEECAP have a great potential for medical applications.Therefore,this work provides theoretical basis for expanding the application of Mg-RE alloys and has a important application value.