Microstructure And Mechanical Properties Of Extruded + Rolled Mg-Y-Zn-Ni(Co) Alloys

In recent years,with the rapid development of the global transportation industry,the demand for high-performance and light-weight alloy are increasing in manufacturing industry and aerospace.Compared with traditional materials,Mg alloys have high specific strengths,low density,good electromagnetic shielding performance,damping and shock absorption,which has great potential for development in the above fields.However,the hexagonal close-packed structure of Mg alloys results in poor strength and plasticity at room temperature,which limits their wide development and application.By adding rare earth elements（RE）and transition elements（TM）to Mg alloys,a long period stacking ordered structure phase（LPSO）can be formed,which can effectively improve the mechanical properties of Mg alloys.Therefore,it is of great application significance to investigate the microstructure and strengthening mechanism of LPSO for Mg alloys.The structural type,morphology,distribution and volume fraction of LPSO phases can be changed through alloying and hot working,thus affecting the mechanical properties of Mg alloys.However,most of the current research are only focus on the effects of addition of single transition elements or single hot working,while there are few studies on microstructure and properties of Mg alloys using composite transition elements and composite processing technology.Therefore,in this paper,Mg-2Y-0.5Zn-0.5Ni（MYZN）and Mg-2Y-0.5Zn-0.5Ni-0.5Co（MYZNC）（at.%）alloys were prepared by composite processing（hot extrusion and hot rolling）,and the main contents are as follows:Firstly,the microstructures and mechanical properties of MYZN alloy are investigated by composite processing（extrusion+rolling）.The results show that the microstructure of extruded MYZN alloy is mainly composed ofα-Mg,lamellar and blocky 18R LPSO phase,fine strip-like 14H LPSO phase,and white granular Mg₂Y and Mg₂₄Y₅ phases.After hot rolling,the alloy occurs an uneven plastic deformation.With the increase of the rolled temperature,the grain size of the alloy increases while the volume fraction of the second phase decreases gradually.The tensile test results indicate that the extruded+rolled alloy at 350℃exhibits the best tensile properties at room temperature.The yield strength(σ_0.2),tensile strength(σ_UTS)and elongation to failure（ε_f）are 372 MPa,409 MPa and 8.4%,respectively.Compared with the extruded alloy,σ_0.2 andσ_UTS increase by 10.7%and 4.1%,respectively.After hot rolling at350℃,a large number of recrystallization regions form and the recrystallization grain and particle phases are also significantly refined and crushed.The average grain size slightly increases.The volume fraction of LPSO phase increases and its distribution becomes more dispersed.Simultaneously,the strong basal texture appears in the rolled alloy at 350℃.The high tensile properties of extruded+rolled MYZN alloy are mainly attributed to the precipitation strengthening and kink strengthening of LPSO phase,grain boundary strengthening aroused from a large number of low angle grain boundaries,and texture strengthening.This composite processing could provide theoretical basis and technical guidance for the design and processing of Mg alloys with high performance.Secondly,the as-cast,homogenized and extruded MYZNC alloys are prepared by adding Zn,Ni and Co to Mg-2Y alloy.It is found that the as-cast MYZNC alloy is mainly composed ofα-Mg,gray lamellar 18R LPSO phase,white irregular granular Mg₂Y phase,Mg Y（Co,Zn,Ni）₄ phase and Mg₃（Co,Zn,Ni,Y）phase.After heat treatment,part of the 18R LPSO phase is transformed into 14H LPSO phase,which precipitates in the grain in a strip-like shape.After hot extrusion,these second phases in MYZNC alloy are dispersedly distributed along the extrusion direction,and the grain size is also refined to～3.03μm.σ_UTS,σ_0.2 andε_f of the extruded MYZNC alloy are 280 MPa,225MPa and 11.7%,respectively,and the hardness of the alloy reaches 64.7 HV.The better mechanical properties of the extruded MYZNC alloy than that of the as-cast and homogenized ones is mainly attributed to the fine grain structure and the high volume fraction of LPSO phase in the extruded alloy.The order of strain hardening capacity of three alloy is as-homogenized MYZNC alloy（n=0.35,Hc=1.13）>as-cast MYZNC alloy（n=0.33,Hc=0.97）>extruded MYZNC alloy（n=0.18,Hc=0.36）.The higher strain hardening ability of as-cast and as-homogenized MYZNC alloys is because of the presence of a large number of second phase particles and strip-like 14H LPSO phase.In contrast,the low strain hardening capacity of the extruded MYZNC alloy may be low dislocation density due to the existence of a large number of recrystallization regions,and the low expansion rate of dislocations due to the existence of fine grains and textures in the alloy.Finally,in order to further improve the mechanical properties of the extruded MYZNC alloy,the extruded alloy is rolled at 2,4 and 8 passes at 400℃.The effects of rolling passes on the microstructures and mechanical properties of extruded MYZNC alloy are studied.The tensile results at room temperature show that the mechanical properties of MYZNC alloy firstly increase and then decrease with the increase of rolling passes.The best mechanical properties of MYZNC alloy are obtained at 2 pass,whoseσ_UTS,σ_0.2 andε_f are 350 MPa,305 MPa and 10.6%,respectively.Compared with extruded alloy,σ_UTS andσ_0.2 increase by 25%and 35.6%,respectively.The elongation of 4 pass and 8 pass rolled alloys remains stable and does not decrease obviously(ε_f-4pass=10.4%,ε_f-8pass=10.5%),indicating that the alloy has a good strength-plasticity.The high mechanical properties are due to the LPSO phase strengthening and part of the bending of the phase also coordinates the plastic deformation of the alloy.Also,the High Angle grain boundaries（HABs）are transformed into the low Angle grain boundaries（LABs）during rolling,this also hinders dislocation movement and improves the strength of the alloy.In addition,the softening effect of the alloy during the rolling and the existence of LABs with high density（～76.5%）further improve the elongation of the alloy.