| Recently,it is the increasing demand for high strength and toughness magnesium alloys used as structural materials in national defense and military industries,especially for 450 MPa grade magnesium alloy components.Looking at the development history of magnesium alloys,magnesium alloys containing LPSO structure phase have obvious reinforcement and toughening,and have attracted extensive attention.However,its application is limited due to the lack of systematic understanding of the whole process of magnesium alloy containing LPSO structural phase from material to component preparation,especially the effects of LPSO structural phase morphology and volume fraction on the properties,and the microstructure evolution during large plastic deformation are still unclear.In this paper,the Mg-8.5Gd-4.5Y-xZn-0.4Zr(x=0,0.7,1.5,3 wt.%)alloys(hereinafter referred to as 0Zn,0.7Zn,1.5Zn and 3.3Zn alloys)were designed,the microstructure and properties of the alloys during as-cast,homogenization,extrusion and aging were studied,especially for the effects of LPSO phase morphology and volume fraction on the properties,after that the preliminary application verification was carried out.The microstructures of as-cast alloys were studied.It was found that 0Zn alloy was mainly composed of α-Mg matrix and Mg5(GdY)phase.With the increase of Zn content,the volume fraction of Mg5(GdY)phase gradually decreased,while the volume fraction of LPSO structure phase gradually increased.The lamellar structure in 0.7Zn,1.5Zn and 3.3Zn alloys was LPSO structure phase or stacking fault phase,and the square phase in the four alloys was rare earth-rich phase.When the as-cast alloy was tensioned,the cracks mainly originated at the interface of Mg/LPSO structure phase or in the bulk LPSO structure phase.The results showed that the Mg5(GdY)phase transition temperature was 552.4℃ and the LPSO phase transition temperature was 520℃.After homogenization treatment,the Mg5(GdY)phase could be completely redissolved in the matrix,the lamellar LPSO phase could be mostly redissolved in the matrix,but the bulk LPSO phase could only be redissolved in a small part.The homogenization parameters of the four alloys were determined.The hot deformation behaviors of the alloys at high temperature were studied,and the constitutive equations and processing maps were established.The deformation activation energy of 0.7Zn,1.5Zn and 3.3Zn alloys with LPSO structure phase was above 220 kJ/mol,which was generally higher than that of magnesium alloys without LPSO structure phase The microstructure evolution mechanisms in stability domain were dynami recrystallization and lamellar structure precipitation,while the instability mechanisms in instability domain were 45° shear cracking,pores and microcracks,and local plastic flow With the increase of Zn content,the easy-to-process area in the processing map gradually changed from the high temperature and medium deformation rate area to the low temperature and low deformation rate area.The optimal hot deformation parameters of the four alloys were determined to be 400℃/0.001 s-1.Under the optimal deformation conditions,the dynamic recrystallization ratio gradually increased with the increase of Zn content.Under the optimum deformation conditions,the extrusion experiments of the alloy were carried out.It was found that the microstructures of all the alloys were completel recrystallized after extrusion.The grain sizes of 0Zn,0.7Zn,1.5Zn and 3.3Zn alloys wer 7.9 μm,4.9μm,5.9 μm and 5.7 μm,respectively.The strengths of the alloy increased firs and then decreased with the increase of Zn content,while the elongation after fracture doe not change obviously with the increase of Zn content.The mechanical properties of 0.7Z alloy were the best.The aging experiments of as-extruded alloys were carried out.It was found that th aging strengthening effect of alloys decreased gradually with the increase of Zn conten and the corresponding peak aging parameters of 0Zn,0.7Zn and 1.5Zn alloys wer obtained,while the aging strengthening effect of 3.3Zn alloy was not obtained.The tensil strength of 0.7Zn alloy was 468 MPa and the elongation after fracture is 8.5%.The hig performance of the alloy was due to the synergistic strengthening of lamellar structure an phaseβ’.During the corrosion process,the LPSO structure phase accelerated the corrosio dissolution of the alloy matrix.The basic research on multi-directional forging of 0.7Zn alloy was carried out,whe the samples deformed at the initial forging temperature of 500℃,the grain sizes after 3,9 and 12 passes were 44μm,2.8 μm,2.2 μm and 1.49 μm,respectively.After the lamella structure was precipitated,kinking occurred in the subsequent deformation process,and th block LPSO structure phase gradually be broken during forging process.With the increas of deformation passes,the texture gradually wreaked and the mechanical propertie gradually increased,but the increase of the mechanical properties in the first 6 passes wa larger,and then the increase was slower.Based on the actual production condition,the application verification of the multi-directional forging was carried out.The surface quality of the blank after 6 passes of multi-directional forging was good.The precision forming of the forging was realized under the forming temperature field of the high temperature die and low temperature of the blank.After aging treatment at 200℃/28 h,the tensile strength of the forging was 478 MPa,and the elongation after fracture was 7.0%. |
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