Influence of Ca on the corrosion properties of magnesium for biomaterials

The corrosion properties of a Mg alloy with Ca in Hank's solution were investigated by the hydrogen evolution test. In the test results, the hydrogen evolution rate increased with increasing Ca content. This phenomenon was caused by the precipitation of Mg₂Ca phase in large amounts on the Mg matrix. In the case of Mg-Ca alloy, the formation of a micro-galvanic cell between the Mg₂Ca phase and Mg matrix accelerated the preferred dissolution of the matrix. These results demonstrated that the corrosion resistance and hydrogen evolution rate are controlled by the Mg₂Ca phase distribution in the Mg-Ca alloy system.     

氢的比焓特性对液氢贮罐静态蒸发率测试的影响

介绍了中国现有静态蒸发率测试的标准和适用范围,并对液氢贮罐适用性问题进行了讨论。通过对液氢与液氧、液氮等低温流体热物理特性进行对比和分析,发现氢的比焓特性是制约液氢贮罐在标准适用时的关键影响因素。最后提出了一种可适用于液氢真空多层绝热贮罐的静态蒸发率测试和计算方法。     

Benefits of carbon addition on the hydrogen absorption properties of Mg-based thin films grown by Pulsed Laser Deposition

Mg-Ni thin films were grown using Pulsed Laser Deposition. In situ optical changes from shiny metallic to transparent states were observed for films deposited in vacuum and under an Ar/H₂ gas mixture (93/7%), respectively. Optical changes were also achieved by ex situ hydrogenation under hydrogen gas pressure of 15 bars at 200 °C. However, after ex situ hydrogenation, the optical transmittance of the Mg-based hydrogenated thin films did not exceed 25%. Such limitation was attributed to oxygen contamination, as deduced by High Resolution Transmission Electron Microscopy observations, showing the co-existence of both Mg-based and MgO phases for as-deposited films. A significant decrease in oxygen contamination was successfully achieved with the addition of carbon, leading to the preparation of (Mg-based)-C_x (x < 20%) thin films showing a faster and easier hydrogenation.     

Testing and mesoscale modelling of hydrogen assisted cracking of magnesium

To rationalise the results of stress corrosion cracking tests on smooth tensile specimens of a magnesium alloy, performed as constant extension rate tensile tests, a mesoscale fibre bundle model is employed in which the material being tested is represented by a bundle of parallel fibres. The effect of hydrogen embrittlement on the stress-strain curves measured at various strain rates is simulated by assuming that the hydrogen is generated in localised corrosion pits and subsequently diffuses into the bulk, thereby reducing the strain-to-failure of individual fibres. The stress-strain curves obtained from these simulations show the same strain rate effect as was experimentally observed.     

Influence of MgO nano-crystals on the thermodynamics, hydrogen uptake and kinetics in Mg films

Hydrogen uptake in thin sputtered magnesium films covered with a Palladium layer was studied by resistance measurements. During growth, oxygen was introduced into the chamber in small amounts and the effect on the resistance, measured in situ, while growing, was monitored. This resulted in the formation of a mixture of MgO nanocrystals and layers in the Mg films. The measurements were made in situ in the sputtering chamber where the samples were grown. The aim of the study was to study the effect of oxygen contamination on hydrogen uptake in Mg films, as well as studying the uptake kinetics. Previous work on clean Mg films has shown that hydride formation at the surface reduces greatly the rate of hydrogen uptake further inside the film. These measurements show that the presence of oxygen contamination initially increases the rate of uptake greatly but decreases it when the Mg film is contaminated further with more oxygen.     

Corrosion mechanism and hydrogen evolution on Mg

Magnesium (Mg) dissolution is distinct from other engineering metals, as Mg can support cathodic hydrogen evolution on its surface during anodic polarisation. The phenomenon of cathodic hydrogen evolution upon anodically polarised Mg is characterised by the rate of the hydrogen evolution reaction (HER) increasing with anodic polarisation, a phenomenon called the negative different effect (NDE). Mg has a tendency to aggressively corrode in aqueous solutions, impairing its application as a durable engineering material or a predictable electrode material, which is also influenced by the NDE. Over the last century a number of different theories have sought to explain the NDE. However, recent progress in research upon Mg utilising contemporary methods including advanced electrochemical techniques, on-line elemental analysis and cross-sectional electron microscopy, have not only refined the understanding of Mg dissolution, but discredited almost a century of alternate theories. During anodic polarisation, a bilayered MgO/Mg(OH)₂ film forms on Mg, appearing as a dark region on visual inspection. This film gradually occupies the bulk of the previously pristine Mg surface, and importantly sustains (and enhances) the HER. This phenomenon of cathodic activation may also be catalysed by an enrichment of noble elements or impurities on the Mg surface, which could play an important role in promoting the HER. A phenomenological model for the dissolution of Mg encompassing the current opinion of many researchers is presented herein.     

Influence of microstructure on the in-vitro degradation behaviour of magnesium alloys

The study shows that the microstructural difference between the fine-grained die-cast and coarse-grained sand-cast magnesium-based alloys has no significant effect on the in-vitro degradation behaviour. However, the post-degradation analysis of the alloys suggest that the high volume fraction of secondary phase particles in the die-cast alloy may not be suitable for biodegradable implant applications, primarily due to the high stability of the secondary phase particles in physiological conditions.     

Influence of hydrogen-doped MgO thin films on the discharge characteristics in plasma display panels

In order to improve the discharge characteristics of Magnesium Oxide (MgO) thin films, hydrogen was doped to MgO thin films using an ion plating technique. Changes in the surface morphology, crystal orientation, optical properties, secondary electron emission coefficient, and defect states were studied with increasing hydrogen flow rates during the growth using field emission scanning electron microscope, X-ray diffraction, ellipsometry, γ-focused ion beam, and photoluminescence analysis. The change in firing voltage and delay time in plasma display panels (PDP) with the hydrogen-doped MgO thin films were also investigated. The results indicated that optimal hydrogen doping conditions can affect the surface structure and defect states: resulting in a significant reduction in the firing voltage and delay time of the PDP.     

蒸发速率对ITO光学常数的影响

由于ITO具有复杂的微观结构和吸收机制,很难准确测量出它的光学常数.采用变入射角光度椭偏仪(VASE)建立了一套简单的拟合模型,成功拟合出ITO光学常数在可见光范围内随波长的变化关系.并在此基础上分析了电子束蒸发法蒸发速率对ITO薄膜光学常数的影响.根据Hall效应测量了ITO在不同蒸发速率下的载流子浓度,分析了ITO光学常数随蒸发速率增加而增加的机理.     

Influence of processing and reinforcement on microstructure and impact behaviour of magnesium alloy AM100

Reinforcing magnesium alloys with a discontinuously dispersed ceramic phase has engineered a new family of materials that are marketed under the trade name “metal-matrix composites”. Continuous research efforts in the processing of these materials have provided the necessary impetus for their emergence and use in structural, automotive and even aerospace-related components. In this paper we report the results of a study aimed at understanding the role of short-fibre reinforcements (discontinuously dispersed through the metal-matrix of magnesium alloy AM 100) on impact deformation and fracture behaviour. In particular, the role of volume fraction of the reinforcing phase on impact energy and fracture behaviour is presented and discussed. An increase in short-fibre reinforcement content in the magnesium alloy metal-matrix is observed to have a detrimental influence on impact energy when compared to the unreinforced counterpart. Micro cracking in the metal-matrix coupled with failure of the reinforcing fibres, both independently dispersed and in clusters, dominates the fracture sequence at the microscopic level. The final fracture behaviour of the composite material is discussed in the light of the concurrent and mutually interactive influences of nature of loading, local stress state, intrinsic microstructural effects and deformation characteristics of the composite constituents.     

Electrochemical corrosion and mechanical behaviors of the charged magnesium

The electrochemical corrosion and mechanical behaviors of the charged magnesium were investigated using Mott–Schottky (M–S) test and slow strain rate test (SSRT), respectively. The results showed that the hole carrier and vacancy concentrations in the corrosion film increased after cathodic charging due to the formation of magnesium hydroxide. The increasing vacancy concentration caused the increase of the permeation rate of hydrogen to the interior of matrix. When the inner stress caused by synergistic effect of hydrogen pressure and expansion stress of the formation of magnesium hydride was above the fracture strength, crack initiated and propagated. It indicated that hydrogen embrittlement (HE) mechanism for the stress cracking corrosion (SCC) of magnesium and its alloys. After cathodic charging, the corrosion resistance and mechanical properties of matrix deteriorated.     

Reactive mechanical grinding of magnesium in hydrogen and the effects of additives

The use of reactive mechanical grinding (MG under H₂) of magnesium powder improves the hydrogen sorption properties. The hydrogenation of Mg starts in situ during the milling process, thus circumventing the activation procedure that is generally required for Mg. The effects of the addition of various elements or compounds have been studied. The hydriding is determined to be a two-step process: nucleation and diffusion. A direct relationship exists between the nucleation duration and the specific surface area of the magnesium powder. A critical milling time exists up to which the diffusion process is improved and above which no more improvement is observed (the maximum internal stress in the powder is also reached at this critical time). The diffusion is controlled by the number of crystallites per particle that can be reduced by increasing the milling time up to 10 hr. The addition of Co (catalyst), YNi (hydrogen pump), or oxides (abrasive elements and nucleation centers) leads to an improvement of the hydrogen sorption properties (but a strong dependence upon the milling time is reported). Finally, the sorption properties of these mixtures are comparable with those reported for MgH₂-metal mixtures.     

Influence of Mg concentration, pH value and specimen parameter on the hemolytic property of biodegradable magnesium

In this study, we first evaluated the hemolytic rate of pure magnesium and MgZn alloy, and the experimental results indicated that both of these two materials showed severe hemolysis. Furthermore, a revised hemolysis test was designed to assess the impact of the Mg²⁺ concentration and the pH value on the hemolytic rate of magnesium. It was found that a Mg²⁺ concentration of 11.4 ppm in the normal saline extract with a pH value of 7.35 or 4.93 did not demonstrate any hemolysis. However, the extract with the same Mg²⁺ concentration but a pH value of 12.01 or 2.48 could result in heavy hemolysis. In addition, compared to the pervious study, it was suggested that there should be a preferred choice between the mass and the surface area of the specimen that used in the hemolysis test according to its different medical applications.     

Influence of loading direction on the anisotropic fatigue properties of rolled magnesium alloy

The influence of loading direction on the fatigue behavior of rolled AZ31 alloy was investigated by conducting fully reversed stress-controlled fatigue tests along the rolling direction and normal to the rolling plane. Alternating twinning and detwinning behavior during initial cycling was found to cause asymmetric hysteresis loops, resulting in a compressive strain in the rolling direction and a tensile strain normal to the rolling plane. A transition in the dominant deformation mechanism from twinning–detwinning to slip occurs at around five cycles under both conditions due to cyclic hardening, thus making their loops symmetric. The lower twinning stress in tension along the normal direction leads to an increase in fatigue damage by plastic strain, resulting in a lower fatigue resistance than along the rolling direction.     

The oxidation kinetics of magnesium at low temperatures and low oxygen partial pressures

The initial oxidation of magnesium at oxygen partial pressures between 1.3 × 10^− 8 Pa and 1.3 × 10^− 5 Pa and at temperatures just above room temperature has been investigated in situ with X-ray photoelectron spectroscopy (XPS) and ellipsometry. Quantitative analysis of the XPS spectra showed that the initially formed oxide has a higher Mg/O ratio (> 1.3) than bulk MgO. Ellipsometry measurements indicated that the band gap values of the oxide layers are considerably smaller than the value expected for bulk MgO ( 2.5 eV vs. 7.8 eV). From the XPS and ellipsometry data recorded as a function of oxidation time the oxidation kinetics have been determined. The kinetics has been described quantitatively with a coupled currents model, involving simultaneous transport of electrons and ions through the oxide layer.     

The surface chemistry of 3-mercaptopropyltrimethoxysilane films deposited on magnesium alloy AZ91

Magnesium and its alloys have desirable physical and mechanical properties for a number of applications. Unfortunately, these materials are highly susceptible to corrosion, particularly in the presence of aqueous solutions. The purpose of this study is to develop a uniform, non-toxic surface treatment to enhance the corrosion resistance of magnesium alloys. This paper reports the influence of the coating bath parameters and alloy microstructure on the deposition of 3-mercaptopropyltrimethoxysilane (MPTS) coatings on magnesium alloy AZ91. The surface chemistry at the magnesium/MPTS interface has also been explored. The results indicate that the deposition of MPTS onto AZ91 was influenced by both the pH and MPTS concentration in the coating bath. Furthermore, scanning electron microscopy results showed that the MPTS film deposited uniformly on all phases of the magnesium alloy surface. X-ray photoelectron spectroscopy studies revealed that at the magnesium/MPTS interface, the molecules bond to the surface through the thiol group in an acid-base interaction with the Mg(OH)₂ layer, whereas in the bulk of the film, the molecules are randomly oriented.     

Review of the atmospheric corrosion of magnesium alloys

Mg atmospheric corrosion is induced by a thin surface aqueous layer. Controlling factors are microgalvanic acceleration between different phases, protection by a continuous second phase distribution, protection by corrosion products, and degradation of protective layers by aggressive species such as chloride ions. The Mg atmospheric corrosion rate increases with relative humidity (RH) and concentrations of aggressive species. Temperature increases the corrosion rate unless a protective film causes a decrease. O₂, SO₂ and NO₂ accelerate the atmospheric corrosion rate, whereas the corrosion rate is decreased by CO₂. The traditional gravimetric method can evaluate effectively the corrosion behavior of Mg alloys.     

Influence of Pd addition on the creep behavior of AZ61 magnesium alloy

The effect of Pd addition (0, 2, and 4 wt%) on the microstructure and creep properties of permanent mold AZ61 (Mg-6Al-1Zn) alloy has been studied. The results indicate that Pd addition introduces a lamella-shaped Al₄Pd phase at the grain boundary, in addition to the Mg₁₇Al₁₂ (β) phase. The addition of Pd also suppresses the precipitation of the Mg₁₇Al₁₂ phase and residual Al at grain boundaries during solidification. These effects lead to an improvement in the creep behavior of AZ61. Moreover, extended steady-state creep and reductions in both the minimum creep rate and total creep strain are also observed in the case of 4 wt% Pd addition.     

Failure mode and fatigue behavior of weld-bonded lap-shear specimens of magnesium and steel sheets

Failure modes and fatigue behaviors of ultrasonic spot welds in lap-shear specimens of magnesium AZ31B-H24 and hot-dipped-galvanized mild steel sheets with and without adhesive were investigated. The spot welded specimens failed from the kinked crack growth mode. The adhesive-bonded specimens failed from the cohesive failure through the adhesive and the kinked crack growth through the magnesium sheet. The weld-bonded specimens failed from the cohesive failure through the adhesive, the interfacial failure through the spot weld, and the kinked crack growth through the magnesium sheet. The estimated fatigue lives for the adhesive-bonded and weld-bonded specimens failed from the kinked crack growth mode are lower than the experimental results.