| Magnesium alloys have broad application prospects in the field of lightweight structural materials due to their low density and high specific strength.However,its close-packed hexagonal structure has the disadvantages of low symmetry and few slip systems that can start at the same time,resulting in poor plasticity and anisotropy,which restricts the application range of magnesium alloy.Under the background of increasingly stringent equipment requirements,it is of great practical significance to develop materials with high strength and high plasticity at the same time.Adding alloying elements is one of the effective ways to improve the comprehensive properties of magnesium alloys.Due to the solid solution strengthening effect of Bi elements,Mg Bi alloys can solve the problem of low strength to a certain extent,but still have the disadvantage of poor plasticity.Therefore,adding the third component element on the basis of Mg-Bi alloy can further give consideration to both high strength and high plasticity,and can also provide support for the composition design of magnesium alloy.In this paper,by means of optical microscope（OM）,X-ray diffraction（XRD）,electron backscatter diffraction（EBSD）,microhardness tester and electronic universal testing machine,the microstructure of Mg-2Bi x Zn（x = 0.51,1.03,2.04wt%）alloy as cast,homogeneous and extruded and the changes of mechanical properties under different extrusion ratios（i.e.22:1,19:1）at extrusion temperature of 200 ℃ and extrusion rate of 1.7mm/s were studied.It was found that with the increase of Zn content,the average as-cast grain size decreased from297.57μm to 245.36μm.When the extrusion ratio is 22:1,with the increase of Zn content,the tensile fracture elongation increases and Mg-2Bi-2.04 Zn reaches the maximum（～ 19.2%）.However,the yield strength first increased and then decreased.Mg-2Bi-1.03 Zn alloy reached the peak of yield strength,which was 280.2MPa.When the extrusion ratio is 19:1,the yield strength and elongation increase gradually with the increase of Zn content,and the tensile strength（～ 223.7MPa）and elongation（～ 24.1%）of Mg-2Bi-2.04 Zn alloy reach the maximum.Therefore,the addition of Zn will improve the fracture elongation of Mg-2Bi alloy.Afterwards,the microstructure of Mg-2Bi-y Zr（y = 0.12,0.51,0.65wt%）alloy as cast,homogeneous and extruded and the changes of mechanical properties under different extrusion ratios（i.e.22:1,19:1）of extrusion temperature 200 ℃ and extrusion rate 1.7mm/s were studied.It was found that the average as-cast grain size decreased from 264.78μm to 90.87μm when the added Zr content increased.When the extrusion ratio was 22:1,the tensile yield strength and tensile strength decreased gradually from 236.5MPa and 250.9MPa to 135.7MPa and 211.7MPa respectively.The fracture elongation increases gradually,and Mg-2Bi-0.65 Zr reaches the maximum（～ 21.6%）.When the extrusion ratio was 19:1,the Zr content was added from0.12wt% to 0.65wt%,and the tensile strength,yield strength and elongation decreased gradually from 28.8%,203.3MPa and 197.8MPa to 15.3%,185.1MPa and 176.4MPa respectively.Therefore,the addition of Zr reduces the strength of the Mg-2Bi alloy.Finally,the microstructure of Mg-2Bi-z Gd（z = 0.23,0.46,1.13,1.84wt%）alloy as cast,homogeneous and extruded and the changes of mechanical properties under different extrusion ratios（i.e.22:1,19:1）at extrusion temperature of 200 ℃ and extrusion rate of 1.7mm/s were studied.It was found that with the increase of Gd content,the as cast grain boundary gradually changes from fluvial to locally continuous.When the extrusion ratio is 22:1,with the increase of Gd content,the tensile strength and yield strength first increase,then decrease and then increase.When the mass fraction of Gd element is 0.46%,the tensile strength（～ 256.7MPa）and yield strength（～ 233.3MPa）reach the maximum.The fracture extension first increased and then decreased,and Mg-2Bi-1.13 Gd reached the maximum（～ 9.7%）.When the extrusion ratio was19:1,the tensile strength and yield strength first decreased and then increased,the tensile strength（～ 220.2MPa）and yield strength（～ 201.5MPa）of Mg-2Bi-1.84wt% Gd reached the peak,while the elongation first increased and then decreased.In conclusion,Mg-2Bi-2.04wt% Zn alloy can meet the performance requirements of high strength and high plasticity at the temperature of 200 ℃,extrusion rate of 1.7mm/s and extrusion ratio of 22:1.The tensile strength,yield strength and elongation at break are321.0MPa,267.9MPa and 19.2% respectively. |
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