Microstructure And Properties Of Biomedical Degradable Zn-1Mg-xSr Alloys

Biodegradable metals have good biocompatibility and degradability,reduce the risk of secondary surgery and medical costs,and the degradation products have no side effects on human body,which has become a research hotspot in the field of medical materials.Zinc is an essential trace element for human body.Its corrosion rate is between magnesium and iron.It has good biocompatibility and suitable degradation rate.It is considered as a new kind of biodegradable biomaterial with great research value and development prospect.However,the mechanical properties of as cast pure zinc are poor,far from the performance requirements of clinical use.It is necessary to use alloying,heat treatment and plastic deformation to control the properties of the alloy,so as to meet the medical performance requirements.In this paper,in order to develop a kind of zinc alloy material with excellent tissue and performance for bone implantation,the microstructure,mechanical properties and degradation characteristics of zn-lmg-xsr alloy were studied through the research idea of Sr alloy,which provided the research basis for the development of a new type of zinc alloy material for bone implantation.The main conclusions are as follows:(1)In the as cast Zn-1Mg alloy,the matrix is α-Zn,the grain boundary is α-Zn and Mg2Zn11,and there are a small amount of non-equilibrium phase MgZn2.After adding Sr element,as cast Zn-1Mg-xSr(where x=0.25,0.5,1 wt.%),SrZn13 phase was formed and the grain size became smaller.The reason is that the solid solubility of Sr in Zn is very low and it is difficult to dissolve in Zn matrix.In the solidification stage,Sr enriched at the front of the solid/liquid interface,increased the supercooling degree of Zn alloy,and increased the nucleation rate.The results show that Sr can significantly refine the structure of as cast Zn-1Mg alloy,and 0.5wt.%Sr has the most obvious refining effect on the alloy,with an average grain size of 26.2μm.After homogenization treatment,the non-equilibrium phase MgZn2 dissolved back into the matrix,the eutectic layer structure disappeared,and the intragranular structure was more uniform.(2)The alloy was hot-rolled at 300℃,and an 8 mm thick plate was rolled to 2 mm with a deformation of 75%.During the rolling process,the Mg2Zn11 phase at the grain boundary was broken and elongated along the rolling direction,and the SrZn13 phase with irregular shape was broken.The dynamic recrystallization occurred in the alloy,and the recrystallization structure fraction of Zn-1Mg-0.5Sr alloy was the largest.(3)With the increase of Sr content,the Vickers hardness and tensile strength of as cast Zn-1Mg-xSr(x=0,0.25,0.5,1 wt.%)alloy increase continuously.The tensile strength of as cast alloy is 128.12MPa,148.65MPa,158.31 MPa and 159.75MPa respectively.After hot rolling,the microhardness and tensile strength of the alloy are higher than that of the as cast alloy due to the synergistic effect of fine grain strengthening and second phase strengthening.The tensile strength of the alloy is increased to 162.78MPa,191.63MPa,211.96MPa and 182.61 MPa respectively.(4)The electrochemical test results show that the dynamic corrosion behavior of Zn-1Mg-xSr alloy is basically the same.With the increase of Sr content in the alloy,the potential is continuously negative shifted.This is because with the increase of Sr content,more SrZn13 phases are distributed in the matrix.As the anode of galvanic corrosion,SrZn13 phase promotes the corrosion of Zn-1Mg alloy and improves the corrosion rate of the alloy.The results show that the corrosion rate fluctuates in the early stage of immersion,and the final corrosion rate decreases and tends to be stable.The corrosion rates of as cast Zn-1Mg-xSr(x=0,0.25,0.5,1 wt.%)alloys were 0.169mm/a,0.171 mm/a,0.174mm/a,0.203mm/a respectively after immersion for 32 days.The corrosion rates of as rolled alloy were 0.155mm/a,0.162mm/a,0.174mm/a,0.196mm/a respectively.In conclusion,in Zn-1Mg-xSr(x=0,0.25,0.5,1wt.)alloys,Zn-1Mg-0.5Sr alloy has the smallest grain and the highest mechanical properties,which is close to the mechanical properties of natural bone.The degradation rate is 0.174mm/a,which can meet the requirement that the corrosion rate of the degradable biological alloy should be less than 0.5mm/a in the simulated environment in vitro.It is expected to become a new type of degradable bone fixation material.