Microstructure And Properties Of High Plasticity Mg-Bi(-Zr) Alloy At Room Temperature

This study carried out the casting heat treatment extrusion deformation processing of magnesium alloy.In this paper,by adding Bi and Zr elements into Mg,changing the content of alloy elements and deformation processing parameters,the room temperature plasticity of Mg alloy is improved.The effects of Bi and Zr content and extrusion process on the microstructure and mechanical properties of Mg-Bi(-Zr)alloy were studied.Finally,the high ductility Mg-Bi(-Zr)alloy with room temperature tensile elongation higher than40%was obtained,and its high plasticity mechanism was studied.The microstructure of as cast Mg-Bi alloy consists of two parts?-Mg matrix and the second phase Mg₃Bi₂ particles are formed together.With the increase of Bi content in the alloy,the amount of the second phase also increases.After adding Zr element,the grain size of the as cast alloy becomes smaller obviously;After solution treatment,the dendrite segregation of as cast alloy decreases significantly,the second phase is partially dissolved,and the grain size does not change significantly.After the solution treatment,the extruded Mg-Bi(-Zr)alloy has a very fine recrystallization structure with a grain size of micron.The alloy extruded at low temperature and low extrusion speed has a dual-mode structure.Compared with the as cast alloy,its mechanical properties have been greatly improved,especially the plasticity,which is higher than the commonly used magnesium alloy,and the room temperature elongation reaches 30%.Under the same extrusion process,with the increase of Bi content,the proportion of recrystallization zone increases first and then decreases,the yield strength and tensile strength do not change significantly,both in a certain range,while the room temperature tensile elongation increases first and then decreases with the increase of Bi content,and reaches the maximum when the Bi content is 2.5wt.%.The as cast magnesium alloy can be refined by adding Zr element.The room temperature plasticity of the as cast magnesium alloy is significantly improved after extrusion.The results show that different extrusion processes have different effects on the microstructure and properties of the alloy.On the whole,the alloy with high extrusion ratio(25:1)has higher recrystallization ratio and more uniform microstructure.The plasticity of the alloy is generally better than that with low extrusion ratio(12:1),and the yield strength is lower.The addition of Zr shows the opposite rule;When the extrusion speed is in the range of 0.5?2.0 mm/s,with the increase of extrusion speed,the elongation of the alloy increases first and then decreases,and reaches the maximum at 1mm/s;When the extrusion temperature is in the range of 125?300?,the elongation first increases and then decreases with the increase of temperature.The elongation of the alloy extruded at 150?is the largest and the room temperature plasticity is higher.After annealing treatment,the microstructure of as extruded alloy will change obviously,and the recrystallization will increase significantly.It can be clearly seen that there is no bimodal structure in the alloy and the grain size increases.The higher the annealing temperature,the shorter the complete recrystallization time and the larger the grain size;The higher the annealing temperature is,the more obvious the work hardening phenomenon is,the lower the room temperature tensile elongation is,and the range of uniform strain first increases and then decreases.The as extruded Mg-2.5Bi alloy with room temperature elongation of 37.4%can be obtained by extruding with extrusion ratio of 25:1 and extrusion speed of 1 mm/s at 150?.After annealing at 200?for 60 min,the room temperature elongation of the alloy can reach 40%,which is much higher than that of the commonly used magnesium alloy.And the strain rate sensitivity of the alloy at room temperature is very obvious.Through the strain rate"jump"experiment,the strain rate sensitivity index m is about 0.23(strain rate is 1.0×10^-4s^-1?1.0×10^-5s^-1),This indicates that grain boundary slip has a significant contribution to the high ductility of Mg-2.5Bi alloy at room temperature.The reason why Mg-Bi(-Zr)alloy has high plasticity at room temperature is that it has fine recrystallized grains and high recrystallization ratio,which promotes the grain boundary sliding at room temperature.