Research On Microstructure Control And Properties Of Mg-3Y Alloy

Magnesium alloy has characteristics of low density,high specific strength and stiffness.It has broad application prospects in transportation,3C products,medical treatment,aerospace and other fields.A large number of studies have shown that rare earth elements play an obvious role in the addition of alloying elements,and after plastic processing,it can significantly improve the mechanical properties and corrosion resistance of magnesium alloys.The forming process has an important influence on the microstructure and properties of the alloy.However,there is a lack of systematic research on the mapping relationship between forming process and parameters,microstructure and properties of the alloy.In this paper,as-cast,homogenized,extruded,rolled and annealed at 250?,300?,350?,400?Mg-3Y alloys were studied.The microstructure and phase composition of were observed and analyzed by optical electron microscope(OM),scanning electron microscope(SEM),energy dispersive spectroscopy(EDS)and X-ray diffractometry(XRD).The mechanical properties of the alloy were tested by tensile equipment.The corrosion resistance of the alloy was studied by electrochemical measurements and immersion test.In order to find out the technological process for best comprehensive properties of the alloy,the mapping relationship among process,microstructure and properties of the alloy was studied by comprehensive quantitative analysis of the test results of the alloy under different processes.The main research results are as follows:(1)Analysis of microstructure results:Mg-3Y alloy is mainly composed of?-Mg phase and a small amount of second phase Mg₂₄Y₅ under different processing conditions.The average grain size of the as-cast and subsequent homogenized alloys is larger,about553?m and 591?m,respectively.After hot extrusion deformation,the alloy undergoes complete dynamic recrystallization,and the average grain size is greatly reduced to about7.8?m,which is 98%lower than that of the as-cast alloy.After rolling,a large number of deformation twins appeared in the structure and the grains were drawn into strips along the rolling direction,and the average grain size increased to about 77.17?m.After annealing,the average grain size of the alloy decreases to varying degrees.After annealing at 250°C,there is little difference from the rolled state.After annealing at 300°C,fine recrystallized grains are produced in the structure,and the average grain size is about15.78?m,which is 97%lower than that of the as-cast alloy.After annealing at 350?,the alloy undergoes complete static recrystallization,and the average grain size is decreases to about 8.78?m;Further increasing the annealing temperature to 400?,the recrystallized grains grow rapidly,and the average grain size increases to 21.56?m.(2)Analysis of mechanical properties test results:after homogenization treatment,the tensile strength of as-cast alloy increases slightly,while the elongation decreases.After hot extrusion,the tensile strength of the alloy increases greatly to 201 MPa,with an increase of 121%;the elongation increases sharply to 28.7%,with an increase of 222%,indicating that the mechanical properties of the alloy can be significantly improved by hot extrusion.During annealing,the tensile strength of the alloy decreases gradually with the increase of temperature,and the elongation first increases and then decreases.The mechanical properties of the alloy are the most optimal when annealing at 300?.It tensile strength is 222 MPa,which is 144%higher than that of as cast alloy,and the elongation is 15.75%,which increases by 77%.(3)Analysis of corrosion resistance test results:after homogenization treatment,the corrosion resistance of the alloy is obviously improved,and further improved after hot extrusion treatment.The corrosion resistance of as rolled alloy is poor before annealing,but it is improved after annealing.With the increase of annealing temperature,the corrosion resistance of the alloy first increases,then decreases,and then increases.It is the optimal annealing at 300?,and the change range is the largest than that of as cast alloy.The self-corrosion current decreased by 71%to 8.91?A cm^-2.The self-corrosion potential is-1.55V.The charge transfer resistance increased by 78%,to 1153?;The film resistance increased by 187%,to 1036?;The weight loss rate decreased by 99.6%to0.3289 mg·cm^-2·d^-1.It shows that the corrosion resistance of the alloy be improved effectively under a certain annealing temperature and a constant annealing time.(4)Comprehensive properties analysis:according to the analysis of the microstructure,mechanical properties and corrosion resistance test results of the alloy,the corresponding relationship between the"process-structure-comprehensive properties"of the alloy is established.It can be determined that among the Mg-3Y alloys treated by different processes,the alloy annealed at 300°C has the best overall performance.