Research On The Microstructure Evolution And Mechanical Properties Of Mg-Zn-RE-Zr Alloys During Extrusion Processes

In this doctoral dessertation,the effect of rare earth elements?Nd,Ce,Gd and Y?and extrusion processes on the Mg-6Zn-0.5Zr?ZK60?alloy were studied.The evolution of microstructures,substructures and the morphology of intermetallic phases,the crystal orientation relationship of intermetallic phases and ambient temperature and elevated temperatures mechanical properties were studied in different conditions,which include as-cast,as-extruded and ageing conditions.The key research of interests is listed as follows:1.Microstructure refinement effect of rare earth elements on ZK60 alloy and the difference among various rare earth elements in refining ZK60 alloy.2.Effects of various processing techniques?such as extrusion temperatures and aging processes?on microstructure and mechanical properties of magnesium alloys.3.Evolution of composition and morphology of intermetallic compounds and crystal orientation relationship between two intermetallic compounds during extrusion and aging process.ZK60 alloy and Mg-6Zn-1.5Nd-0.5Zr?ZKNd602?alloy were prepared by gravity casting using a steel mould with water cooling device,then they were extruded into rods with extrusion temperatures of 290?,305?,320?,340?and 360?,extrusion speed of 0.4 mm/s and extrusion ratio of 18.It is found that the grain size of the as cast and extruded alloys can be significantly reduced by rare earth Nd addition.With the increase of extrusion temperature,the dynamic recrystallization grains of the two alloys all grow up,while the volume fraction of the non-recrystallized zone decreases.However,the change of the dynamic recrystallization and volume fraction of the non-recrystallized zone of ZKNd602 alloy is smaller than those of ZK60 alloy.When the extrusion temperature was increased from 290?to 360?,dynamic recrystallization grain size of ZKNd602 alloy increased from less than 1?m to 2?m,the volume fraction of non-crystallized zone decreased from 28%to 23%.For ZK60alloy,the two data are from less than 4?m to 9?m and decreased from 53%to 11%.It is shown that extrusion temperature has less influence on the microstructure of ZKNd602 alloy than that of ZK60 alloy.The dynamic recrystallization grain size of the extruded ZKNd602 alloy is measured to be 0.42?m by the bright-field TEM image and grain size statistics,and the grain size changes little during the aging process.The type of dislocations of the dynamic recrystallization region in ZKNd602 alloy is mainly<a>type dislocation with a small amount of<c+a>type dislocation.There are still some deformation twins,such as{10-12}twin.There are a large amount of subgrains and dislocations in the non-recrystallized region,the size of the subgrain is approximately0.56?m,and the distortion angle between adjacent subgrains is less than 4 degrees.It is noted that most of the dislocations in sub-grains are<a>type,but<c+a>type on sub-grain boundaries.Compared with the dislocations in the dynamic recrystallization grains,there are much more<c+a>typed dislocations in non-recrystallized zone.The intermetallic compound in the ZKNd602 alloy is Mg₃Zn₃Nd₂ phase?W phase?and(Mg_1-x Zn_x)₁₁Nd phase?T phase?,which is detected by HAADF-STEM,and they follow the following crystal orientation relationship as:?110?_W//?002?_T and[111]_W//[100]_T.The small angle X-ray scattering?SAXS?analysis showed that there were new precipitates precipitated in the extruded ZKNd602 alloy during the aging treatment process,and hardly grew up with the increase of aging time.It is confirmed that the new phase precipitated in the matrix is Mg₄Zn₇ phase during the aging process and the size is about 4nm by TEM/HRTEM.The mechanical properties of two kinds of extruded alloy were tested,and it was found that the alloy had higher strength when the extrusion temperature was lower.When the extrusion temperature is 290?,the tensile strength and yield strength of ZKNd602 alloy are 421MPa and 402MPa,and those of ZK60 alloy are 371MPa and 338MPa respectively.Compared with ZK60 alloy,the tensile strength and yield strength of ZKNd602 alloy increase by 13%and 19%respectively.After aging treatment,the strength of ZKNd602 alloy increases,especially the yield strength,but the elongation decreases significantly.The high temperature strength of the extruded ZKNd602 alloy is obviously higher than that of ZK60 alloy,the tensile strength and yield strength in 150?tensile test of ZKNd602?E290?alloy are 34%and 37%higher than those of ZK60?E290?alloy,respectively,and which in200?tensile test of ZKNd602?E290?alloy are 41%and 43%higher than those of ZK60?E290?alloy.The effect of Ce,Gd and Y elements on the microstructure and mechanical properties of ZK60 alloy was also investigated in this dessertation.The conclusions are as follows:The related factors of grain refinement effect of the studied four rare earth elements on ZK60 alloy in an order from strong to weak were Y>Nd>Gd>Ce,and the ability to precipitate intermetallic compounds in ZK60 alloy from strong to weak was Nd>Ce>Y>Gd.The strength of the extruded alloys containing rare earth elements under theoptimizedextrusionprocessfromhightolowwas ZKCe602>ZKNd602>ZKY602>ZKGd602.Based on the casting results,it can be concluded that Ce addition will significantly exacerbate the hot tearing behavior of the ZK60 alloy.So it was hard to be casted or extruded.Ce element is not a good choice for strengthening ZK60 alloy.The tensile test results of ZK60 alloy,ZKNd602 alloy and ZKGd602 alloy were fitted by Hall-Petch relationship,the results indicated that:the grain sensitivity of the alloys containing rare earth elements is stronger than that of ZK60 alloy.Meanwhile,the solid solution strengthening effect of ZKNd602 alloy is lower than that of ZKGd602 alloy,but the grain refinement strengthening effect of ZKNd602 alloy is higher than that of ZKGd602 alloy.In this dessertation,the relationship between the dynamic recrystallization grain size d and the extrusion deformation absolute temperature T was fitted according to the linear relationship between the d and the Z parameter,which is according to the Zener-Hollomon parameter.