Researches On Preparation,Deformation And Superhydrophobic Modification Of Biomedical Degradable Rare-earth Magnesium Alloys

Compared with the biological implant materials,biomedical degradable magnesium and its alloys have the advantages of non-toxicity,fast biodegradation,second operation avoiding and no stress shielding.However,the corrosion rate of the magnesium is too fast to use in human body.To improve the corrosion resistance of the magnesium and its alloys,adding small amount of rare earth(RE)element and adopting suitable plastic deformation which could improve the microstructure and the corrosion form of magnesium alloys are used.The most effective and direct way is the surface treatment,which can provide a coating inhibiting on the magnesium surfaces the corrosion in a period of time.In this paper,the experiments were started from the smelting and RE-doped biodegradable Mg-Gd-Nd-Zn-Zr magnesium alloys have been designed.To find out the optimalizing alloy composition,the effect of Gd element to the microstructures and property of this alloy was investigated.Moreover,the influence of the equal channel angular pressing(ECAP)on the evolution of microstructures,texture and corrosion mechanism was explored.Then the superhydrophobic surfaces were fabricated on Mg-Gd-Nd-Zn-Zr alloys through hydrothermal treatment,micro-arc oxidation(MAO)and polymer plating,and the corrosion resistace relationship of the superhydrophobic surface with different contact modes was studied.The main conclusions were as follows:(1)The Mg-xGd-1.5Nd-0.5Zn-0.5Zr alloys(Mg-xGd,x=1,3,5)were fabricated by gravity casting and the solution-treated Mg-xGd alloys with different Gd was obtained by heating at 540 °C for 16 h.The as-cast Mg-xGd alloys was made up of ?-Mg,island-like eutectic ?1-(Mg,Zn)3RE phase dispersed around the grain boundaries and cuboidal phases(REH2).With the increasing of Gd content,the grain sizes of the alloys were decreased.Comparatively,the eutectic ?1 phase disappeared and cuboidal REH2 phases still existed in the solution-treated alloys.What's more,large amount of flower-like ZnZrx phase emerged from the substrate during the quenching.The tensile strength of the solution-treated alloys increased with the Gd content,which of the Mg-5Gd alloys reached 217.5 MPa.The immersion test results showed that the as-cast Mg-3Gd alloy presented the best corrosion resistance with the corrosion rate of 0.285±0.05 mm/a.Due to the fine flower-like ZnZrx phase,which formed the galvanic interaction with the ?-Mg,the corrosion resistance of solution-treated Mg-3Gd alloy was the worst(0.973±0.10 mm/a).(2)The as-cast and solution-treated Mg-3Gd alloys were extruded for different passes through ECAP.The fine ?1 phase precipitated from the as-cast during ECAP.Dynamic recrystallization appeared at 4 passes,and the grain was refined to about 2.5 ?m.With the passes increased,the refining effect in the grain was obvious and the precipitation became coarse.In the solution-treated alloys,large amount of ?1 phase precipitated at 1 passes.The yield strength and tensile strength reached the largest after 8 passes(231.6 MPa,274.1 MPa)owing to the texture strengthening,grain refinement and precipitation strengthening.Meanwhile,the elongation of solution-treated alloy presented better(8 passes,25.2 %)resulted by the texture weakening and grain refinement.(3)The corrosion property of the as-cast and solution-treated Mg-3Gd alloys after ECAP was studied,conclusion could be made that the corrosion resistance of the as-cast alloys climbed up and then declined with the passes increasing,the corrosion rate of 4 passes sample was the lowest(0.145 ± 0.017 mm/a).On account of the grain size,the quantity of second phase and the crystal orientation,the corrosion form of the as-cast alloys have changed from the localized corrosion with large pitting holes to the uniform corrosion with small and fine pitting holes.Oppositely,the corrosion resistance of the solution-treated alloys increased with the passes increasing.The corrosion rate arrived at 0.228 ± 0.023 mm/a at 8 passes.As a consequence of grain size,the quantity and distribution rule of second phase,the corrosion form of the solution-treated alloys have changed from general corrosion with large area to the combination of intergranular corrosion and couple corrosion.(4)A novel one-step hydrothermal treatment was developed to fabricate the superhydrophobic coating with rough structure and low surface energy materials on the Mg-3Gd alloy.The composition of these coatings was mainly calcium stearate and hydroxyapatite.The coating consisted of hierarchical structure with different size of slice sheets.The top layer is comparatively thick,which is made up of the combination of calcium stearate and hydroxyapatite.The bottom layer is thinner,which is composed of calcium stearate.The contact angle of this hydroxyapatite coating on the Mg-3Gd alloy surface with the red of 6:4 is 152.8 °.The corrosion current of this coating in Hank's solution was 2.03×10-9 A·cm-2,which reduced four orders of magnitude compared with the substrate.The hemolysis rate of this coating is just 0.016 %,which satisfied the requirement of biomedical materials.(5)To investigate the effect of water contact modes on the corrosion resistance of superhydrophobic surfaces,the superhydrophobic coatings with Wenzel mode and Cassie-Baxter mode on the Mg-3Gd alloy and ME20 M alloy were fabricated by MAO + polymer plating and MAO + etching + polymer plating,respectively.The result of electrochemical test showed that the corrosion resistance of the superhydrophobic surface with Cassie-Baxter mode is apparently better than that with Wenzel mode.Their ratio is equal to their actual surface contact area with the liquid.