Study On The Microstructure Evolution And Mechanical Properties Of The Mg-12Gd-4.5Y-2Zn-0.4Zr Alloy Produced By Server Plastic Deformation

Magnesium alloy is known as the lightest metal structural material,which has low density,high specific stiffness and high specific strength.The addition of rare earth elements can further improve the mechanical properties of magnesium alloys,and greatly expand the application fields of magnesium alloys.However,as-cast rare earth magnesium alloys have a series of disadvantages such as inhomogeneous microstructure and poor mechanical properties,which limits its application to some extent.The severe plastic deformation(SPD)is one of the methods of strengthening and toughening materials by increasing the amount of accumulated strain to achieve grain refinement and texture weakening,and to improve mechanical properties.Most of the SPD processes are complex and have high production costs.In view of the limitations of the repetitive upsetting extrusion process,a new server plastic deformation method of one-step continuous upsetting-extrusion-upsetting short process is proposed,which can effectively refine the grain and weaken the texture.This method only needs one-step heating deformation to realize upsetting,extrusion and extrusion-upsetting composite deformation,shorten the process and improve on efficiency,which has obvious engineering significance.However,so far,the microstructure evolution and its influence on the mechanical properties are still unclear.In this paper,the Mg-12Gd-4.5Y-2Zn-0.4Zr alloy is the research object.The microstructure of the as-cast alloy is analyzed by optical microscopy,scanning electron microscopy,transmission electron microscopy and electron backscattered diffraction.The influence of different homogenization treatment process parameters(temperature and time)on the microstructure of the alloy are studied.The effect of different deformation process parameters on the microstructure and mechanical properties of the alloy are analyzed during the isothermal repetitive upsetting extrusion.The influence of different deformation temperatures on the microstructure and mechanical properties of the alloy are studied during the one-step upsetting-extrusion-upsetting process.The results showed that the second phase in the as-cast Mg-12Gd-4.5Y-2Zn-0.4Zr alloy was mainly composed of fine-lamellar LPSO phase,block-shaped LPSO phase,Mg₅(Gd,Y,Zn)eutectic compound and a small amount of RE-rich phase.After homogenization treatment at520?for 16 hours,the eutectic compound was completely transformed into a discontinuously distributed block-shaped LPSO phase,and the fine-lamellar LPSO phases were completely dissolved.The tensile mechanical properties of the alloy at room temperature were higher than the as-cast alloy.The tensile strength was 236.8MPa,the yield strength was 114.6MPa,and the elongation after fracture was 6.4%at room temperature.The ?20×50mm sample was used for isothermal repetitive upsetting extrusion test.With the increase of the deformation passes and temperature,and the decrease of the deformation speed,the grains were gradually refined.The number fraction of the low angle grain boundaries was decreased,and the high angle grain boundaries was increased,the average dislocation angle gradually was increased.The maximum pole density of the texture was gradually decreased,and the degree of the maximum pole density region of the(0001)crystal plane from the equatorial zone to the two poles was increased.With the increase of cumulative strain,the dynamic recrystallization process was continued.The fine-lamellar LPSO phase and the precipitated particle phase effectively hindered the growth of recrystallization grains and the microstructure was refined.The ?53×210mm sample was used for a one-step continuous upsetting-extrusion-upsetting test.In the upsetting section,a strong base texture was produced.In the extrusion section,the grains were elongated along the extrusion direction,resulting in an extrusion texture.In the extrusion-upsetting section,the grains were significantly refined.When the deformation at 480?,the average grain size was refined to 9.5?m,and the volume fractions of recrystallization reached 76.8%.The discontinuously distributed block-shaped LPSO phase at the grain boundary was broken into small blocks,and a large number of Mg₅(Gd,Y,Zn)phase particles distributed homogeneous with the size of less than 2?m were precipitated.With the increase deformation temperature,the maximum pole density of the texture was gradually decreased.At 440?,the cumulative strain reached 4.0,the average grain size of the alloy obtained by the repetitive upsetting extrusion process was 29.2?m,the ultimate tensile strength was309.8MPa,and the elongation at break was 8.4%;while the average grain size of the alloy obtained by the one-step continuous upsetting-extrusion-upsetting process was refined to14.7?m,the ultimate tensile strength was increased to 371.4MPa,and the elongation at break was increased to 12.3%.The comprehensive mechanical properties were better than that of the repetitive upsetting extrusion.