Research On The Mechanical Properties And Strengthening Mechanism Of Precipitated Phases Reinforced Zn Implants

As an implant material,Zinc（Zn）has attracted extensive attention from researchers due to its good biocompatibility and moderate degradability.However,the Zn grains obtained by the conventional process are relatively coarse,which are difficult to effectively hinder the dislocation movement during plastic deformation.Moreover,the lattice of Zn alloy belongs to the close-packed hexagonal system with few slip systems,which leads to the mechanical strength and ductility are difficult to meet the requirements of bone repair.For these reasons,in this study,laser powder bed fusion（LPBF）was used to prepare Zn implants to refine the microstructure due to its rapid solidification properties.At the same time,the precipitation phase and nanophase were introduced to Zn alloy to strengthen and toughen the implant through grain boundary strengthening,dislocation shearing and load transfer.The main research contents and innovations of this study were as follows:1.The effect of precipitation on the mechanical properties of LPBF-prepared Zn implants was investigated.It is found that the solute element Cu dissolved in Zn liquid phase during melting,andε-Cu Zn₅ precipitated preferentially during cooling.Subsequently,theε-Cu Zn₅acted as heterogeneous nucleation sites and reacted with the surrounding liquid phase to generate refinedη-Zn grains.On the other hand,the unprecipitated Cu element in the solute precipitated into the Cu enriched Zn solid solution.Due to precipitation strengthening,the compressive yield strength,ultimate tensile strength and microhardness of Zn-Cu implants were increased to 165.7 MPa,223.4 MPa and 97.2HV,respectively.The in vitro cell experiments proved that the implants had good biocompatibility.2.In order to improve the weak interface between the precipitates and Zn matrix,the formation mechanism of semi-coherent precipitates during LPBF process and its correlation with mechanical properties were studied.It is found that the（Fe,Mn）Zn₁₃ phase formed by preferential co-precipitation of Mn,Fe and Zn element can serve as nucleation substrate of Mn Zn₁₃ to remarkably refine the precipitation phase and Zn matrix.More importantly,the formed semi-coherent interface solved the problem of weak interfacial bonding between the precipitated phase and Zn matrix.Therefore,the precipitates can effectively shear and transfer dislocations during plastic deformation,thereby improving the mechanical properties of Zn implants.In addition,the degradation behavior was modified from localized pitting corrosion to uniform degradation due to the large amount of secondary precipitates and grain refinement.3.In view of the low ductility of Zn,the mechanical toughening mechanism of nanophase CNTs@Ag on Zn implants was investigated.CNTs are ideal toughening material due to their ultrahigh Young’s modulus and large aspect ratio,but their reinforcing effect is limited by their easy agglomeration and weak interfacial bonding with Zn matrix.It was found that Ag nanoparticles grown on CNTs reacted with Zn to precipitate Ag Zn₃interface phase during the LPBF process,which effectively improved both the dispersibility and interfacial compatibility.The ductility and ultimate tensile strength of the implants were improved to 16.1%and 221 MPa due to the additionally activated slip systems and enhanced load transfer effect of CNTs.Moreover,the Zn-CNTs@Ag implants also exhibited excellent antibacterial activity due to the broad-spectrum bactericidal effect of Ag ions.