Microstructure Evolution Of Mg-13Gd-3.5Y-2Zn-0.5Zr Alloy Fabricated By Repetitive Upsetting And Extrusion

The equipments with lightweight and high performance have always been an important research direction in the field of aerospace,military and other high-tech technology.It is necessary for high-performance equipments to use lightweight and high-strength materials and design the large-scale integrated complex structure.Rare earth（RE）containing Mg alloys have become atrractive materials due to their high strength,excellent heat resistance and good creep resistance.At present,high performance and large-scale cone-shaped magnesium alloy componets are promising for aerospace industry.Because of the special structure and mechanical properties of such components,it is necessary to prepare the billet by plastic deformation at first to improve the microstructure uniformity of the billet and enhance its mechanical properties for subsequent component forming.The repetitive upsetting-extrusion（RUE）technology belongs to the severe plastic deformation,which can not only refine the grain significantly and improve the mechanical properties of the alloy,but also ensure the profile of the billet to be almost invariable.Thus,it is an ideal method for processing the cylindrical bar billets.However,up to now,there are few researches focused on the preparation of Mg-Gd-Y-Zn-Zr alloys by RUE technology,and the application of RUE in large-scale bar billet has not been reported.So it is urgent to carry out related investigation on the issues.In this paper,the Mg-13Gd-3.5Y-2Zn-0.5Zr（wt.%）alloy was used as the research material.The original as-cast microstructure of the alloy was characterized and the effect of different homogenization process parameters（temperature and time）on the microstructure and mechanical properties of the alloy were studied.The thermal compression behavior and microstructure evolution of as-homogenized alloy were studied by the thermal compression simulation test,and the reasonable processing parameters were determined according to the thermal deformation processing diagram.The effects of different deformation parameters on the deformation load,microstructure and mechanical properties of the alloy during the process were studied by isothermal RUE simulation experiments.Based on the experimental results,the RUE with decreasing temperature technology was performed on two bar billets with different scale,and the microstructure evolution and mechanical properties of the alloy during the process were investigated.The results showed that the as-cast Mg-13Gd-3.5Y-2Zn-0.5Zr alloy was mainly composed ofα-Mg matrix,eutectic compounds consisting of Mg₅（Gd,Y,Zn）phase and intergranular long period stacking ordered（LPSO）phases,a small amount of intragranular lamellar LPSO phase and rich-RE cubic phase.After homogenizing treatment（515°C×16h）,the eutectic compounds were completely transformed into discontinuous distributed block-shaped LPSO phase and the rich-RE cubic phases were dissolved.The tensile mechanical properties of the as-homogenized alloy at room temperature were obviously improved,with the yield strength of 172.1MPa,the tensile strength of 212.3MPa,the elongation of 3.1%,respectively.In addition,it still exhibited good mechanical properties at high temperature that tensile strength was 237.2MPa at 200°C.The thermal deformation activation of the homogenized alloy is Q=263.165 kJ/mol.The constitutive equation was established:（?）..The thermal process parameters of the alloy were determined by establishing the heat processing diagram with strain of 0.5:380⁴40°C/0.001⁰.006s^-1 and 460⁵00°C/0.001⁰.05s^-1,respectively.The peak power dissipation,reached to 44%,and the dynamic recrystallization（DRX）ratio at high temperature reached to 17.3%.The specimens of Ф10×25mm were used for the isothermal RUE process test and the temperature was in the range of 420°C⁴80°C.As the deformation velocity was constant,the higher the temperature was the lager the load was needed.During the process,with the increase of cumulative strain,the load gradually decreased,and the effect of deformation temperature on load was weakened.As the deformation velocity was in the range of0.03mm/s to 0.17mm/s,for the various cumulative strains condition,the extrusion load kept a decline trend with decreasing deformation velocity.Under a certain condition,the dynamic recrystallization proceeded continuously when the cumulative strain was increased.Moreover,the existence of the lamellar LPSO phase and precipitated particles on the grain boundaries retards the growth of the DRXed grains,leading to that region of fine grains expanded and replaced the original coarse grains gradually.In the process of RUE deformation at different temperature and deformation velocity,the process of microstructure refinement was obviously different,which affects the refining efficiency of the whole microstructure and the refinement of recrystallized grain.The Ф50×200mm bar was fabricated by repetitive upsetting-extrusion process with decreasing temperature and the temperature was decreased from 480°C to 370°C pass by pass.The results showed that the process was an efficient grain refinement method which exhibited high level of overall microstructure refinement efficiency.This may attributed to the inhibit effects on the grain growth by amount of precipetied particles and low deformation temperature.In the initial stage of deformation,there was a strong baseal texture and uneven deformed microstructure of the alloy.With the increase of the RUE passes,the texture was gradually weakened,and the heterogeneity of microstructure was improved significantly.After 6 passes,the cumulated strain was 8.1,the temperature was 370°C,the average grain size was decreased to about 3.4μm,and the ultrafine grain region（<1μm）was obtained.The block LPSO phases at the grain boundaries were broken into small bars and uniformly dispersed along the extrusion direction.Moreover,fine rods with length of 2μm to 4μm were distributed like a band in the local region.The lamellar LPSO phase in the grains were gradually dissolved into the matrix and precipitated in some DRXed regions again.A large number of Mg₅（Gd,Y,Zn）phase particles precipitated dispersely which mean size was less than 2μm.The 3-passes RUE with decreasing temperature was conducted on the Ф330×1000mm bar for billet preformation,and the high quality billet for subsequent forming was successfully obtained.The room temperature tensile strength,yield intensity and elongation rate reached 385MPa,342.1MPa and 7.1%,respectively.Meanwhile,the homogeneity of the microstructure and mechanical properties of the billet were significantly improved.The difference of tensile strength,yield strength and elongation between center and edge of the billet from 37.5MPa,27.1MPa and 0.4%after the first pass of RUE,and finally was reduced to only 12.4MPa,8.1MPa and 0.3%,respectively.