Biodegradation Study Of Magnesium Alloy For Biodegradable Cranial Fixation System

Magnesium(Mg)and its alloy are very promising biomedical implant materials not just for temporary orthopedic and cardiovascular stents but also for cranial fixation system because of their biodegradability,biocompatibility and bioactivity properties.Mg is also naturally found in the body,meaning that it offers a potential degradable material that can support bone remodeling,and its gradual degradation in the physiological environment encourages its acceptability as a biodegradable implant and also reduces the risk associated with secondary surgery as in the case of permanent metallic implants.However,a very high degradation rate can post a problem.There is an optimal level of biodegradability for every application because it is not desirable for the material to degrade completely before tissue healing.In order to develop an alloy with an appropriate degradation response,the relationship between microstructure and bio-corrosion rate needs to be known before its potential deployment as a biomedical implant.In this study,the corrosion behaviors of pure Mg and Mg-2Zn-0.5Nd alloy under different processing routes and parameters are presented,and the stress corrosion mechanism under pre-deformation was also systematically studied to reveal their corrosion response after tensile and compressive deformations.The microstructural characterizations were analyzed using optical microscopy(OM),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),X-ray diffraction(XRD)and electron backscattered diffraction(EBSD)while the corrosion response was analyzed using potentiodynamic polarization(PD),electrochemical impedance spectroscopy(EIS),and immersion test.The major findings are presented below.The effect of recrystallization and local misorientation induced by various rolling parameters on the biodegradation of pure Mg was studied systematically.Results reveal that increase in recrystallization fraction and reduction in local misorientation of grains as the rolling temperature increased were beneficial to reduce the degradation rate of Mg.There is basal texture variation arising from the occurrence of high twinning activity at a lower rolling temperature which suppressed the maximum texture intensity and also increased the degradation rate.The degradation rate generally increased with decreasing rolling temperature(i.e.350?<250?<100?),indicating that the higher rolling temperature was beneficial to reduce the degradation rate.The influence of twinning and local misorientation on the biodegradation behavior of Mg sheets was systematically investigated at different rolling directions,i.e.,cross rolling and unidirectional rolling.The in vitro degradation rate of the unidirectional rolled sheets was less than that of the cross rolled sheet due to the detrimental effect of residual stress arising from twinning and local misorientation in the grains of the cross rolled Mg sheet.Stamping processing is commonly used to manufacture medical devices and implant,however,for biodegradable Mg alloy,the stamping will influence the degradation behavior because of the change in microstructure after stamping.So in this study,the as-rolled Mg-2Zn-0.5Nd alloy was processed by stamping,the microstructure,crystallographic orientation and corrosion performance of this processing method were investigated to reveal the influence of the stamping process on the degradation rate of the rolled Mg-2Zn-Nd alloy.The in vitro degradation result showed that the degradation rate of the rolled Mg-2Zn-0.5Nd increased from 0.2mm/year to 0.5mm/year after stamping processing.The result revealed that the activation of the {1012} twin during stamping can remarkably weaken the {0001} basal texture and have a significant influence on the degradation rate of the stamped Mg-2Zn-0.5Nd sheet.After removing the deformation by annealing,the degradation rate was reduced to 0.15mm/year.In the present study,pure Mg,Mg-2Zn,Mg-2Zn-0.5Nd and Mg-2Zn-0.5Nd-0.5Zr were subjected to tensile and compressive loading before immersion in Hank's solution to evaluate their degradation behaviors after pre-deformation.The electron backscattered diffraction technique used to analyze the microstructure after plastic deformation revealed that the addition of alloying elements Zn,Nd and Zr to Mg affected the grain size,which played a major role in the twinning activity,residual stress and dislocation density after plastic deformation.The high twin volume fraction in Mg and Mg-2Zn after plastic deformation escalated their corrosion rate,while the refined grain in Mg-2Zn-0.5Nd and Mg-2Zn-0.5Nd-0.5Zr suppressed the twinning activity,which was beneficial to their corrosion resistance after pre-deformation.The kernel angular misorientation(KAM)value revealed a higher concentration of residual stress and dislocation density in both pure Mg and Mg-2Zn after tensile and compressive loadings as compared to Mg-2Zn-0.5Nd and Mg-2Zn-0.5Nd-0.5Zr,which also increased the degradation rate of pure Mg and Mg-2Zn after plastic deformation.