Biodegradable behaviors of AZ31 magnesium alloy in simulated body fluid

Magnesium alloys have unique advantages to act as biodegradable implants for clinical application. The biodegradable behaviors of AZ31 in simulated body fluid (SBF) for various immersion time intervals were investigated by electrochemical impedance spectroscopy (EIS) tests and scanning electron microscope (SEM) observation, and then the biodegradable mechanisms were discussed. It was found that a protective film layer was formed on the surface of AZ31 in SBF. With increasing of immersion time, the film layer became more compact. If the immersion time was more than 24 h, the film layer began to degenerate and emerge corrosion pits. In the meantime, there was hydroxyapatite particles deposited on the film layer. The hydroxyapatite is the essential component of human bone, which indicates the perfect biocompatibility of AZ31 magnesium alloy.     

Effect of D-fructose on the in-vitro corrosion behavior of AZ31 magnesium alloy in simulated body fluid

The effect of addingd-fructose to simulated body fluid(SBF) on the corrosion behavior of AZ31 magnesium(Mg) alloy at 37.C and at a pH of 7.4 was studied by potentiodynamic polarization(PDP), electrochemical impedance spectroscopy(EIS), potentiostatic polarization and hydrogen(H2) collecting techniques,Raman spectroscopy technique, scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), X-ray diffraction(XRD), X-ray photoelectron spectroscopy analysis(XPS) and Fourier transformed infrared(FTIR). The results demonstrated that the addition of fructose enhanced the deposition of phosphates forming thick and compact corrosion products, which inhibited the transmission of aggressive ions into the Mg substrate. As a result, both the anodic dissolution of Mg and negative difference effect(NDE) were suppressed. Thus, the corrosion resistance of AZ31 Mgalloy in SBF was significantly improved.     

电轧AZ31镁合金在模拟海水中腐蚀行为研究

为对比研究高能电脉冲轧制工艺和冷轧工艺对AZ31镁合金腐蚀性能的影响,采用腐蚀形貌观察、动电位极化测试、电化学阻抗谱与腐蚀速度测试等方法系统地研究了高能电脉冲轧制和冷轧AZ31镁合金带材在模拟海水(3.5%NaCl)中的腐蚀行为.结果表明:在同样变形量下,与冷轧AZ31镁合金相比,电轧AZ31镁合金的耐腐蚀性略有提高.这与电轧AZ31镁合金再结晶比例大,位错密度小,具有较低能态的位错组态及能形成较稳定的腐蚀产物膜有关.     

Electrochemical noise analysis on the pit corrosion susceptibility of biodegradable AZ31 magnesium alloy in four types of simulated body solutions

Magnesium alloys have been investigated as biodegradable implant materials since the last century. Non-uniform degradation caused by local corrosion limits their application, and no appropriate technology has been used in the research. In this study, electrochemical noise has been used to study the pit corrosion on magnesium alloy AZ31 in four types of simulated body solutions, and the data have been analyzed using wavelet analysis and stochastic theory. Combining these with the conventional polarization curves, mass loss tests and scanning electron microscopy, the electrochemical noise results implied that AZ31 alloy in normal saline has the fastest corrosion rate, a high pit initiation rate, and maximum pit growth probability. In Hanks’ balanced salt solution and phosphate-buffered saline, AZ31 alloy has a high pit initiation rate and larger pit growth probability, while in simulated body fluid, AZ31 alloy has the slowest corrosion rate, lowest pit initiation rate and smallest pit growth probability.     

Electrochemical characteristics of electric-arc aluminum coatings on AZ31 magnesium alloy

The possibilities of improvement of the corrosion resistance of AZ31 magnesium alloy by spraying AlSi5 electric-arc coatings with their subsequent melting by an Nd : YAG laser of quasicontinuous action are analyzed. The electrochemical characteristics of the surface layers in aqueous media are studied. It is shown that the AlSi5 electric-arc coatings significantly increase the corrosion resistance of the magnesium alloy. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 5, pp. 112–114, September–October, 2008.     

Fatigue behavior of AZ31 magnesium alloy produced by solid-state recycling

Mechanical properties of AZ31 Mg alloy produced from machined chips by the solid-state recycle method were compared to those of the reference alloy which was produced from an as-received AZ31 Mg alloy block under the same conditions with the recycled alloy. Tensile properties of the recycled alloy were comparable to those of the reference alloy, however, the recycled alloy exhibited poorer fatigue resistance than the reference alloy.     

The semi-solid tensile deformation behavior of wrought AZ31 magnesium alloy

The semi-solid tensile deformation behavior of wrought AZ31 magnesium alloy has been studied through applying a set of low strain rate (0.001 s⁻¹) hot tension tests at temperature range of 300–500 °C. The results indicated a ductility drop at ∼450 ± 25 °C. This was attributed to the occurrence of eutectic reaction (L → α + γ) and the partial melting of intermetallic γ phase. The ductility was started to improve by increasing the temperature to 500 °C. The latter was explained considering the effect of liquid phase on stress relaxation through accommodation of the grain boundary sliding phenomena. To further investigating the semi-solid tensile deformation behavior of the experimental alloy, the cavitations characteristics of the alloy were also examined.     

Post-welding formability of AZ31 magnesium alloy

The plastic flow behaviour and formability of friction stir welded AZ31 magnesium alloys were widely investigated. Flow curves were obtained in extended ranges of temperature (250–350 °C) and strain rate (0.001–0.1 s⁻¹) by means of uniaxial tensile tests; furthermore, forming limit curves were determined using the hemispherical punch method in the same range of temperature but with a constant crosshead speed of 0.1 mm/s. The results were compared with those obtained, under the same experimental conditions, on the base material. The flow stress levels of joint and base material are very similar up the peak of the flow curve although the equivalent strains at the peak and to failure are usually lower than those of base material. However, at the highest temperature and lowest strain rate investigated (350 °C and 0.001 s⁻¹), the flow behaviour of the welded joint tends to be similar to the one of the base alloy. Finally, formability of the friction stir welded material, evaluated in terms of forming limit curves, is usually lower than the one of the base material.     

Localized deform behavior of AZ31B magnesium alloy by numerical simulation

The dynamic compression behavior of AZ31B magnesium alloy with hat shaped specimen was investigated at high strain rate in this paper. Based on the Johnson‐cook constitutive model and fracture model, the interaction of temperature, stress and strain fields of AZ31B magnesium alloy with hat shaped specimen were numerically simulated by using ANSYS/LS‐DYNA software under different strain rates, which was validated by experiment. It is found that the plastic strain is highly concentrated on the corner of the hat shaped specimen, which leads to large localized deformation. The voids are nucleated and extended by compression stress. Work harden effect is caused by remained plastic strain, which is located around adiabatic shear band. The stress collapse is discovered in gauge section, which is also discovered in experiment. Thermal soften effect is suppressed with the strain rate increased.     

Corrosion fatigue behavior of extruded magnesium alloy AZ31 in sodium chloride solution

In the present study, corrosion fatigue experiments were done using the extruded magnesium alloy AZ31 in the 3% sodium chloride solution to clarify the corrosion fatigue characteristics of the material. Corrosion fatigue lives greatly decreased as compared with those in laboratory air. It was also clarified that most of the corrosion fatigue life (70–80%) at the lower stress amplitude is occupied with the period of the corrosion pit growth. Corrosion fatigue lives were evaluated quantitatively by dividing the corrosion fatigue process into the following two periods, i.e. (1) the corrosion pit growth period preceding the crack initiation from the pit and (2) the crack growth period before the specimen failure. In the analysis, the law of the corrosion pit growth proposed by authors was used to deal with the above first period. The evaluated results corresponded well to the experimental results.     

EBSD analysis of fatigue crack initiation behavior in coarse-grained AZ31 magnesium alloy

Plane bending fatigue tests had been conducted to investigate fatigue crack initiation mechanism in coarse-grained magnesium alloy, AZ31, with hexagonal close-packed (hcp) crystallographic structure. The initial crystallographic structure was analyzed by an electron backscatter diffraction (EBSD) method. Subsequently, a fatigue test was periodically terminated and time-series EBSD analyses were performed. Basal slip and primary twin operated predominantly. In a twin band, secondary twin operated, and resulted in the fatigue crack initiation. The crack initiation was strongly affected by Schmid factors in the grains and twin bands.     

AZ31镁合金非等温拉深性能的研究

针对AZ31镁合金等温拉深性能差的问题,提出了AZ31镁合金的非等温拉深工艺.通过平底杯形冲头拉深试验研究了不同冲头温度和板料温度对AZ31镁合金非等温拉深性能的影响,确定了使AZ31镁合金具有最佳拉深性能的板料和冲头温度范围.实验结果表明,除了板料和冲头温度之外,拉深速度和润滑条件对AZ31镁合金的非等温拉深性能也有重要影响.     

In-vitro degradation behaviour of WE54 magnesium alloy in simulated body fluid

Rare earths containing magnesium alloy, WE54, exhibited a marginally higher in-vitro degradation resistance than that of pure magnesium. Heat-treatment procedure had an influence on the degradation behaviour. However, comparing with AZ91 magnesium alloy the in-vitro degradation resistance of WE54 magnesium alloy was significantly lower, which suggests that the passivating capacity of rare earths is inferior to that of aluminium under in-vitro condition.     

Microstructural features of spray-formed AZ31 magnesium alloy

Abstract

Mg alloy AZ31 was spray-formed using an indigenously developed spray atomisation and deposition unit under protective atmosphere and various processing parameters were optimised. The microstructural features of the bell shaped AZ31 spray-formed deposit were characterised using optical microscope, scanning electron microscope/energy dispersive spectrometer, X-ray diffraction and high resolution transmission electron microscope. It was observed that the microstructural features are critically dependent on location in the spray-formed deposits. Under optimised processing conditions, the central region of the bell shaped deposit exhibited minimal porosity and a uniform fine grained equiaxed microstructure with fine Mg₁₇Al₁₂ intermetallics preferably located at the grain boundaries. However, the peripheral regions of the spray-formed deposit indicate higher porosity with distinct microstructural characteristics different from those in the central region. These microstructural features, observed at different locations in the spray-formed deposit, have been analysed and their evolution is discussed in the light of variations in thermal and solidification conditions of the droplets in flight, during impingement as well as those of the deposition surface.     

Influence of rolling temperature on static recrystallization behavior of AZ31 magnesium alloy

Poor formability of rolled magnesium (Mg) alloys extremely restricts applications in form of sheets originating from formation of strong basal texture. Recently, we found that increasing rolling temperature from 723 to 798 K for a AZ31 Mg alloy can significantly improve stretch formability due to remarkable texture weakening after annealing. In this study, static recrystallization behaviors of AZ31 alloy sheets rolled at 723 and 798 K were investigated by electron backscattered diffraction analyses at different annealing stages in order to understand the origin of high temperature rolling on texture weakening. For both sheets, similar deformation microstructures with approximately the same types and fractions of twins exist in the as-rolled condition and recrystallized grains are mainly formed at pre-existing grain boundaries due to discontinuous recrystallization during subsequent annealing. However, only the basal texture of the latter remarkably weakens due to the formation of new recrystallized grains with well-dispersed orientations. Non-basal slips enhanced during high temperature rolling at 798 K are most likely responsible for the texture randomization as a result of rotations of recrystallization nuclei.     

Dynamic recrystallization behavior of AZ31 magnesium alloy processed by alternate forward extrusion

One of the important factors that affect the microstructure and properties of extruded products is recrystallization behavior. Alternate forward extrusion (AFE) is a new type of metal extrusion process with strong potential. In this paper, we carried out the AFE process experiments of as-cast AZ31 magnesium alloy and obtained extrusion bar whose microstructure and deformation mechanism were analyzed by means of optical microscopy, electron backscattered diffraction and transmission electron microscopy. The experimental results indicated that homogeneous fine-grained structure with mean grain size of 3.91 μm was obtained after AFE at 573 K. The dominant reason of grain refinement was considered the dynamic recrystallization (DRX) induced by strain localization and shear plastic deformation. In the 573-673 K range, the yield strength, tensile strength and elongation of the composite mechanical properties are reduced accordingly with the increase of the forming temperature. Shown as in relevant statistics, the proportion of the large-angle grain boundaries decreased significantly. The above results provide an important scientific basis of the scheme formulation and active control on microstructure and property for AZ31 magnesium alloy AFE process.     

In vitro degradation behavior of M1A magnesium alloy in protein-containing simulated body fluid

Magnesium alloys possess unique advantages to be used as biodegradable implants for clinical applications. In this study, in vitro cells responses and degradation behaviors of magnesium alloy M1A in simulated body fluid (SBF) and albumin-containing SBF (A-SBF) were systematically investigated. Cell responses, in terms of Cell morphology and cell proliferation, imply that M1A possesses good viability for MG63 cells. The corrosion behaviors of M1A are strongly affected by the addition of albumin through the combined effects of adsorption and chelation. Electrochemical testing indicates that such an absorbed albumin layer makes M1A to be more noble with a smaller corrosion current. Corrosion rate monitored by hydrogen evolution rate suggests that the quickly adsorbed albumin serves as an effective protective layer, resulting in a much slower hydrogen release rate at initial stage. With increasing immersion time, a higher corrosion rate is observed since the chelation effect exerts more significant acceleration effects on the removal of the passivation layer. The corrosion mode evaluated by surface morphology of the samples changes from a nonuniform-anisotropic mode for M1A in SBF to a uniform-isotropic mode for M1A in A-SBF.     

Fluoride treatment and in vitro corrosion behavior of an AZ31B magnesium alloy

As a new class of biodegradable material, magnesium alloys have attracted much attention in recent years. In order to improve the corrosion resistance, a fluoride coating was prepared on the surface of AZ31B magnesium alloy. The surface characterization analysis showed a dense coating with some irregular pores was formed. The TF-XRD analysis indicated that the coating was mainly composed of MgO and MgF₂. Electrochemical and immersion tests proved that the fluoride conversion coating significantly improved the corrosion resistance of AZ31B. Three-point bending test revealed that the degradation behavior of the fluoride treated AZ31B could meet the requirement as a biodegradable material.     

Simulation of extrusion process of AZ31 magnesium alloy

The extrusion process of AZ31 magnesium alloy has been simulated. First, a series of tests were designed to obtain the simulation parameters: stress–strain curves, friction factors and heat transfer coefficient. Then, a special extrusion process was carried out and simulated by DEFORM-2D simultaneously. The results reveal that the simulated temperature–time curve agrees well with the measured one, indicating the good accuracy of the simulation parameters. Accordingly, the extrusion of AZ31 at different conditions was analyzed in detail. The extrusion loads and temperature distribution of billets were obtained.