Commercial Purity Magnesium Research Articles

Tribological behavior of electroless Ni–B coatings on magnesium and AZ91D alloy

Ni–B coatings have been deposited directly on commercial purity magnesium and AZ91D alloy and characterized using glow discharge optical emission spectroscopy, scanning electron microscopy, X-ray diffraction and, for tribological and mechanical evaluation, dry sliding wear and friction testing and microhardness measurement. The boron content of the coating on Mg was slightly lower than that of the coating on the AZ91D alloy. The wear rates were similar for both coatings and two magnitude orders lower than those of the substrates. The coefficients of friction were similar for both coatings, in the range ∼0.31 to 0.34. The values were close to that of the uncoated magnesium, while that for the AZ91D alloy was slightly lower, namely ∼0.25. The wear mechanism of the electroless Ni–B coatings was governed by the presence of abrasive wear, delamination and tribo-corrosion processes.

Texture Evolution of the Mg/Al Laminated Composite by Accumulative Roll Bonding at Ambient Temperature

Abstract The Mg/Al laminated composites were prepared by the accumulative roll bonding (ARB) at ambient temperature using the commercial pure magnesium and pure Al sheets as starting materials. Microstructure of both Mg and Al layer were refined during ARB processing. Neutron diffraction was employed to investigate the texture evolution of this kind of laminated composite. Both the Mg and Al layer of the primary sandwich exhibited dominant shear texture. During the following accumulative roll bonding, the texture of Mg layer transformed to typical rolling texture while the Al layer showed a combined texture type including evident β fiber and the Rotate Cube component.

Internal friction and microplastic deformation behavior of pure magnesium processed by equal channel angular pressing

Equal channel angular pressing (ECAP) was performed on the as-extruded commercial pure magnesium at 250°C for 4 passes. The internal friction of the ECAPed pure Mg as a function of strain amplitude was investigated by dynamic mechanical analyzer (DMA), and the cyclic microplasticity of pure Mg was investigated by tensile loading and unloading test. After ECAP processing, the grain size is significantly refined, the texture component with basal planes parallel to extrusion direction is replaced by a new stronger one with basal planes having a tilting angle of about 40° to the extrusion direction. The stress in microplastic region is reduced with increasing ECAP passes, while the internal friction increases. The internal friction of Mg at high strain amplitude is closely related to microplastic deformation and can be interpreted by dislocation mechanism. The Granato and Lücke model only satisfies in anelastic regions, while the internal friction in microplastic deformation region should be explained in terms of the internal friction model postulated by Peguin. The internal friction related to microplasticity can be divided into two parts with different activation volumes of dislocation motion, which correspond to the two regions of microplasctic deformation process. The initial stage associated with dislocation motion on basal plane shows larger activation volume and lower friction stress of dislocations. The second stage related to the annihilation and tangle of dislocations is characterized by larger hardening exponent and friction stress.

Effect of grain size on cyclic microplasticity of ECAP processed commercial pure magnesium

Equal channel angular pressing (ECAP) was performed on the extruded commercial pure magnesium at 250 °C for 4 passes. Heat treatments were carried out to modify the microstructures. The cyclic plastic deformation behavior of pure Mg with different grain sizes in microstrain region was studied by tensile loading and unloading experiments. The microplastic deformation process of pure Mg can be divided into two stages. In the first stage, pronounced plastic deformation associated with dislocation motion on basal plane is initiated at several MPa. The materials are softened and characterized by low friction stresses and hardening exponents. The microplastic deformation enters into region II above the strain of about 8 × 10−4. Annihilation and tangle of dislocations lead to the increase of hardening exponents and friction stresses. Pure Mg shows a very pronounced anelastic behavior during cyclic microplastic deformation, which results in a rapid increase of modulus defect, effectively decreasing the elastic modulus by up to 60 %. Grain size has a marked effect on microplastic deformation behavior of pure Mg. With increasing the grain size, the specimen shows a more pronounced microstrain and anelastic behavior.

Characterization and mechanical properties of coatings on magnesium by micro arc oxidation

Abstracts The commercial pure magnesium was coated by micro arc oxidation method in different aqueous solution, containing sodium silicate and sodium phosphate. Micro arc oxidation process was carried out at 0.060 A/cm2, 0.085 A/cm2 and 0.140 A/cm2 current densities for 30 min. The thickness, phase composition, morphology, hardness, adhesion strength and wear resistance of coatings were analyzed by eddy current, X-ray diffraction (XRD), scanning electron microscope (SEM), micro hardness tester, scratch tester and ball-on disk tribometer, respectively. The average thicknesses of the micro arc oxidized coatings ranged from 27 to 48 μm for sodium silicate solution and from 45 to 75 μm for sodium phosphate solution. The dominant phases formed on the pure magnesium were found to be a mixture of spinel Mg2SiO4 (Forsterite) and MgO (Periclase) for sodium silicate solution and Mg3(PO4)2 (Farringtonite) and MgO (Periclase) for sodium phosphate solution. The average hardnesses of the micro arc oxidized coatings were between 260 HV and 470 HV for sodium silicate solution and between 175 HV and 260 HV for sodium phosphate solution. Adhesion strengths and wear resistances of coatings produced in sodium silicate solution were higher than those of the ones in sodium phosphate solution due to high hardness of coatings produced in sodium silicate solution.

Low frequency damping capacities of commercial pure magnesium

The amplitude-dependent and temperature-dependent low frequency damping capacities of magnesium with 99.96% purity were studied by a dynamic mechanical analyzer. The pure magnesium alloys include CPM1 and CPM2 castings having textures of columnar grains which extraordinarily influence the damping behaviours. The commercial pure magnesium alloy CPM was re-melted to obtain equiaxed grains, which could remove the effect of texture orientation on the damping behaviours of these pure magnesium alloys. The results of strain amplitude-dependent damping spectrums of these pure magnesium alloys show that the pure magnesium with equiaxed grains possesses the highest damping capacity. In temperature-dependent damping plot for all these three pure magnesium alloys, there are two damping peaks P1 and P2 located at 80 and 230 °C, respectively. These two damping peaks are considered to be caused by the interaction between dislocation and point defects, and the movement of grain boundaries, respectively.

Grain refinement of commercial purity Magnesium processed by Ecap (Equal Channel Angular Pressing)

Grain refinement in magnesium is evaluated in the present paper. Equal Channel Angular Pressing is used to process commercially pure magnesium. Processing was carried out at 523 K which is lower than the temperature used in other papers on the literature. The grain structure was evaluated throughout the deformation zone. The low processing temperature prevents significant grain growth. The evolution of the grain structure is compared to a recent model for mechanism of grain refinement in magnesium. The present results confirm the validity of the model.

Microstructure and damping behaviour of consolidated magnesium chips

Chips produced by turning a commercial grade pure magnesium billet were consolidated by solid state recycling technique of cold compaction followed by hot extrusion. The cold compacted billets were extruded at four different temperatures: 250 degrees C, 300 degrees C, 350 degrees C and 400 degrees C. For the purpose of comparison, cast magnesium (pure) billets were extruded under similar conditions. Extruded products were characterized for damping properties. Damping capacity and dynamic modulus was measured as a function of time and temperature at a fixed frequency of 5 Hz 10 to 14% increase in damping capacity was observed in chip consolidated products compared to reference material. Microstructural changes after the temperature sweep tests were examined. Chip boundaries present in consolidated products were observed to suppress grain coarsening which otherwise was significant in reference material. The present work is significant from the viewpoint of recycling of machined chips and development of sustainable manufacturing processes. (C) 2012 Elsevier B.V. All rights reserved.

Effect of foil orientation on damage accumulation during irradiation in magnesium and annealing response of dislocation loops

The effect of foil orientation on damage accumulation behavior in commercial purity magnesium is investigated by in situ electron and ion irradiation. Transmission electron microscope has been used to study the dislocation loops formed by the agglomeration of point defects during irradiation. It has been observed that the ratio of prism plane to basal plane defects increases as the foil orientation is changed from basal to the prism foil. The ratio of vacancy to interstitial defects also increases in prism foils as compared to the basal foils. This point defect accumulation behavior is reversed when magnesium is irradiated with 1MeV Kr2+ ions and the formation of basal plane dislocation loops were only observed in prism foils and did not take place in the basal foils. Analysis showed that all the basal plane dislocation loops have Burgers vector of the type 16〈202¯3〉 and are interstitial in nature whereas prism plane dislocation loops have Burgers vector of the type 13〈112¯0〉 and are of mixed interstitial/vacancy in character.In situ annealing experiments at different temperatures performed on electron irradiated magnesium foils suggest that those dislocation loops that become thermodynamically unstable anneal out in a matter of few seconds whereas other stable dislocation loops continue to shrink by absorbing surrounding vacancy clusters. The activation energy for the shrinkage of the interstitial dislocation loops has been derived and the results show that the shrinkage of interstitial dislocation loops takes place by the mechanism of vacancy assisted self diffusion.

Microstructure and compression behavior of chip consolidated magnesium

Abstract

Fabrication of Multi-Phases Composite Mg-Ni Alloys by High Current Pulsed Electron Beam

Promising hydrogen storage can be obtained by hydride composites, the present investigation employed high currency pulsed electron beam (HCPEB) to deposit nano Ni powder on a commercial purity magnesium producing Mg-Ni alloy of multi-phases. Different roughness surface of magnesium were made and subjected to HCPEB treatment firstly, a content of Ni powder were coated on magnesium bases subsequently. Acceleration voltage of 25kV and 10 pulses were used for Mg-Ni alloying by HCPEB. XRD results show a composite of Mg, Mg2Ni and MgNi2 after HCPEB processing. It concludes that the deformation introduced by HCPEB treatment and adhesion of Ni powder are favorable for the formation of hydrogen storage phase Mg2Ni.

Bioactive Grain Refined Magnesium by Friction Stir Processing

Magnesium and its alloys are promising candidates for temporary implant applications due to their combination of mechanical properties, biocompatibility and biodegradation. But higher degradation rate restricts their wider applications. Recently friction stir processing (FSP) has emerged as a promising tool to attain near surface fine grain structure in materials. In the present work commercial purity magnesium was processed by FSP to obtain fine grain structure and the effect of the grain refinement on the bioactivity was investigated. The microstructural observations were carried out at different locations of the processed regions, from an original grain size of 1500μm, grain refinement was achieved to a level of 6.2μm at the nugget zone. Microhardness was measured across the processed regions and improvement was observed at the nugget zone. Contact angle measurements were carried out to estimate the wettability of the material and the measurements indicate increased wettability due to the increased surface energy induced by grain refinement. For studying the bioactivity the FSPed samples were immersed in simulated body fluids (SBF 5X) for different intervals of time. The phases formed on the samples were investigated by X-ray diffraction (XRD) method, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. The phases on the samples after 72hr of immersion were confirmed as magnesium hydroxide, hydroxyapatite and magnesium phosphate by XRD. Controlled degradation due to formation of these phases was observed. FSPed samples have more deposition of Ca/P than non FSP samples which implies better control over the degradation. Hence grain refinement by FSP can be a simple technique to control the degradation of magnesium for temporary implant applications.

Nickel–boron plating on magnesium and AZ91D alloy by a chromium-free electroless process

Ni–B coatings have been deposited directly on commercial purity magnesium and AZ91D magnesium alloy by a chromium-free electroless process and characterized using scanning electron microscopy, glow discharge optical emission spectroscopy, gravimetric measurements and, for corrosion evaluation, hydrogen evolution. The surfaces of the coatings reveal a typical cauliflower-type appearance and form at a rate of ~8 to 13μm/h. The nickel and boron contents of the coatings are relatively uniform across the whole thickness. The addition of NH4HF2 to the electroless bath improves the corrosion resistance provided by the coatings during immersion of coated substrates in sodium chloride solution.

Softening and dynamic recrystallization in magnesium single crystals during c-axis compression

Commercial purity (99.8%) magnesium single crystals were subjected to plane strain compression (PSC) along the c-axis at 200 and 370 °C and a constant strain rate of 10 −3 s −1. Extension was confined to the 〈 1 1 2 ¯ 0 〉 direction and the specimens were strained up to a logarithmic true strain of −1. The initial rapid increase in flow stress was followed by significant work softening at different stresses and comparable strains of about −0.05 related to macroscopic twinning events. The microstructure of the specimen after PSC at 200 °C was characterized by a high density of { 1 0 1 ¯ 1 } and { 1 0 1 ¯ 3 } compression twins, some of which were recrystallized. After PSC at 370 °C, completely recrystallized twin bands were the major feature of the observed microstructure. All new grains in these bands retained the same c-axis orientation of their compression twin hosts. The basal plane in these grains was randomly rotated around the c-axis, forming a fiber texture component. The obtained results are discussed with respect to the mechanism of recrystallization, the specific character of the boundaries between new grains and the initial matrix, and the importance of the dynamically recrystallized bands for strain accommodation in these deformed magnesium single crystals.

In vitro biodegradation behavior of magnesium and magnesium alloy

Magnesium has the potential to be used as degradable metallic biomaterial. For magnesium and its alloys to be used as biodegradable implant materials, their degradation rates should be consistent with the rate of healing of the affected tissue, and the release of the degradation products should be within the body's acceptable absorption levels. Conventional magnesium degrades rapidly, which is undesirable. In this study, biodegradation behaviors of high purity magnesium and commercial purity magnesium alloy AZ31 in both static and dynamic Hank's solution are systematically investigated. The in vitro test results show that magnesium purification and selective alloying are effective approaches to reduce the degradation rate of magnesium. In the static condition, the corrosion products accumulate on the materials surface as a protective layer, which results in a lower degradation rate than the dynamic condition. Anodized coating can significantly further reduce the degradation rate of magnesium. This study strongly indicates that magnesium can be used as degradable implant material as long as the degradation is controlled at a low rate. Magnesium purification, selective alloying, and anodized coating are three effective approaches to reduce the rate of degradation.

Characterization and corrosion behavior of ceramic coating on magnesium by micro-arc oxidation

In this study, the commercial pure magnesium was coated in different aqueous solutions of Na 2SiO 3 and Na 3PO 4 by the micro-arc oxidation method (MAO). Coating thickness, phase composition, surface and cross sectional morphology and corrosion resistance of coatings were analyzed by eddy current method, X-ray diffraction (XRD), scanning electron microscope (SEM) and tafel extrapolation method, respectively. The average thickness of the coatings ranged from 52 to 74 μm for sodium silicate solution and from 64 to 88 μm for sodium phosphate solution. The dominant phases on the coatings were detected as spinal Mg 2SiO 4 (Forsterite) and MgO (Periclase) for sodium silicate solution and Mg 3(PO 4) 2 (Farringtonite) and MgO (Periclase) for sodium phosphate solution. SEM images reveal that the coating is composed of two layers as of a porous outer layer and a dense inner layer. The corrosion results show the coating consisting Mg 2SiO 4 is more resistant to corrosion than that containing Mg 3(PO 4) 2.

Irradiation damage in commercial purity magnesium

In-situ electron irradiation damage within the transmission electron microscope has been studied and compared with ion irradiation on commercial purity magnesium. Magnesium has the same crystal structure (hcp) and a similar c/a ratio to that of zirconium and is considered a suitable model system and useful analogue to zirconium (an important structural material for nuclear reactors). The results presented in this paper suggest that microstructure developed during irradiation is modified by the nature of the irradiating particles. The basal plane dislocation loops with Burgers vector 1 / 6 〈 2 0 2 ¯ 3 〉 are formed by electron irradiation and analysis showed that all of these dislocation loops are interstitial in nature. Vacancy type prism plane dislocation loops with Burgers vector 1 / 3 〈 1 1 2 ¯ 0 〉 also form but to a lesser degree than the basal plane dislocation loops. Ion irradiation for a dose up to 0.1 dpa produces a majority of interstitial type prism plane dislocation loops with Burgers vector 1 / 3 〈 1 1 2 ¯ 0 〉 . Void formation also takes place in ion irradiated samples subjected to a higher dose of 0.7 dpa.

Native Air-Formed Oxide Film and its Effect on Magnesium Alloys Corrosion

The present work uses X-ray photoelectron spectroscopy (XPS) analysis to compare the chemical composition of native oxide films formed spontaneously on commercial pure magnesium and AZ31, AZ80 and AZ91D magnesium alloys. The study considers both the outer surface and inner regions of the films with the assistance of argon ion bombardment. Possible relationships are established between the alloy Al content and the native oxide film characteristics. The Al content is very similar in the oxide films on all three studied alloys. XPS identifies a much greater film thickness on AZ80 and AZ91D specimens than on AZ31 and pure Mg specimens, which seems to be related with the presence ofphase (Mg17Al12) on the AZ91D alloy surface and of the eutectic � -Mg/� on the AZ80 alloy surface. Considerable Ca segregation is observed (directly related with the calcium impurities content in the bulk material) towards the outer surface of the metal, where it appears in the form of calcium oxide. Direct correspondence is found between the thickness of the native oxide film formed spontaneously on the surface of magnesium and its alloys and their subsequent corrosion resistance in exposure to a 3.5 wt% NaCl solution.

Chloride-induced filiform corrosion of organic-coated magnesium

In-situ scanning Kelvin probe (SKP) potentiometry is used to study the underfilm corrosion of organic-coated, commercial purity (CP) magnesium under conditions of high relative humidity. Following initiation by the application of aqueous hydrochloric acid (HCl) to a penetrative coating defect, underfilm corrosion is characterised by the onset of coalesced filiform-like features, which lengthen with time and propagate at a rate which is largely independent of the quantity of initiating HCl. The potentials of the advancing underfilm corrosion-front range from −1.35 to ca. −1.45 V vs. SHE, with corrosion potentials in the tail region largely similar to those of the intact coated Mg surface. The rate of filiform corrosion (FFC) advance is shown to be insensitive to the presence of oxygen, but highly dependent upon the relative humidity of the holding environment. It is proposed that the mechanism of propagation is not governed by differential aeration, but rather a differential “electrocatalytic activation” phenomenon involving anodic Mg dissolution at the leading edge of the corrosion front, galvanically coupling with hydrogen evolution on a cathodically activated corroded region behind. This proposed mechanism also accounts for observations of FFC upon immersion of CP-Mg in dilute chloride-containing electrolyte, where differential aeration would not be expected to exert an influence.

Microstructure and Properties of Pure Mg/ZK60 Laminate Processed by Accumulative Roll Bonding

Commercial pure magnesium with excellent damping capacity and ZK60 magnesium alloy with high strength were accumulative roll bonded (ARBed) at 300oC up to 3 cycles to fabricate Mg laminate consisting of alternating layers of pure Mg and ZK60. Microstructure, tensile properties and damping capacity of the sheets were analyzed. The research suggests that Mg sheet having both high strength and excellent damping capacity can be developed by accumulative roll bonding of pure Mg and ZK60 Mg alloy.