Zhangzhong Wang

Microstructure and corrosion resistance of as cast Mg–Nd–Gd–Sr–Zn–Zr alloys for biomedical applications

Abstract Mg–(4–x)Nd–xGd–0·3Sr–0·2Zn–0·4Zr (wt-%, x = 0, 1, 2 and 3) alloys were prepared by gravity casting. Microstructure was analysed by X-ray diffraction, optical microscopy and scanning electron microscopy respectively. The α-Mg and Mg41Nd5 phases existed in the as cast alloys with 4% and 3% Nd addition, while the new Mg3Gd was found in the alloys with 3% Gd addition. The volume fraction of the second phase reduced with increasing Gd addition and the microstructure was refined by the Gd addition. The corrosion resistance of the alloys was evaluated by mass loss, hydrogen evolution and Tafel polarisation tests. The results showed that the corrosion rate of the alloys decreased with increasing Gd addition even though the corrosion rates of the alloys obtained by different methods showed different values. Moreover, the corrosion morphologies of the alloys were uniform.

Effects of solution treatment on yield ratio and biocorrosion behaviour of as-extruded Mg-2?7Nd-0?2Zn-0?4Zr alloy for cardiovascular stent application

Abstract Biodegradable magnesium alloy with proper yield strength, high ultimate strength and elongation is a potential candidate for cardiovascular stent. Nevertheless, magnesium alloy with high ultimate tensile strength always exhibits high yield strength, which is not suitable for the plastic deformation of cardiovascular stent during implantation. Solution treatment was conducted on the as extruded Mg–2·7Nd–0·2Zn–0·4Zr alloy in order to reduce the high yield ratio, and the biocorrosion behaviour of the alloy was also evaluated in artificial plasma. The results show that the grains of the alloy grow, and the second phase reduces with increasing solution treatment temperature. The yield strength decreases gradually, and the ultimate tensile strength still keeps a high level due to the remarkable workhardening. The yield ratio of the alloy reduces from 92 to 57%. The corrosion resistance of the alloy after solution treatment is improved slightly when the temperature is <475°C.

Influence of Gd and Ca on microstructure, mechanical and corrosion properties of Mg–Gd–Zn(–Ca) alloys

Abstract Microstructures, mechanical and corrosion properties of the Mg–xGd–1Zn (x=3, 6, and 9 wt%) and Mg–9Gd–1Zn–yCa (y=0, 0.2, and 0.6 wt%) alloys were studied and compared for biomedical applications. The results show that the microstructures of the alloys are composed of primary α-Mg, eutectic phase, lamellar long period stacking ordered (LPSO) phase, and MgxGd compounds. The grain size is apparently refined and the volume fraction of the eutectic phase increases with increasing Gd. The grain size is not reduced and the divorced eutectic phase is formed with Ca addition. Gd plays a more positive role in the improvement of yield strength while Ca shows a higher enhancement on microhardness. The higher Gd addition results in worse corrosion resistance of the Mg–xGd–1Zn alloy, nevertheless, the alloy with 0.6 wt%Ca has the best corrosion resistance among the Mg–9Gd–1Zn–yCa alloys. The as-cast Mg–3Gd–1Zn alloy exhibits the lowest corrosion rate, and is a promising alloy for biodegradable applications.

Corrosion behaviour of Nd ion implanted Mg–Gd–Zn–Zr alloy in simulated body fluid

Abstract Neodymium (Nd) ion implantation was conducted on the Mg–Gd–Zn–Zr alloy to improve its corrosion resistance in simulated body fluids for biomedical applications. X-ray photoelectron spectroscopy reveals that a hybrid structure layer composed of Nd2O3, Gd2O3 and metallic Nd was formed in the implanted layer after Nd ion implantation. The electrochemical test and hydrogen evolution test demonstrate that the enhanced corrosion resistance of the implanted alloy was achieved under a proper dose. Nevertheless, the corrosion characteristics of the alloys obtained by the two test methods showed a different trend, and the hydrogen evolution test provided a greater reliability of evaluated result. It was found that the corrosion rate decreased first with increasing dose and then increased rapidly and even higher than that of the bare alloy.

Microstructure and biocorrosion behaviors of solution treated and as-extruded Mg–2.2Nd–xSr–0.3Zr alloys

Abstract Mg–2.2Nd– x Sr–0.3Zr alloys ( x =0, 0.4 and 0.7, mass fraction, %) were prepared by gravity casting. Solution treatment was conducted on the as-cast alloys to homogenize microstructure, and hot extrusion was subsequently conducted. Microstructure was observed using an optical microscope and a scanning electron microscope. Biocorrosion behaviors of the alloy in simulated body fluid were analyzed by mass loss, hydrogen evolution and Tafel polarization experiments. The results show that the amount of residual eutectic phase of the solution treated alloys increases with increasing Sr addition, and the grains are significantly refined after hot extrusion. The corrosion resistance of the solution treated alloys deteriorates apparently with increasing Sr addition, while the corrosion resistance of the as-extruded alloys is improved with Sr addition. Nevertheless, the biocorrosion behavior of the as-extruded alloys obtained by Tafel polarization shows different trends from those obtained by the other two methods.

Effect of heat treatment on microstructure, mechanical properties and in vitro degradation behavior of as-extruded Mg-2.7Nd-0.2Zn-0.4Zr alloy

Abstract Mg-2.7Nd-0.2Zn-0.4Zr (mass fraction, %) alloy was designed for degradable biomedical material. The ingots of the alloy were solution treated and then hot extruded. The extruded rods were heat treated with aging treatment, solution treatment and solution+aging treatment, respectively. Microstructures of the alloy were observed by optical microscopy (OM) and scanning electron microscopy (SEM). Mechanical properties at room temperature were tested. In vitro degradation behavior of the alloy immersed in simulated body fluid was measured by hydrogen evolution and mass loss tests. The degradation morphologies of the alloy with and without degradation products were observed by SEM. The results show that the grains grow apparently after solution treatment. Solution treatment improves the elongation of as-extruded alloy significantly and decreases the strength, while aging treatment improves the strength and reduces the elongation of the alloy. The yield ratio is reduced by heat treatment. The in vitro degradation results of the alloy show that solution treatment on the as-extruded alloy results in a little higher degradation rate and aging treatment on the alloy can reduce degradation rate slightly.

Enhanced mechanical and corrosion properties of NZ20K alloy by double extrusion and aging processes for biomedical applications

In order to refine and homogenise microstructure and to improve mechanical properties of the magnesium alloys for biomedical application, the Mg–2.0Nd–0.1Zn–0.4Zr (wt-%, NZ20K) alloy with low amount of alloying elements was processed by double extrusion and aging processes (DEAP). Compared to the NZ20K alloy after once extrusion and aging processes (OEAP), the microstructure becomes finer and more homogeneous, and the elongation is raised up ∼121% by DEAP. Moreover, the yield strength and ultimate tensile strength are improved ∼27 and ∼16% respectively. Additionally, the corrosion rate of the NZ20K alloy by the DEAP is a little slower than that of the OEAP. It can be concluded that the mechanical properties, as well as the corrosion resistance of the NZ20K alloy, are able to be enhanced by the double extrusion and aging processes.

Electric field assisted chemical conversion process of AZ91D magnesium alloy in HCO3−/CO32− solution

Abstract A new method for synthesizing Mg–Al hydrotalcite conversion coating on AZ91D Mg alloy was developed by the application of electric field (EF). By using EF technique, the formation time of the coating can be significantly reduced. The SEM results indicate that a continuous and compact Mg–Al hydrotalcite coating is formed on the surface of Mg alloy after short time EF treatment. However, a long time treatment would make the coating partially exfoliate. The corrosion current density ( J corr ) of the coated sample (EF1+1 h) is approximately two orders of magnitude lower than that of Mg alloy substrate. The test of electrochemical impedance spectroscopy (EIS) and immersion corrosion also suggest that the coating can effectively protect Mg alloy against corrosion.

Surface nanocrystallization by mechanical punching process for improving microstructure and properties of Cu-30Ni alloy

Abstract A punching process was employed to produce nanocrystalline on the surface of Cu-30Ni alloy. Compared with the original sample, corresponding changes of the grain size, mechanical properties and corrosion resistance of surface nanocrystalline induced by punching process were characterized by atomic force microscopy, nanoindentation, microhardness, electrochemical behavior and electron work function. It was observed that the grain size was about 40 nm in the topmost surface. The increasing mechanical properties were also demonstrated. Additionally, the dynamic polarization test showed that the surface nanocrystallization improved the resistance to electrochemical attacks. Such increasing corrosion resistance was identified with the higher EWF value. This study demonstrates that surface nanocrystallization can be generated by punching process, and thus mechanical properties and corrosion resistance of alloy can be improved.

Effect of ageing on mechanical and impact properties of non-quenched and tempered steel wire strengthened by cold forming

The effect of ageing on the mechanical and impact properties of non-quenched and tempered steel wire strengthened by cold forming to an area reduction of 25.5% was investigated by tensile testing, charpy impact tests and scanning electronic microscopy. It was found that when the specimen was aged at 300 °C for 2 h, its tensile strength, yield strength, elongation and impact absorption energy increased by 10.8, 18.1, 44.8 and 19.6%, respectively. In addition the ductile-brittle transition temperature decreased by 5 °C. These improved properties are attributed to the decreasing dislocation density, the formation of sub-grains in a stable condition and the uniform dispersion of a fine precipitation phase during the ageing treatment.