Development Of Biodegradable Zn-Ag-Zr Alloys And Their Properties Evaluation

In recent years,biodegradable alloys have attracted much attention from researchers because of their excellent mechanical properties and good biodegradability.Because of the proper degradation rate which is between that of the Mg based alloy and that of the Fe based alloy,the biodegradable Zn based alloy is considered to be a biodegradable implant material with great potential.But the current research on Zn based biodegradable alloy is relatively lacking and it is just in the stage of exploration currently.The purpose of this project is to design a type of Zn based biodegradable alloy which is non-toxic to humans,with good mechanical properties and appropriate degradation rate.To develop the biodegradable alloys which fit the clinical requirements.We selected the non-toxic silver?Ag?and zicornium?Zr?elements,and improve the mechanical properties of Zn based alloys by alloying methods.In the meantime,to provide reference for medical use,we will study the micro-structure,the mechanical properties and the biodegradability of Zn based alloy.With the addition of Ag and Zr,the second phase precipitates were formed in the as cast Zn-Ag-Zr alloys.Two inter-metallic compounds were formed by Zn and Zr:Zn₂Zr₃,a long strip of inter-metallic compound,and an irregular and circular inter-metallic compound Zn₂Zr.With the increase addition of Zr,the Zn₂Zr₃ of the elongated inter-metallic compound increased.Zn and Ag formed a kind of points like inter-metallic compound,AgZn.There is some Ag solid-solutionized in the Zn matrix.With the increase of Ag content,the AgZn of the intermetallic compounds increased gradually.The content of Zn-1Ag-2Zr is the largest in the strip of intermetallic compound Zn₂Zr₃ and exhibited the highest hardness value,up to130.7HV_0.1.The tensile strength of reached 206MPa,which is 10 times as that of the cast pure zinc.The elongation of the Zn-1Ag-2Zr alloy is 11.06%.In the simulated body fluid medium,the degradation rate of Zn-Ag-Zr alloy and pure zinc was studied by weight loss method.Zn-Ag-Zr alloy and pure zinc were degraded rapidly at the initial stage?1st-5th days?,but it achieved the steady state gradually.The degradation rate of pure zinc was higher for 1E-4 g/?m²·d?.The degradation rate of Zn-Ag-Zr alloys were close,in the range of 6.7E-6 g/?m²·d?to 9.7E-6 g/?m²·d?.The degradation rate of pure zinc is two orders of magnitude higher than that of Zn-Ag-Zr alloy.The experimental results showed that the degradation rate of Zn-Ag-Zr alloy could be adjusted by the addition of alloying elements.For the studying of degradation of the alloys in stress condition,the experiment about stress corrosion of Zn-Ag-Zr alloy by slow strain rate tensile were done both in air and in simulated body fluid medium in the tensile stress state.It was observed that the mechanical properties of Zn-Ag-Zr alloy were decreased and the tensile strength was decreased by 20MPa under the stress state,which indicated that there is the stress corrosion tendency in the Zn-Ag-Zr alloy.The tensile strength of Zn-1Ag-2Zr alloy was 189MPa in air,and it was 157MPa in simulated body fluid,and the stress corrosion sensitivity coefficient is 0.2144.The experimental results showed that Zn-1Ag-2Zr alloys had the less tress corrosion susceptibility and the minimum corrosion tendency under the stress state,which confirms the conclusion that the Zn-1Ag-2Zr alloys had the lowest degradation rate by weight loss method.The degradation behavior of Zn-Ag-Zr alloys was also investigated by electrochemical experiments in simulated body fluid and saline medium.Compared to Zn-1Ag-0.5Zr and Zn-1Ag-2Zr,with the same Ag content but Zr content increased,the impedance radius of the alloy becomes larger,indicating that addition of alloying elements Zr can improve the corrosion resistance of the alloy.The EIS impedance spectra were measured in saline medium:compared to Zn-1Ag-0.5Zr and Zn-1Ag-2Zr,the same Ag content,Zr content changed had little effect on impedance of the alloy;that the addition of Zr,Cl^—will make little influence on the degradation of alloy.Zn-0.5Ag-1Zr and Zn-2Ag-1Zr,compared with the same Zr content,with the increase of Ag content,the impedance radius becomes larger,indicating that alloy degradation rate was slow,that the addition of Ag element can improve the degradation performance in the environment of Cl^—.The degradation rate of Zn-0.5Ag-1Zr is the fastest,0.171 mm/year,indicating the effect of Cl^—on Zn-1 Ag-0.5Zr;Zn-1Ag-2Zr;Zn-1.5Ag-1.5Zr;Zn-2Ag-1Zr,the degradation rate is about0.075 mm/year,indicating that Cl^—has little effect on the four kinds of alloy.