Shaokang Guan

In vitro degradation and mechanical integrity of Mg-Zn-Ca alloy coated with Ca-deficient hydroxyapatite by the pulse electrodeposition process.

The key to manufacturing magnesium-based alloys that are suitable as biodegradable orthopaedic implants is how to adjust their degradation rates and mechanical integrity in the physiological environment. In this study, to solve this challenge, a soluble Ca-deficient hydroxyapatite (Ca-def HA) coating was deposited on an Mg-Zn-Ca alloy substrate by pulse eletrodeposition. This deposition can be demonstrated by X-ray diffractometry and energy dispersion spectroscopy analyses, and the Ca/P atomic ratio of as-deposited coating is about 1.33 (within the range from 1.33 to 1.65). By regulating the appropriate pulse amplitude and width, the Ca-def HA coating shows better adhesion to Mg-Zn-Ca alloy, whose lap shear strength is increased to 41.8+/-2.7 MPa. Potentiodynamic polarization results in Kokubos simulated body fluid (SBF) indicate that the corrosion potential of Mg alloy increases from -1645 to -1414 mV, while the corrosion current density decreases from 110 to 25 microA/cm(2), which illustrates that the Ca-def HA coating improves the substrate corrosion resistance significantly. Since orthopaedic implants generally serve under conditions of stress corrosion, the mechanical integrity of the Mg-Zn-Ca alloy was measured using the slow strain rate tensile (SSRT) testing technique in SBF. The SSRT results show that the ultimate tensile strength and time of fracture for the coated Mg-Zn-Ca alloy are higher than those of the uncoated one, which is beneficial in supporting fractured bone for a longer time. Thus Mg-Zn-Ca alloy coated with Ca-def HA is be a promising candidate for biodegradable orthopaedic implants, and is worthwhile to further investigate the in vivo degradation behavior.

Microstructure and corrosion properties of as sub-rapid solidification Mg-Zn-Y-Nd alloy in dynamic simulated body fluid for vascular stent application

Magnesium alloy stent has been employed in animal and clinical experiment in recent years. It has been verified to be biocompatible and degradable due to corrosion after being implanted into blood vessel. Mg–Y–Gd–Nd alloy is usually used to construct an absorbable magnesium alloy stent. However, the corrosion resistant of as cast Mg–Y–Gd–Nd alloy is poor relatively and the control of corrosion rate is difficult. Aiming at the requirement of endovascular stent in clinic, a new biomedical Mg–Zn–Y–Nd alloy with low Zn and Y content (Zn/Y atom ratio 6) was designed, which exists quasicrystals to improve its corrosion resistance. Additionally, sub-rapid solidification processing was applied for preparation of corrosion-resisting Mg–Zn–Y–Nd and Mg–Y–Gd–Nd alloys. Compared with the as cast sample, the corrosion behavior of alloys in dynamic simulated body fluid (SBF) (the speed of body fluid: 16xa0ml/800xa0mlxa0min−1) was investigated. The results show that as sub-rapid solidification Mg–Zn–Y–Nd alloy has the better corrosion resistance in dynamic SBF due to grain refinement and fine dispersion distribution of the quasicrystals and intermetallic compounds in α-Mg matrix. In the as cast sample, both Mg–Zn–Y–Nd and Mg–Y–Gd–Nd alloys exhibit poor corrosion resistance. Mg–Zn–Y–Nd alloy by sub-rapid solidification processing provides excellent corrosion resistance in dynamic SBF, which open a new window for biomedical materials design, especially for vascular stent application.

Enhanced photocatalytic degradation properties of nitrogen-doped titania nanotube arrays

Abstract Nitrogen-doped TiO 2 nanotubes array were synthesized to improve the photocatalytic efficiency by annealing the anodized titania nanotubes with ammonia at 500 °C. Detailed structural analysis revealed that the nitrogen-doped titania nanotubes are of highly ordered structure, and exhibit a decreased phase transformation temperature compared with those that are not doped, as evidenced by the decrease in full width at half maximum (FWHM) of the (110) peak of rutile phase and the occurrence of the typical Raman peaks of rutile phase at 196, 235, 442, 610 cm −1 . According to the photocatalytic degradation of methyl orange under visible light irradiation, the nitrogen-doped TiO 2 nanotubes exhibit enhanced photocatalytic efficiency compared with their non-doped nanotubes, which might be a result of either the nitrogen doping induced band gap narrowing or the synergistic effect produced by both nitrogen and fluorine dopants.

Fabrication and characterization of bioactive composite coatings on Mg–Zn–Ca alloy by MAO/sol–gel

High corrosion rate and accumulation of hydrogen gas upon degradation impede magnesium alloys’ clinical application as implants. In this work, micro-arc oxidation (MAO) was used to fabricate a porous coating on magnesium alloys as an intermediate layer to enhance the bonding strength of propolis layer. Then the composite coatings were fabricated using sol–gel method by dipping sample into the solution containing propolis and polylactic acid at 40°C. The corrosion resistance of the samples was determined based on potentiodynamic polarization experiments and immersion tests. Biocompatibility was designed by observing the attachment and growth of wharton’s jelly-derived mesenchymal stem cells (WJCs) on substrates with MAO coating and substrates with composite coatings. The results showed that, compared with that of Mg–Zn–Ca alloy, the corrosion current density of the samples with composite coatings decreased from 5.37xa0×xa010−5 to 1.10xa0×xa010−6xa0A/cm2 and the corrosion potential increased by 240xa0mV. Composite coatings exhibit homogeneous corrosion behavior and can promote WJCs cell adhesion and proliferation. In the meantime, pH value was relatively stable during the immersion tests, which may be significant for cellular survival. In conclusion, our results indicate that composite coatings on Mg–Zn–Ca alloy fabricated by MAO/sol–gel method provide a new type bioactive material.

Flow stress and microstructure evolution of semi-continuous casting AZ70 Mg-alloy during hot compression deformation

To evaluate and predict flow stress and set up hot forging process of AZ70 magnesium alloy, hot compression tests of AZ70 magnesium alloy were carried out on Gleeble 1500D thermo-mechanics tester at 300–420 °C and strain rates of 0.001–1 s−1 with different compression degrees. It is indicated that temperature and strain rate are the main factor affecting the flow stress and microstructure. Stress increases but average grain size decreases with temperature decreasing and strain rate increasing. The stress model, constituted by introducing temperature-compensated strain rate, the Zener-Hollomon parameter, has a good fitness with the proof stress value under the experimental condition. The reciprocal of grain size at true strain of 1.0 has a linear relation with natural logarithm of Z parameter, and the correlation coefficient, R=0.95, is very significant by examination. The hot deformation activation energy Q of AZ70 alloy is 166.197 kJ/mol by calculation.

Microstructures and mechanical properties of double hot-extruded AZ80+xSr wrought alloys

Abstract The effects of Sr addition on microstructures and tensile properties of the as-cast and hot-extruded AZ80 alloys were studied by OM, SEM, EDS, XRD, DSC and Instron tester. The results show that the microstructures of as-cast alloys consist of α-Mg and β-Mg 17 Al 12 phase. Sr gathers on the boundaries, and dissolves into β-Mg 17 Al 12 phase or forms Mg 17 Sr 2 phase. The grains of as-cast alloys are refined and discontinuous net-shaped structure is formed. The compound phases on the boundaries become thicker with increasing Sr content. The ultimate tensile stress(UTS) and elongation are improved compared with the corresponding Sr-free alloy. After preliminary hot-extruding, the UTS is up to 308–320 MPa and elongation reaches 8.0%-13.5%. After double hot-extrusion, the dynamic recrystallization completes totally, and the UTS is up to 310–355 MPa, but the elongation does not change apparently. The alloy with 0.02%Sr (mass fraction) obtains the best comprehensive performance with the UTS of 355 MPa and elongation of 13.2%. The SEM morphology of fracture surface shows that the alloys with Sr present good ductility after double hot-extrusion.

Effects of silicocalcium on microstructure and properties of Mg-6Al-0.5Mn alloy

The effects of silicocalcium on the microstructure and mechanical properties of casting magnesium alloy Mg-6Al-0.5Mn(AM60) were studied. The results show that the microstructure of AM60 casting magnesium alloy is effectively refined by adding small amount of silicocalcium. The grain size decreases from 180 μm to 80 μm with 1.8% Si addition, while the size increases with 2.5% Si addition. The AM60+Si-Ca alloys mainly contain Mg matrix, β-Mg17Al12 phase, Al8Mn5 phase and a small polygonal type Mg2Si phase in matrix. Al8Mn5 can act as the heterogeneous nucleation for the Mg2Si phase. With the increase of silicocalcium, the content of Mg2Si phase increases gradually, the Mg2Si particles grow up and change coarse gradually. The microhardness of AM60 matrix increases with silicocalcium addition. The peak values of the tensile strength, elongation and impact toughness appear simultaneously with 1.8% silicocalcium addition, and the tensile strength, elongation and impact toughness are heightened respectively by 13.9%, 28.5% and 100%.

Effect of Al Addition on Formation and Mechanical Properties of Mg-Cu-Gd Bulk Metallic Glass

The effect of partial substitution of Al for Cu on the glass forming ability(GFA) and mechanical properties of Mg65Cu(subscript 25-x)Al(subscript x)Gd10 (x=0, 1, 3 and 5, molar fraction, %) alloys were studied by X-ray diffractometry(XRD), differential scanning calorimetry(DSC) and uniaxial compression test. The result reveals that GFA of the alloys changes slightly with increasing x from 0 to 3, and then abruptly decreases with x increasing up to 5. The plasticity can be greatly improved with appropriate substitution of Cu by Al (3%, molar fraction) in Mg65Cu25Gd10 bulk metallic glass, and the resultant fracture strength, total strain to failure, and plastic strain are 898MPa, 2.19% and 0.2%, respectively.

Mg Alloys Development and Surface Modification for Biomedical Application

The development of biodegradable implants has grown into one of the important areas in medical science (Mani et al., 2007), since they can be gradually dissolved, absorbed, consumed or excreted in human body environment, and then disappear spontaneously after the bone tissues heal. The biodegradable materials available in the current market are mainly made of polymeric or ceramic materials, while these implants have an unsatisfactory mechanical strength when used for load-bearing parts (Staiger et al., 2006). Compared with the currently approved biomaterials, Mg alloys have a lot of advantages (Song et al., 2007; Witte et al., 2007; Witte et al., 2005; Song et al., 2007). First, with high strength/weight ratio, Mg alloy exhibits an appropriate mechanical integrity and is more suitable for load-bearing implantation. Its fracture toughness is higher than ceramic biomaterials (e.g. HA), and the elastic modulus and compressive yield strength of magnesium are closer to those of natural bone than other metallic implants (Table 1.1). Thus it will help to reduce or avoid “stress shielding effects” that can lead to reducing stimulation of new bone growth and remodeling. Moreover, magnesium has little toxicity to human body. Magnesium is essential to human metabolism and is naturally found in bone tissue. It is the fourth most abundant cation in the human body, with an estimated 1 mol of magnesium stored in the body of a normal 70 kg adult. Approximately, half of the total physiological Mg is stored in bone tissue. Thirdly,

Microstructure and synthesis mechanism of Al-Ti-C-Sr master alloy

Abstract Al-5Ti-0.5C-8Sr (mass fraction, %) master alloy was prepared using a melt reaction method. The microstructure and synthetic process of the master alloy were investigated by optical microscopy, X-ray diffraction, scanning electron microscopy and X-ray energy-dispersive spectrum. The results show that the master alloy is composed of a (Al), TiAl 3 , TiC, Al 4 Sr and Al-Ti-Sr phases. The synthesis mechanisms of the master alloy are as follows: TiAl 3 is formed through the reaction between K 2 TiF 6 and Al melt at 850 °C; when the melt was heated up to 1 200-1 300 °C, TiC was formed through the reaction: Ti+C(s)=TiC(s); Al 4 Sr was formed through the binary uniform reaction when Sr was added into the melt; after the following solidification process in the peritectic reaction: L(Al, Sr)+α(TiAl 3 )→β(Al-Ti-Sr), the enwrapped structure was formed with the outer layer of Al-Ti-Sr phase and the internal layer of TiAl 3 phase.