Segregation Of Magnesium Research Articles

Local damage in grain boundary stabilized nanocrystalline aluminum

The excess free volume in grain boundaries of pure nanocrystalline aluminum leads to the grain coarsening, which degrades their desirable mechanical properties. The preferential segregation of dopants to the grain boundaries has been proposed to enhance the stability of the nanostructure. To shed light on the stabilization mechanisms, we present results from virtual mechanical tests on nanocrystalline aluminum samples with similar grain sizes but different magnesium dopant contents. The segregation of magnesium decreases the excess free volume within the grain boundaries and prohibits the nucleation and growth of intergranular cracks.

Grain-boundary segregation of magnesium in doped cuprous oxide and impact on electrical transport properties

In this study, we report the segregation of magnesium in the grain boundaries of magnesium-doped cuprous oxide (Cu2O:Mg) thin films as revealed by atom probe tomography and the consequences of the dopant presence on the temperature-dependent Hall effect properties. The incorporation of magnesium as a divalent cation was achieved by aerosol-assisted metal organic chemical vapour deposition, followed by thermal treatments under oxidizing conditions. We observe that, in comparison with intrinsic cuprous oxide, the electronic transport is improved in Cu2O:Mg with a reduction of resistivity to 13.3 ± 0.1 Ω cm, despite the reduction of hole mobility in the doped films, due to higher grain-boundary scattering. The Hall carrier concentration dependence with temperature showed the presence of an acceptor level associated with an ionization energy of 125 ± 9 meV, similar to the energy value of a large size impurity−vacancy complex. Atom probe tomography shows a magnesium incorporation of 5%, which is substantially present at the grain boundaries of the Cu2O.

Thermal and spectroscopic characterization of sol–gel-synthesized doped lanthanum gallate

In this study, a complex oxide solid solution consisting of lanthanum gallate with partial substitutions for strontium and magnesium (La0.9Sr0.1Ga0.8Mg0.2O2.85) was synthesized by the sol–gel route, aiming to obtain a sinter active powder and a final material that could be stoichiometrically controlled. The thermal behavior of the synthesized powder involves several steps of decomposition. The linear shrinkage of green compacts up to 1500 °C was 31%. The bulk conductivity of sintered specimens increases with sintering temperature up to 1400 °C. The stoichiometry was maintained for sintering temperatures up to 1450 °C. Elemental mapping obtained by energy-dispersive spectroscopy evidenced magnesium segregation at the grain boundaries. The overall results evidence the suitability of the synthesis method for preparing doped lanthanum gallate.

Hot cracking in autogenous welding of 6061-T6 aluminum alloy by rectangular pulsed Nd:YAG laser beam

Due to high hot crack susceptibility of Al-Mg-Si alloys, autogenous welding by rectangular pulsed laser beam has not been generally successful in the removal of cracks. In this research, the effect of pulsed Nd:YAG laser parameters and preheating on the creation of hot cracks in the 6061-T6 aluminum alloy was investigated. The sample that was fabricated by the laser parameters including 1 Hz, 0.12 mm/s, 10 ms, and without preheating exhibited the highest cooling rate and the smallest dendrite arm spacing but no hot crack was observed. The tensile test specimens of this sample fractured at a point far from the weld metal and a decrease in the precipitation of silicon and magnesium in the inter-dendritic space and the reduction of thermal stresses resulted in the elimination of hot cracks. However, according to macro-scale models for the creation of hot cracks, preheating decreased the tensile stresses in the BTR (brittle temperature range), but increasing the preheating temperature led to increasing rather than decreasing the hot crack length. In this case, the formation of hydrogen porosity, the segregation of silicon and magnesium, and the creation of low melting point compounds were the important parameters affecting the hot crack initiation and growth.

Evaluation of microstructure stability at the interfaces of Al-6061 welds fabricated using ultrasonic additive manufacturing

Ultrasonic additive manufacturing (UAM) is a solid-state additive manufacturing process that uses fundamental principles of ultrasonic welding and sequential layering of tapes to fabricate complex three-dimensional (3-D) components. One of the factors limiting the use of this technology is the poor tensile strength along the z-axis. Recent work has demonstrated the improvement of the z-axis properties after post-processing treatments. The abnormally high stability of the grains at the interface during post-weld heat treatments is, however, not yet well understood. In this work we use multiscale characterization to understand the stability of the grains during post-weld heat treatments. Aluminum alloy (6061) builds, fabricated using ultrasonic additive manufacturing, were post-weld heat treated at 180, 330 and 580 °C. The grains close to the tape interfaces are stable during post-weld heat treatments at high temperatures (i.e., 580 °C). This is in contrast to rapid grain growth that takes place in the bulk. Transmission electron microscopy and atom-probe tomography display a significant enrichment of oxygen and magnesium near the stable interfaces. Based on the detailed characterization, two mechanisms are proposed and evaluated: nonequilibrium nano-dispersed oxides impeding the grain growth due to grain boundary pinning, or grain boundary segregation of magnesium and oxygen reducing the grain boundary energy.

Metallurgical investigations of laser remelted additively manufactured AlSi10Mg parts

AbstractSelective laser melting is gaining importance to manufacture reliable and highly complex parts. However, the surfaces of the selective laser melted parts exhibit for many applications an insufficient high roughness, thus require subsequent post processing steps. A relatively new way to reduce the surface roughness is the laser polishing technique. In the present paper, additively manufactured AlSi10Mg samples were polished with different laser intensities and laser modes. The investigations contain the potential of roughness reduction and enhancement of the surface appearances, which can be achieved by laser polishing of the as‐built surfaces. An initial arithmetic mean roughness of 8.43 μm was remarkably reduced up to 98 %. The compositions of the polished surfaces were detected and the surface appearances were examined. Reasons and mechanisms were explained and depicted for the occurred shade formations on the polished surfaces. High laser intensity led to segregation of silicon and magnesium on the surface. A higher laser intensity enabled an increased melt depth within the conture layer of the selective laser melting structure. Through increasing melt depth, a porosity of max. 1.7 % was detected in the remolten area. Hardness investigations of the initial and laser remolten cross section revealed no significant reduction in hardness.

Influence of diffusion on the coating adhesion of zinc-magnesium thin films onto steel

Abstract In this study the zinc and magnesium diffusion in ZnMg thin PVD films is examined and linked to the crash adhesion performance of the coating-substrate system. The investigation is conducted at 175 °C because this temperature conforms to both the temperature range during production of the films and the post process steps at automotive customers. The results confirm that zinc is the faster diffusive specie and although the movement of zinc results in a phase transformation, first from magnesium into MgZn2 and later from MgZn2 into Mg2Zn11 this phase change does not seem to alter the crash adhesion performance of the coating substrate system. Additionally, the magnesium seems to diffuse along grain boundaries towards the steel substrate interface. This segregation of magnesium and the enrichment of magnesium at the interface results in a deterioration of the crash adhesion resistance. The time until the samples fail the crash adhesion test is determined by the distance the magnesium has to travel from the Mg containing layer to the steel-substrate interface.

Adhesion enhancement of titanium nitride coating on aluminum casting alloy by intrinsic microstructures

Aluminum casting alloys have excellent castability, high strength and good corrosion resistance. However, the presence of silicon in these alloys prevents surface finishing with conventional methods such as anodizing. Hard coating with titanium nitride can provide wear and corrosion resistances, as well as the aesthetic finish. A critical factor for a durable hard coating is its bonding with the underlying substrate. In this study, a titanium nitride layer was coated on LM25 casting alloy and a reference high purity aluminum substrate using Ion Assisted Deposition. Characterization of the coating and the critical interface was carried out by a range of complementing techniques, including SIMS, XPS, TEM, SEM/EDS and nano-indentation. It was observed that the coating on the aluminum alloy is stronger compared to that on the pure aluminum counterpart. Silicon particles in the alloy offers the reinforcement though mechanical interlocking at microscopic level, even with nano-scale height difference. This reinforcement overcomes the adverse effect caused by surface segregation of magnesium in aluminum casting alloys.

RETRACTED ARTICLE: Investigation on heterogeneous nucleation of MgAl2O4 in Al-Mg alloys

This work investigates the mechanisms of grain refinement of Al-Mg alloys through ultrasonic treatment and varying Mg content. MgAl2O4 particles display an equiaxed and faceted morphology with {111} planes and when exposed to {111} of α-Al, can act as heterogeneous and potent nuclei for aluminium. Intensive vibrations by ultrasonic treatment lead to significant grain refinement through effective dispersion of MgAl2O4 particles. The equiaxed and faceted morphology of MgAl2O4 along with a well-defined cube-on-cube orientation relationship $$(1\bar 11)[110]$$ MgAl2O4// $$(1\bar 11)[110]$$ α-Al, provide improved refinement. Furthermore, multiplication of solidification sites by the ultrasonic pulses enhances heterogeneous nucleation. The resulting improved grain refinement relates in all probability to the increased nucleation sites, due to grain multiplications and wetting of impurities, the lowering of surface tension due to the segregation of magnesium to the surface of liquid aluminium, the appropriate growth restriction factor and the low lattice mismatch coupled with strong dispersion of oxides.The filtration test confirms these particles as potent refiners.

Initial formation of corrosion products on pure zinc and MgZn2 examinated by XPS

A corrosion behaviour study of pure phases of zinc and zinc–magnesium (MgZn2) exposed to humid air at 35°C for 4days is presented. For zinc, hydroxide formation at the surface and zinc oxide in the first innerlayers is observed. The corrosion of the MgZn2 leads to the segregation of magnesium at the surface to mainly form magnesium hydroxycarbonate: the presence of magnesium modifies the corrosion products. At larger depths, metallic zinc coexists with magnesium oxide and hydrozincite. The higher reactivity of MgZn2 alloy can be attributed to the interaction with carbonate ions.

Thermal-spray deposition of enamel on aluminium alloys

Traditional enamelling requires high temperatures, which soften high-performance aluminium substrates and limit their application in engineering. Thermal spraying can, in principle, overcome this problem by enabling low substrate temperatures during deposition but it was found that its rapid cooling conditions produced enamel with high residual stress and poor adhesion. The research shows that thermal-spray enamelling of aluminium can be successfully achieved by introducing a preheating step prior to spraying, so that the enamel can flow for long enough to relax the residual stress. In this study, preheating was undertaken by flame scanning as a means of raising the enamel above its glass transition temperature and allowing sufficient flow. Furthermore, it was found that in the Al–Mg alloy substrate under investigation, the magnesium segregated to the substrate–enamel interface and this also adversely affected adhesion. The preheating temperature therefore needs to be optimized to give adherence but avoid excessive substrate softening and magnesium segregation. The results showed that optimally flame-sprayed enamel successfully increased the wear resistance of the Al–Mg alloy by a factor of ten. The thermal-spray enamelling of generic aluminium alloys is discussed.

Mixed Magnesium and Zinc Oxide Prepared by Co-precipitation and Analyzed by XPS

Two Zinc-Magnesium Oxide powders were prepared by means of a sol-gel method based on the mixing of nitrates of Mg and Zn, followed by complexation with citric acid. The samples are characterized by a nominal (i.e. calculated from the weighted amount of precursors) Zn/Mg atomic ratio of 0.01 (specimen 1, Accession #1232), and 0.5 (specimen 2, Accession #1233). The surface properties and the influence of the Zn/Mg atomic ratio are investigated by means of XPS (using a standard Al Kα). Besides the wide scan spectra, detailed spectra for the Mg 1s, Zn 2p3/2 and O 1s regions and related data are presented and discussed. XP analysis confirms the presence of Mg (II) and Zn (II). The XPS Zn/Mg atomic ratio is always lower than the nominal one; this is particularly evident in specimen 1 (Accession #1232) thus suggesting a certain surface segregation of magnesium. The O/(Mg + Zn) atomic ratio is higher than the nominal one and decreases with increasing the Zn/Mg atomic ratio (from 1.5 for [Zn/Mg]nom = 0.01 to 1.3 for [Zn/Mg]nom = 0.5).

ChemInform Abstract: Segregation of Calcium and Magnesium into Different Substructures. Ca4Ag0.948Mg and Other Compounds with Gd4RhIn‐Type Structure.

AbstractThe compounds Ln4PdMg (Ln: Ca, Eu, Tb—Lu), Ln4AgMg (Ln: Ca, Yb), Ln4PtMg (Ln: Eu, Tb—Lu), and Ln4AuMg (Ln: Ca, Eu, Yb) are prepared by induction melting of the elements.

Action of Magnesium on Carbide Spheroidization in Annealed Bearing Steel

The effect of magnesium on the morphology and composition of the carbide in annealed bearing steel was studied. The morphology of carbide was observed and statistical analysis was carried out. The results showed that the non-uniform distribution of carbides in bearing steel was obviously improved with increasing the content of magnesium. Segregation thermodynamics indicated that the segregation of magnesium to grain boundaries prevented element such as Cr, C from enriching on grain boundary, which was helpful for the reduction of network carbide precipitation. The chemical analysis results proved this viewpoint.

Segregation of Calcium and Magnesium into Different Substructures. Ca4Ag0.948Mg and Other Compounds with Gd4RhIn-type Structure

Segregation of Calcium and Magnesium into Different Substructures. Ca4Ag0.948Mg and Other Compounds with Gd4RhIn-type Structure

Pyramidal defects in Mg-doped GaN in light of strain-energy minimization

The planar segregation gives rise to stress and strain fields which are approximated by a uniaxial character in a displacement controlled system. In this condition, the elastic strain energy is proportional to Young’s modulus. Young’s modulus of GaN is minimized when the directions normal to a conical segregation surface make about 48° with the c-axis of hexagonal GaN, which is close to the angle 47.3° between the c-axis and the directions normal to the {112¯3} planes. This implies that the formation of pyramidal defects in magnesium-doped GaN can be a compromise between minimization of the elastic strain energy due to segregation of magnesium and the planar segregation.

Effect of Yttrium and Magnesium Segregation on Microstructure and Mechanical Properties of Heavy Section Ductile Iron Castings

Ductile iron specimen with dimensions of Φ590mm×800mm were prepared by treating the melt with an yttrium-containing nodulizer. Cooling curves at four locations____the center location, the location 85mm distant from the center, the location 170mm distant from the center and the location 255mm distant from the center of the heavy ductile iron casting Φ590mm×800mm was obtained. The effect of yttrium and magnesium on microstructure and mechanical properties was investigated, and the factors on the distribution of chunky graphite in the specimen were discussed as well. The results show that the largest amount of chunky graphite and the lowest mechanical properties appear at the location 170mm distant from the center. Excessive yttrium and magnesium content at the location 170mm distant from the center is the main factor for the large amount of chunky graphite. The characteristics of the solidification mode____pasty solidification and the considerably long solidification of the casting are the basic reasons for the distribution of chunky graphite of the heavy ductile iron casting Φ590mm×800mm.

Research on Friction and Wear Properties of Annealed 5052 Al-Mg Alloy

At annealing temperature, Al-Mg alloys have grain boundary and surface magnesium segregation and their strength are also impossibly enhanced by heat treatment. Friction and wear properties of cold-rolled 5052 aluminum alloy annealed at different temperature were studied in this paper. X-ray diffract analysis(XRD) indicates that metallurgical phase has no obvious transformation with different annealing temperature. Auger electron (AE) fines that magnesium segregation and oxide layer thickness of surface are different along the plate thickness direction at different annealing temperatures. Results show that the higher oxide layer of the surface, the smaller wear can be got. Friction and wear testing shows that 5052 aluminum alloy annealed at different temperatures almost have the same coefficient of friction under the same experimental condition , but the coefficient friction of the same sample will increase while temperature rises.

Investigation on the magnesium segregation in low‐magnesium aluminium alloys

AbstractThis paper studies the magnesium migration phenomenon during annealing in 8xxx series alloy with magnesium bulk content in the range of 20–240 ppm. X‐ray photoelectron spectroscopy and dynamic secondary ion mass spectrometry are used to determine the Mg surface concentration and distribution in the aluminium oxide layer depending on the magnesium bulk concentration and the annealing duration. Copyright © 2010 John Wiley & Sons, Ltd.

Microdeformation behaviour of Al–SiC metal matrix composites

The satisfactory performance of metal matrix composites depends critically on their integrity, the heart of which is the quality of the matrix-reinforcement interface. The nature of the interface depends in turn on the processing of the MMC component. At the micro-level, the development of local concentration gradients around the reinforcement can be very different according to the nominal conditions. These concentration gradients are due to the metal matrix attempting to deform during processing. This plays a crucial role in the micro-structural events of segregation and precipitation at the matrix-reinforcement interface. Equilibrium segregation occurs as a result of impurity atoms relaxing in disordered sites found at interfaces, such as grain boundaries, whereas non-equilibrium segregation arises because of imbalances in point defect concentrations set up around interfaces during non-equilibrium heat treatment processing. The amount and width of segregation depend very much on (a) the heat treatment temperature and the cooling rate, (b) the concentration of solute atoms and (c) the binding energy between solute atoms and vacancies. An aluminium–silicon–magnesium alloy matrix reinforced with varying amounts of silicon carbide particles was used in this study. A method of calculation has been applied to predict the interfacial fracture strength of aluminium, in the presence of magnesium segregation at metal matrix interface. Preliminary results show that the model succeeds in predicting the trends in relation to segregation and intergranular fracture strength behaviour in these materials. Microhardness profiles of reinforced and un-reinforced aluminium alloys are reported. The presence of precipitates at alloy-reinforcement interface identified by Nano-SEM.