Sep 2014, Volume 8 Issue 3

Cover illustration

  • Corrosion image of Mg–6Zn–1Mn–4Sn–0.5Y alloy was characterized by electron probe microanalysis. It exhibits both general corrosion and pitting corrosion, occurred in simulated body fluid during the immersion test. Hydrogen bubbles were released in the crater-like corrosion pit, which initiated at the α-Mg matrix neighboring intermetallic compounds such as Mg2Zn. The seabed-like morphology resulted from the in vitro degradation of α [Detail] ...

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    Guang-Ling SONG,Rong-Chang ZENG
    Xue-Nan GU,Shuang-Shuang LI,Xiao-Ming Li,Yu-Bo Fan

    Magnesium has been suggested as a revolutionary biodegradable metal for biomedical applications. The corrosion of magnesium, however, is too rapid to match the rates of tissue healing and, additionally, exhibits the localized corrosion mechanism. Thus it is necessary to control the corrosion behaviors of magnesium for their practical use. This paper comprehensively reviews the research progress on the development of representative magnesium based alloys, including Mg--Ca, Mg--Sr, Mg--Zn and Mg--REE alloy systems as well as the bulk metallic glass. The influence of alloying element on their microstructures, mechanical properties and corrosion behaviors is summarized. The mechanical and corrosion properties of wrought magnesium alloys are also discussed in comparison with those of cast alloys. Furthermore, this review also covers research carried out in the field of the degradable coatings on magnesium alloys for biomedical applications. Calcium phosphate and biodegradable polymer coatings are discussed based on different preparation techniques used. We also compare the effect of different coatings on the corrosion behaviors of magnesium alloys substrate.

    Zhen-Tao YU,Ming-Hua ZHANG,Yu-Xing TIAN,Jun CHENG,Xi-Qun MA,Han-Yuan LIU,Chang WANG

    Developing the new titanium alloys with excellent biomechanical compatibility has been an important research direction of surgical implants materials. Present paper summarizes the international researches and developments of biomedical titanium alloys. Aiming at increasing the biomechanical compatibility, it also introduces the exploration and improvement of alloy designing, mechanical processing, microstructure and phase transformation, and finally outlines the directions for scientific research on the biomedical titanium alloys in the future.

    Rong-Chang ZENG,Lei WANG,Ding-Fei ZHANG,Hong-Zhi CUI,En-Hou HAN

    The microstructure evaluation, surface morphology, chemical compositions and phase analysis of the biomedical Mg--6Zn--1Mn--4Sn--1.5Nd/0.5Y (ZMT614--1.5Nd/0.5Y) alloys were investigated by means of optical microscopy, EPMA, X-ray EDS, XRD and FTIR. The corrosion behavior was evaluated using weight-loss measurement, hydrogen evolution, electrochemical and pH measurements. The results demonstrate that the microstructure for both ZMT614--1.5Nd alloy and ZMT614--0.5Y alloy is characterized by α-Mg and intermetallic compounds, most of which are distributed along the grain boundaries. These second phases contain Mg2Zn, Mg2Zn11, Mg2Sn and single metal Mn, together with Mg12Nd phase for the ZMT614--1.5Nd alloy, and with Mg24Y5 phase for the ZMT614--0.5Y alloy. Honeycomb-like corrosion product layers form. The corrosion resistance of the ZMT614--0.5Y alloy is higher than that of the ZMT614--1.5Nd alloy, which is ascribed to the addition of the element Y into the alloy delaying the corrosion initiation in comparison to that of Nd element in the alloy.

    Cheng-Long LIU,Yi ZHANG,Chun-Yan ZHANG,Wei WANG,Wei-Jiu HUANG,Paul K. CHU

    In this work, we report on synergistic effect of chloride ion and albumin on the corrosion of pure magnesium through corrosion tests. We show that the adsorption of albumin mainly affects the anodic polarization behavior of pure magnesium in NaCl solution. Low concentration of albumin enhances the reaction reactivity of pure magnesium and the initial evolvement of hydrogen at the initial immersion time. Addition of 1 g/L albumin provides limited corrosion control for pure magnesium in NaCl solution. In comparison with low concentration albumin, addition of 10 g/L albumin can effectively inhibit the further dissolution of pure magnesium in test solutions with NaCl concentration of 0.2--0.8 wt.%, but this effect lowers gradually with increasing the concentration of chloride ion.

    Shi-Jie ZHU,Qian LIU,Ya-Feng QIAN,Bin SUN,Li-Guo WANG,Jing-Min WU,Shao-Kang GUAN

    The biomagnesium alloys have been considered to be one of the most potential biodegradable metal materials due to its good mechanical compatibility, biological compatibility, biological security and biodegradable characteristics. However, the two major problems of high degradation rates in physiological environment and low mechanical properties prevent the development of biomagnesium alloys. In the present work, the samples of Mg--Zn--Y--Nd alloy were prepared by cyclic extrusion compression (CEC) and equal channel angular pressing (ECAP). The microstructures, mechanical properties of alloy and its corrosion behavior in simulated body fluid (SBF) were evaluated. The results reveal that Mg--Zn--Y--Nd alloy consists of equiaxial fine grain structure with the homogeneous distribution of micrometer size and nano-sized second phase, which was caused by the dynamic recrystallization during the ECAP and CEC. The corrosion resistance of alloy was improved. The tensile and corrosion resistance were improved, especially the processed alloy exhibit uniform corrosion performances and decreased corrosion rate. This will provide theoretical ground for Mg--Zn--Y--Nd alloy as vascular stent application.

    Rong SONG,De-Bao LIU,Yi-Chi LIU,Wen-Bo ZHENG,Yue ZHAO,Min-Fang CHEN

    The main purpose of this paper is to investigate the effect of corrosion on mechanical behaviors of the Mg--Zn--Zr alloy immersed in simulated body fluid (SBF) with different immersion times. The corrosion behavior of the materials in SBF was determined by immersion tests. The surfaces of the corroded alloys were examined by SEM. The tensile samples of the extruded Mg--2Zn--0.8Zr magnesium alloy were immersed in the SBF for 0, 4, 7, 10, 14, 21 and 28 d. The tensile mechanical behaviors of test samples were performed on an electronic tensile testing machine. SEM was used to observe the fracture morphology. It was found that with extension of the immersion time, the ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) of the Mg–2Zn–0.8Zr samples decreased rapidly at first and then decreased slowly. The main fracture mechanism of the alloy transformed from ductile fracture to cleavage fracture with the increasing immersion times, which can be attributed to stress concentration and embrittlement caused by pit corrosion.

    Meng-Chang LIN,Shang-Qiu LIN,Jun-Yen UAN

    This study investigated the effect of annealing temperature on the mechanical properties of an as-rolled Mg--9.26wt.%Li--3.03wt.%Al--1.10wt.%Zn (LAZ931) alloy sheet. The dual-phase (α+β) LAZ931 alloy plate of 3 mm in thickness were rolled (67% reduction) and then annealed at temperatures at 100°C--350°C. The alloy's ductility showed a sharp concave downward tendency as a function of annealing temperature. The elongation of the LAZ931 alloy sheet increased with annealing temperature up to 150°C, followed by a sharp decrease of the alloy’s ductility as the annealing temperature higher than 150°C. The specimen exhibited an extremely low elongation (only ~0.5%) at annealing temperature around 300°C. Formation of brittle AlLi particles on boundary resulted in Li depletion zone near by grain boundary, transforming the Li depletion zone into α (hcp) layer. The combined effects including brittle AlLi particles on boundary and the hcp α layer on boundary resulted in the brittlement of the high-temperature-annealing sample.

    Xuan LI,Chao SHI,Jing BAI,Chao GUO,Feng XUE,Ping-Hua LIN,Cheng-Lin CHU

    The degradation behaviors of the novel high-strength AZ31B magnesium alloy wires after surface modification using micro-arc-oxidization (MAO) and subsequently sealing with poly-L-lactic acid (PLLA) in different simulated physiological environments were investigated. The results show the surface MAO micropores could be physically sealed by PLLA, thus forming an effective protection to corrosion resistance for the wires. In simulated gastric fluid (SGF) at a low pH value (1.5 or 2.5), the treated wires have a high degradation rate with a rapid decrease of mass, diameter, mechanical properties and a significant increase of pH value of the immersion fluid. However, surface modification could effectively reduce the degradation rate of the treated wires in SGF with a pH value above 4.0. For the treated wires in simulated intestinal fluid at pH= 8.5, their strength retention ability is higher than that in strong acidic SGF. And the loss rate of mass is faster than that of diameter, while the pH value of the immersion fluid decreases. It should be noted that the modified wires in simulated body environment have the best strength retention ability. The wires show the different degradation behaviors indicating their different degradation mechanisms, which are also proposed in this work.

    Ya-Ming WANG,Jun-Wei GUO,Yun-Feng WU,Yan LIU,Jian-Yun CAO,Yu ZHOU,De-Chang JIA

    The key to use magnesium alloys as suitable biodegradable implants is how to adjust their degradation rates. We report a strategy to prepare biocompatible ceramic coating with improved biocorrosion resistance property on AZ91D alloy by microarc oxidation (MAO) in a silicate--K2ZrF6 solution with and without Ca(H2PO4)2 additives. The microstructure and biocorrosion of coatings were characterized by XRD and SEM, as well as electrochemical and immersion tests in simulated body fluid (SBF). The results show that the coatings are mainly composed of MgO, Mg2SiO4, m-ZrO2 phases, further Ca containing compounds involve the coating by Ca(H2PO4)2 addition in the silicate--K2ZrF6 solution. The corrosion resistance of coated AZ91D alloy is significantly improved compared with the bare one. After immersing in SBF for 28 d, the Si--Zr5--Ca0 coating indicates a best corrosion resistance performance.

    Jun LIANG,Ren-Hui ZHANG,Zhen-Jun PENG,Bai-Xing LIU

    This research demonstrates a novel one-step electrochemical method to fabricate thick bilayer coatings on magnesium alloy in acid phosphate electrolyte containing aniline monomer and styrene-acrylic emulsion (SAE) with pulsed DC voltage. The morphologies, XRD and FTIR results show that the bilayer coating consists of an inner oxide layer and an outer polyaniline (PANI)/SAE composite layer. It is believed that the bilayered structure achieved results from a hybrid process combining electropolymerization (EPM) of aniline, electrophoretic deposition (EPD) of SAE and plasma electrolyte oxidation (PEO) of magnesium alloy substrate. Electrochemical corrosion tests indicate that the bilayer coating can provide superior corrosion protection to the magnesium alloy substrate in 3.5 wt.% NaCl solution.