Fabrication And Degradation Of Gradient Magnesium Alloys For Medical Application

Magnesium and its alloys are becoming more and more popular in the field of medical implant due to their unique advantages,such as being an essential element of human body,density close to human bones and elastic modulus close to human bone.However,the phenomenon of local corrosion leading to structural failure under physiological conditions limits its further development in clinical.Although many researchers proposed to slow down the degradation rate of the material by means of corrosion-resistant coating or surface modification,the combination of general surface treatment and magnesium alloy is not firm or the coating itself is not degradable,which are not conducive to the degradation protection of the outer material on the magnesium alloy body.Meanwhile,the coating treatment material as a medical implant cannot have good biocompatibility.Therefore,with the development of research on biodegradable medical implant materials,it is great significance to develop a new type of medical implant material with good biocompatibility and safe degradation in vivo.In this thesis,pure Mg and ZK60 were selected as the matrix materials.The gradient magnesium alloy material of ZK60 alloy on the outer layer and pure Mg on the inner layer were casted in semi-solid state by the inclined plate melt treatment technology,and the preparation process,nucleation mechanism,micro structure,phase formation,interface diffusion mechanism,heat treatment process,corrosion resistance,degradation performance and biocompatibility of the materials were systematically studied.The conclusions in this investigation are summarized as following:(1)The best semi-solid casting process was to use a 30 0 inclined plate,controled the casting temperature to 670?,and the temperature of melt into the mold was 610?.(2)The microstructure analysis of the material showed that the bonding degree of the inner and outer layers of the solution treated gradient magnesium alloy was stronger than that of the semi-solid nested gradient material,and the diffusion coefficient of the outer layer ZK60 alloy to the Mg side is 1.168,which is higher than that of the Mg side to the outer layer ZK60 alloy,which is 0.213.(3)The electrochemical experiments showed that the corrosion resistance of the gradient material increased gradually from the outside to the inside.The corrosion potential of gradient magnesium alloy increased from-1.977 V in external metal layer to-1.869 V in internal metal layer.The corrosion current density decreased from 7.1732×10-4 A/cm2 in external layer to 1.5450×10-4 A/cm2 in internal metal layer,and the capacitive arc radius decreased gradually.After solution treatment,the corrosion current density decreased from 5.8570×10-4 A/cm2 in external metal layer to 1.5362×10-4 A/cm2 in internal metal layer,and the capacitive arc radius decreased gradually.The corrosion resistance after solution treatment was better than that of semi-solid casting gradient magnesium alloy.(4)The results showed that the corrosion rate of ZK60 alloy in the gradient magnesium alloy was higher than that of the inner layer,and the corrosion degree of the interface was between ZK60 and Mg.The corrosion products were mainly magnesium oxide,zinc oxide,zirconia,magnesium chloride and magnesium carbide.The overall corrosion rate of the material was 0.58 g/a.The pH value of SBF solution increased from 7.4 to 7.85 with the increase of the immersion time of the gradient material.(5)The cytotoxic test showed that the three materials of pure Mg,ZK60 and gradient magnesium alloy had no cytotoxic effect.The biocompatibility of gradient magnesium alloy was close to that of pure Mg,and its cytotoxic reaction was graded as 0,which met the biocompatibility requirements of clinical medical implants.