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A new type of patented biodegradable biomedical magnesium alloy Mg-Nd-Zn-Zr hereafter, denoted as JDBM was prepared in this study. The biocorrosion properties of the as-extruded JDBM alloy were investigated in simulated body ï¬‚uid (SBF) by hydrogen evolution, mass loss and electrochemical tests. The biocorrosion properties of as-extruded AZ31 and as-extruded WE43 alloys as well as the mechanical properties at room temperature were also studied in order to compare with the novel JDBM biodegradable biomedical magnesium alloy. The results show that the as-extruded JDBM alloy not only owns much better mechanical properties at room temperature but also exhibits much better biocorrosion properties in SBF.
Magnesium alloys have been extensively studied as biodegradable implants in the ï¬elds of bone ï¬xation devices, cardiovascular stents and tissue engineering scaffolds in the last decade, due to biodegradation, good mechanical properties and other characteristics . However, the magnesium alloys currently under investigation as implant materials are mostly commercial alloys such as AZ91 [2-4], AZ31 [5-7], WE43 [4,8,9] and WE54 . None of the above mentioned alloys have been originally developed to be a biodegradable implant material. Al3+ ions can combine with inorganic phosphates easily, leading to a lack of phosphate in the human body. Al is a risk factor for Alzheimer's disease and can cause muscle ï¬bre damage . Even though a lot of data of Al containing magnesium alloys in vitro and in vivo are available today, it is recommended that Mg-Al systems should just be used as experimental alloys to investigate the improvements of processing and surface modiï¬cation technologies in biomedical applications. For the use in humans, it is recommended to use Al-free magnesium alloy systems . Rare earth elements (RE) introduced into magnesium alloys can strengthen the material by solid solution strengthening and precipitation strengthening. Nevertheless, the results of evaluation of short-term effects of rare earth used in magnesium alloys on primary cells and cell lines show that the highly soluble Dy and Gd seem to be more suitable than Y, suitable elements with low solid solubility could be Eu, Nd and Pr .
In this study, Mg-3.09Nd-0.22Zn-0.44Zr (wt.%, JDBM) alloy was designed in order to develop a novel biodegradable magnesium alloy for biomedical applications. Neodymium was selected as the main alloying element, accompanied with the microalloyed Zn and Zr to develop a quaternary patent alloy. Nd is one of light rare earth elements. Mg-Nd binary alloys have already exhibited signiï¬cant strengthening effect. Recent research also indicates that Nd doesn't exhibit cell toxicity . Addition of small amount of Zn addition into Mg can enhance the ductility and deformability. Moreover, Zn is also one of the abundant nutritionally essential elements in the human body. Zirconium is a powerful grain-reï¬ning agent in Al-free Mg alloys. The biocompatibility of small amount of zirconium in magnesium alloy has been veriï¬ed . In the present study, the microstructure, mechanical properties at room temperature and biocorrosion properties in SBF of JDBM were studied and compared with the commercial AZ31 and WE43 alloys.
The ingots of JDBM (wt.%: 3.09Nd, 0.22Zn, 0.44Zr, 0.003Fe, 0.001Ni, 0.001Cu, 0.003Si, 0.001Mn and balance Mg), AZ31(wt.%: 2.89Al, 0.92Zn, 0.25Mn, 0.004Fe, 0.002Ni, 0.002Cu, 0.003Si and balance Mg) and WE43(wt.%: 3.94Y, 2.29Nd, 0.88Gd, 0.32Zr, 0.003Fe, 0.001Ni, 0.001Cu, 0.002Si, 0.001Mn and balance Mg) alloys were heated to 350 °C and then were extruded with extrusion ratios of 10 and extrusion rate of 2 mm/s. Tensile tests were carried out on a material test machine at room temperature with a strain rate of 1.7-10−3s−1. The data were the average value of ï¬ve tested specimens.
The specimens for immersion test were polished, cleaned in distilled water and ethanol and then dried in warm air. Simulated body ï¬‚uid (SBF), composed of 8.0 g/L NaCl, 0.4 g/L KCl, 0.35 g/L NaHCO3, 0.2 g/L MgSO4·7H2O, 0.14 g/L CaCl2, 0.06 g/L Na2HPO4 and 0.06 g/L KH2PO4, was used as the test solution. The PH value was adjusted to 7.4 with NaOH or HCl solution before experiments, and the temperature was kept at 37±0.5 °C. The ratio of SBF volume to specimen surface's area is 30 ml/cm2 according to ASTM G31-72. For hydrogen evolution test, the specimens were respectively put in beakers containing SBF. A funnel was placed over the specimen and a burette was mounted over the funnel and ï¬lled with SBF. The test lasted for 240 h and the immersion solution was renewed every 24 h. After the immersion test, the corrosion products were removed in a chromic acid solution (200 g/L Cr2O3+10 g/L AgNO3). Five specimens were measured for each alloy.
Electrochemical impedance spectroscopy (EIS) and polarization curves of the alloys were measured in SBF at 37±0.5 °C using a PARSTAT 2273 instrument. EIS measurement was begun after stabilization of the open circuit potential for an hour. The experiments were performed at open circuit potential with AC amplitude of ±5 mV over the frequency range of 100 kHz to 100 mHz. The polarization curves measurement was subsequently conducted with a scan rate of 1 mV/s. The polarization started from a cathodic potential of −50 mV relative to the open circuit potential and stopped at an anodic potential about −1.3 V.
3. Results and discussion
3.1. Microstructure and mechanical properties
Fig. 1 shows the optical images of the as-extruded alloys. It is visible that the grains of JDBM are much ï¬ner than those of AZ31 and WE43. Moreover, the grain sizes of the JDBM distributes uniformly while those of the AZ31 and WE43 distributes non-uniformly since some large grain size is more than 20 μm but some ï¬ner grain size is less than 2 μm.
Table 1 listed the mechanical properties of Mg alloys extruded under the same condition. It is clear that JDBM exhibits the best strength and elongation among the three alloys. Some amount of the Nd is kept in solid solution and therefore Nd can strengthen the alloy by solid solution strengthening. Nd can form intermetallic phases Mg12Nd with Mg . The intermetallic phase acts as obstacles for the dislocation movement at elevated temperatures and causes precipitation strengthening. Furthermore, Zr is an effective grain-reï¬ning agent in Al-free magnesium alloys and contributes to strengthening due to the formation of ï¬ne grains . Additionally, microalloying of Zn enhances the activation of non-basal dislocations at room temperature which can improve the ductility signiï¬cantly . Therefore, the mechanical properties of the JDBM alloy are much better than the other two alloys.
3.2. Biocorrosion properties
The hydrogen evolution results and mass loss results of the Mg alloys are shown in Fig. 2. It can be seen that the hydrogen evolution (Fig. 2a) of all the three alloys shows the same trend: the hydrogen evolution rate decreases with the increased immersion time. The hydrogen evolution rate of the Mg alloys presents the following order: JDBM b AZ31 bWE43. It means that the corrosion resistance of JDBM is the best in SBF. The same results were obtained from the mass loss test (Fig. 2b), which shows the corrosion rate of JDBM is much slower than AZ31 and WE43. The corrosion morphology of the JDBM after immersion in SBF for 240 h is uniform pitting corrosion and partial polished surface was not corroded while the surface of AZ31 and WE43 undergo severe corrosion.
The polarization curves and EIS spectra of the Mg alloys are shown in Fig. 3a and b, respectively. The polarization curves (Fig. 3a) of the three alloys show the same tendency but the corrosion potential (E corr) and the pitting corrosion potential (Ept) are different. The Ecorr of the JDBM is more positive than AZ31 and WE43. The Ept is an important parameter and indicates the tendency for localized corrosion. A more positive E pt means a less likely localized corrosion . It can be seen that the E pt values of the alloys follow the order: JDBM N AZ31 N WE43, indicating the localized corrosion can be more difï¬cult to occur on JDBM than on AZ31 and WE43. The corrosion current density of the JDBM, AZ31 and WE43 obtained from these curves are 5.25-10−7 A/cm2, 5.70-10−7 A/cm2 and 1.61-10−6 A/cm2, respectively. The EIS results (Fig. 3b) reveal that all the Mg alloys exhibit a similar single capacitive loop except for the difference in the diameter of the capacitive loop. The single capacitive loop was also obtained from AZ31 , AZ91 and Mg-Zn-X (X =Ca, M, Si) alloys . This means the corrosion mechanism of the alloys is the same but the corrosion rate is different. JDBM alloy with larger diameter means better corrosion resistance. The improved biocorrosion resistance of the JDBM is probably due to the proper composition of the alloys and ï¬ner and more homogeneous microstructure.
The biocorrosion properties as well as mechanical properties of the as-extruded Mg-Nd-Zn-Zr (JDBM) alloy were studied and compared with commercial AZ31 and WE43 extruded under the same parameters. The grains of JDBM are ï¬ner than those of AZ31 and WE43. The mechanical properties of JDBM are also much better than AZ31 and WE43. The biocorrosion resistance of the JDBM alloy in SBF obtained by hydrogen evolution, mass loss and electrochemical tests is much better than AZ31 and WE43 alloys. The JDBM alloy with good mechanical properties and biocorrosion resistance will be a promising degradable biomaterial.