The Study On The Microstructure,Tensile Properties And Corrosion Behavior Of Low-alloyed Extruded Mg-Bi Based Alloys

With favorable castability,excellent specific strength and stiffness as well as its similar modulus of elasticity to human bones,Mg alloys are considered as potential biodegradable bone implants.However,uncontrollable corrosion rate and unsufficient mechanical properties of magnesium alloys inhibit the wide-spread use in service for implant production.Generally speaking,it is an effective way to improve the comprehensive properties of magnesium alloys to regulate the microstructure by alloying and plastic deformation.At the same time,low alloying can effectively improve the deformation capacity and comprehensive properties of the alloy.In addition,it can reduce the adverse effect caused by the second relative corrosion and reduce the preparation cost.Based on non-toxic Mg-Bi alloy alloy system as the foundation,through Zn,Ca,Sn,Mn microalloying and extrusion deformation methods to regulate the microstructure of alloy,to conduct the thorough research to the mechanical properties and corrosion behavior of the alloy,reveal the complete alloy corrosion mechanism,the micro structure and finally optimize the design and composition.The research results of this paper are as follows:(1)The extruded Mg-1Bi-1Zn-x Ca(x=0,0.6 and 1.2 wt.%)alloy was designed and prepared.These alloys presented a typical bimodel grain structure,and the texture strength increased with the increase of Ca content.After the addition of Ca,the second phases Mg₃Bi₂at the nanometer scale were uniformly distributed in the alloy and Mg₂Bi₂Ca at the micron scale were distributed along the extrusion direction.The experimental results show that Mg-1Bi-1Zn-0.6Ca alloy has the best combination of mechanical properties and corrosion resistance.The research also shows that the second phase and grain orientation have a great influence on the comprehensive properties of the alloy.(2)The extruded Mg-0.5Bi-0.5Zn-0.5Ca/Sn/Mn alloy was designed and prepared,and the optimization effect of Ca,Sn and Mn on Mg-Bi-Zn alloys was discussed.The content of alloy element was reduced.The results show that Mn element has the best grain refinement results,Ca element has the greatest influence on the second phase of the alloy,and Sn element has the best corrosion resistance under the premise that the mechanical properties of the alloy meet the requirements.(3)Further study on the microstructure of extruded Mg-0.5Bi-0.5Zn-0.5Sn alloy shows that the grain orientation distribution of the alloy was relatively dispersed.The TEM results shows there were few nano-sized Bi-rich phases existed in grains,and the effect of the particles on the corrosion behavior of the studied alloy will not be apparent.During the short-term immersion,pitting will preferentially propagate in the aggregation area of non-basal grains and fine grains after random pitting occurs on the surface of the alloy.During the long-term immersion,the corrosion morphology of studied alloy changed from groove dominated to uniform round pit based due to the balance between dissolution and the precipitation of Mg(OH)₂.The above results show that for low-alloying Mg-Bi alloy,the uniform grain structure with more base grain is beneficial to obtain good performance,and the comprehensive performance of the alloy can be improved by reducing the type and quantity of the second phase as much as possible.The above research results provide some experimental data and theoretical basis for the control of corrosion properties of magnesium alloys by tailoring the microstructure.