MgAl2O4 Phases Research Articles

Relative hardness and corrosion behavior of micro arc oxidation coatings deposited on binary and ternary magnesium alloys

Abstract Magnesium alloys have attracted wide-spread industrial applications in automotive, space, mobile and telecommunication segments. Present investigation reports the influence of Mg-alloy composition (7-different substrate compositions: pure Mg, three binary Mg–Al alloys with varying wt.% Al such as 3%, 6% and 9% and three Mg–Al–Zn ternary alloys such as AZ31, AZ63 and AZ91) on the micro arc oxidation (MAO) coating formation kinetics, coating morphology, elemental and phase compositions, coating hardness and corrosion behavior. The substrate composition significantly influences the coating formation kinetics, coating hardness and corrosion behavior. Although Zn added to Mg alloys does not contribute directly toward the formation of any specific Zn-bearing phase like MgAl2O4 phase as contributed by the Al present in the Mg alloy, it provides dramatic reduction in corrosion rate. A minimum of 3 wt.% Al is essentially required in ternary Mg alloys to exhibit significantly improved relative corrosion resistance (RCR) while 6–9 wt.% of Al is preferred for enhanced relative hardness (RH). The RCR and RH were found to be beneficial criterions for effective design of new Mg-alloys with attractive combination of mechanical properties and corrosion resistance.

Microstructure and mechanical properties of Al2O3/MgAl2O4/ZrO2 eutectic ceramic prepared with induction zone melting

In order to develop low-cost large eutectic ceramic, Al2O3/MgAl2O4/ZrO2 eutectic ceramic crystal was prepared with induction zone melting. The eutectic phases' composition and microstructure were investigated with X-ray diffraction and scanning electron microscopy, respectively, and mechanical properties were tested with micro-indentation and three-point bending method. The crystal consists of only Al2O3, MgAl2O4 and ZrO2 phases, and no pore is discovered along the whole crystal. The eutectic spacing decreases as the growth rate increases. After a heat treatment in 1550°C for 15 hours in the air, both the crystal weight and the phase size do not change. Its flexure strength reaches up to 720 MPa, which is about four times of that of the sintered ceramic. Its fracture toughness reaches 6·0 MPa m−1/2, more than three times of that of the sintered ceramic.

The firing steps and phases formed in Mg–Zr–Al refractory dolomite-based materials

Due to the open pore microstructure, MgO–CaZrO3 refractory composites are considered useful materials in various industrial applications. However, the reactivity of the MgO limits its utilization in acid conditions. The chemical inertness and more balanced mechanical properties of MgAl2O4 in comparison with MgO bring forward CaZrO3–MgAl2O4 refractories as promising alternative materials.The present study was carried out in order to determine the feasibility of using Argentine dolomites in the preparation of CaZrO3–MgAl2O4 by solid state reaction. The thermal and structural changes which occur during the firing of the batches up to 1425°C were studied by the combination of diffractometric and infrared spectroscopic data at the most remarkable reaction steps. The study of phase changes within the investigated thermal range allowed establishing the main differences in the composition of the mixtures in each firing step. According to these results, the final product is composed mainly by MgAl2O4, CaZrO3, and Ca2SiO4 phases and the optimal synthesis temperature would be 1425°C. According to SEM microscopy, the existence of ceramic bonding between CaZrO3 and MgAl2O4 could be confirmed.

Low-Temperature Synthesis of MgAl<sub>2</sub>O<sub>4</sub> by Capsule HIP Using Hydroxides as Starting Materials

Low-temperature synthesis of spinel (MgAl2O4) was investigated by capsule hot isostatic press (HIP) using hydroxides as starting materials; Mg(OH)2, MgO, Al(OH)3, g-AlOOH, g-Al2O3. MgAl2O4 has been conventionally prepared above 1400 °C, whereas the mixtures containing the hydroxides reacted at 400-500 °C for 1 h under 200 MPa of isostatic pressure to form MgAl2O4. The hydroxides showed high reactivity to form MgAl2O4 with high crystallinity. However, the products more or less contained Mg(OH)2. The single phase of MgAl2O4 was obtained by adding 20 mass% of water to the starting mixtures of Mg(OH)2 - Al(OH)3, Mg(OH)2 - g-Al2O3 and MgO - Al(OH)3. The average grain size of the MgAl2O4 was 70 – 100 nm. It was concluded that the transformation of AlOOH or g-Al2O3 was promoted by a small amount of water and that Mg(OH)2 rapidly reacted with the active intermediate compound between AlOOH or g-Al2O3 and a-Al2O3.

Spark plasma sintering of MgAl2O4–YCr0.5Mn0.5O3 composite NTC ceramics

New high temperature negative temperature coefficient (NTC) thermistor ceramics based on a xMgAl2O4–(1 − x)YCr0.5Mn0.5O3 (x = 0.1, 0.4, 0.6) composite system have been successfully fabricated through spark plasma sintering (SPS) with a low sintering temperature and a short sintering period. The X-ray diffraction analysis indicates that the SPS-sintered composite ceramics consist of a cubic spinel MgAl2O4 phase and an orthorhombic perovskite YCr0.5Mn0.5O3 phase isomorphic to YCrO3. The SPS-sintered composite ceramics have high relative density ranging from 94.1 to 97.4% of the theoretical density. X-ray photoelectron spectroscopy analysis corroborates the presence of Cr3+, Cr4+, Mn3+, and Mn4+ ions on lattice sites, which may result in the hopping conduction. The obtained ρ25, B25–150, and B700–1000 of the SPS-sintered composite NTC thermistors are in the range of 1.53 × 106–9.92 × 109 Ω cm, 3380–5172 K, and 7239–9543 K, respectively. These values can be tuned by adjusting the MgAl2O4 concentration.

Evaluation of aluminum dross as raw material for high-alumina refractory

We report on the preparation of high-alumina refractory (HAR) from metallurgical aluminous dross with main crystalline phase of MgAl2O4 and small amounts of CaAl2O4. Samples were characterized using X-ray diffraction, scanning electron microscopy and petrographic analysis. Physical properties such as bulk density, apparent porosity and compressive strength of HAR were investigated. Magnesium aluminate spinel, calcium monoaluminate (CaAl2O4) and calcium bialuminate (CaAl4O7) were the primary phases of the obtained HAR. CaAl4O7 was generated after sintering and formed a matrix to bind the separated magnesia–alumina spinel effectively, which could improve the compactness of the samples. Ti promoted the propagation velocity of phase particles, and could reduce the extent of oxidation and improve HAR product compactness. The as-prepared HAR met the Chinese National Standard (GB/T 2988-2004), indicating its potential application in the refractory field.

Structural, Microstructure, Mechanical and Electrical Properties of Porous Zr4+-Cordierite Ceramic Composites

Zirconium-cordierite ceramic composites have been synthesized by a co-precipitation method using MgCl2·6H2O, NaAlO2, Na2SiO3·5H2O, and ZrOCl2·8H2O as starting materials. XRD, FT-IR, and SEM techniques were employed to study the effect of zirconium on the crystal structure and microstructure of the samples. XRD results revealed that spinel MgAl2O4 and t-ZrO2 phases were predominant in the samples with low Zr4+ content (10 wt.%), whereas zircon ZrSiO4 was predominant with high Zr4+ content (≥15 wt.%). The densification behavior was improved from 30.4 to 44.6% of the theoretical density (2.6 g/cm3) at 15 wt.% of Zr4+. However, microhardness of the sintered samples was enhanced from 7.1 to 7.5 GPa with increasing the Zr4+ dose from 0 to 25 wt.%. On the other hand, the gradual increase in Zr4+ content from 0 to 25 wt.% led to suppression in the electrical resistivity (ρ) from 16.6 to 2.8 × 109 Ω/cm, respectively. In addition, the dielectric permittivity (e) of the pure cordierite was decreased with Zr4+ ion addition. The maximum dielectric permittivity (e) at low frequencies (10 MHz) was 18.7 at 10 wt.% Zr4+ content, whereas at high frequencies (1 GHz) it was 38.8 at 15 wt.% Zr4+ content.

High-Pressure Combinatorial Process Integrating Hot Isostatic Pressing

A high-pressure combinatorial process integrating hot isostatic pressing (HIP) was developed by providing a reaction vessel with a high-pressure tightness based on a commercial flange. The reaction vessel can be used up to 200 MPa and 500 °C under HIP processing condition. Preparation of spinel-type MgAl2O4 from Mg(OH)2, Al(OH)3 and AlOOH was performed using the reaction vessel under 200 MPa and 500 °C as demonstration. The entire powder library was characterized using powder X-ray diffraction patterns, and the single phase of spinel-type MgAl2O4 was obtained from Mg(OH)2+Al(OH)3. These assessments corresponded with previously published data.

Effect of calcination temperature on Mg–Al bimetallic oxides as sorbents for the removal of F− in aqueous solutions

Bimetallic oxides were synthesized from hydrotalcite using increasing calcination temperatures (873, 1073, 1273K). These bimetallic oxides were fully characterized and the sorption density of F− was investigated. X-ray diffraction patterns for the produced bimetallic oxides showed that MgO was the primary phase within the range of investigated calcination temperatures, but MgO crystallinity increased with calcination temperature and an additional MgAl2O4 phase was formed. In the process of F− sorption, the bimetallic oxides were primarily transformed into hydrotalcite with intercalation of F−.The Higher calcination temperature increased the MgAl2O4 phase, which did not contribute to the immobilization of F−. These findings show that optimizing the calcination temperature can be used to maximize the sorption density of this material for F− removal.

MgAl2O4–LaCr0.5Mn0.5O3 composite ceramics for high temperature NTC thermistors

New negative temperature coefficient (NTC) ceramics based on xMgAl2O4–(1 − x)LaCr0.5Mn0.5O3 (x = 0, 0.4, 0.6, 0.8) composition were investigated. The composite ceramics (x = 0.4, 0.6, 0.8) consisted of two phases: a cubic spinel MgAl2O4 phase and an orthorhombic perovskite LaCr0.5Mn0.5O3 phase isomorphic to LaCrO3. The brighter regions were the LaCr0.5Mn0.5O3 and the darker was the MgAl2O4. There was ion diffusion between these two phases. X-ray photoelectron spectroscopy analysis corroborated the presence of Cr3+, Cr4+, Mn3+ and Mn4+ ions on lattice sites, which may result in the hopping conduction. The ρ 300 and B 400/800 constant increased with increasing MgAl2O4 content. The values of ρ 300, B 400/800 and activation energy of NTC thermistors were in the range 1.76–1.22 × 108 Ωcm, 2,646–8,711 K, 0.228–0.746 eV, respectively. This means the electrical properties can be adjusted to desired values, depending on the MgAl2O4 content.

Radiation damage in multiphase ceramics

Four-phase ceramic composites containing 3mol% Y2O3 stabilized ZrO2 (3Y-TZP), Al2O3, MgAl2O4, and LaPO4 were synthesized as model materials representing inert matrix fuel with enhanced thermal conductivity and decreased radiation-induced microstructural damage with respect to single-phase UO2. This multi-phase concept, if successful, could be applied to design advanced nuclear fuels which could then be irradiated to higher burn-ups. 3Y-TZP in the composite represents a host (fuel) phase with the lowest thermal conductivity and Al2O3 is the high thermal conductivity phase. The role of MgAl2O4 and LaPO4 was to stabilize the structure under irradiation. The radiation response was evaluated by ion irradiation at 500°C with 10MeV Au ions and at 800°C with 92MeV Xe ions, to simulate damage due to primary knock-on atoms and fission fragments, respectively. Radiation damage and microstructural changes were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy and computational modeling. Al2O3, Y2O3 stabilized ZrO2 and MgAl2O4 phases exhibit high amorphization resistance and remain stable when irradiated with both Au and Xe ions. A monoclinic-to-tetragonal phase transformation, however, is promoted by Xe and Au ion irradiation in 3Y-TZP. The LaPO4 monazite phase appears to melt, dewet the other phases, and recrystallize under Au irradiation, but does not change under Xe irradiation.

Effect of Duty Cycle on Microstructure and Corrosion Resistance of MAO Coatings Fabricated on AZ91D Magnesium Alloy in an Optimized Dual Electrolyte

Ceramic coatings were fabricated on AZ91D magnesium alloy in a novel optimized dual electrolyte by Microarc oxidation. The influence of duty cycle on coating thickness, surface roughness, phase composition, microstructure and corrosion resistance were studied. It is found that both the thickness and surface roughness of coatings increases gradually with the increasing of duty cycle. XRD results show that all the coatings are mainly composed of MgO, Mg2SiO4 and MgAl2O4. The results of potentiodynamic polarization tests indicate that the coating formed at 40% duty cycle exhibits a better corrosion resistance as a result of its relatively compact microstructure and more relative content of Mg2SiO4 and MgAl2O4 phases.

Microstructure and corrosion resistance of AZ91D magnesium alloy treated by hybrid ion implantation and heat treatment

In order to improve the corrosion resistance of magnesium alloy, solid solution-aging treatment and then nitrogen and aluminum (N/Al) duplex ion implantation was conducted on AZ91D magnesium alloy surface. X-ray diffraction (XRD), scanning electronic microscope (SEM) with energy dispersive X-ray spectrometer (EDX or EDS) analysis, Auger electron spectroscopy (AES), electrochemical workstation and microhardness tester are used to characterize the surface microstructure, phase composition, atomic concentration-depth distribution, corrosion resistance and microhardness, respectively. XRD results show that the surface modified layer includes Mg, Mg17Al12, MgAl2O4 and AlN phase, the diffraction peak position and intensity of Mg and Mg17Al12 shift obviously. AES analysis indicates that 280 nm modified layer exists in the surface and near surface. Compared with the substrate, the microhardness of solid solution-aging sample and solid solution-aging combined with duplex ion implantation sample increases by 16.7% and 36.6%, the polarization resistance Rp increases by 20.6 and 107.3 times, respectively. The corrosion current intensity (icorr) decreases by two orders of magnitude in 0.56 M NaCl solution. SEM observation exhibits that duplex ion implantation sample shows less corrosion pit, but untreated and solution-aging coupons have a large amount of corrosion pit under the same corrosion conditions.

Thermal and luminescent properties of Eu2+-doped aluminates prepared by the sol–gel method

Alkaline earth aluminates with the overall nominal compositions Ca0.5Sr0.5Al2O4, Ca0.5Mg0.5Al2O4 and Mg0.5Sr0.5Al2O4 doped with 1 mol% of Eu2+ ions were prepared by the modified aqueous sol–gel method. The thermal behaviour of the xerogels was studied by the TG/DSC-MS technique under an argon and a reductive atmosphere (Ar/H2–5 %). Appropriate luminescent efficiency of the materials was achieved after annealing at temperatures lower than those in conventional solid state reactions. All three aluminates are mixtures of at least two phases; the monoclinic phase of CaAl2O4, the hexagonal phase of SrAl2O4 and the cubic phase of MgAl2O4 were identified. Solid solubility was recognised in the Ca0.5Sr0.5Al2O4:Eu2+ composition due to the similar ionic radii of Ca2+ and Sr2+. UV excited luminescence was observed in the blue region (λmax = 441 nm) in the aluminates containing the monoclinic phase of CaAl2O4 and in the green region (λmax = 520 nm) in the Mg0.5Sr0.5Al2O4:Eu2+ composition.

Improving Corrosion Resistance of MRI 230D Mg Alloy by Hybrid Coating of Laser Surface Alloying and Plasma Electrolytic Oxidation

A permanent mould cast creep resistant MRI 230D Mg alloy was laser surface alloyed (LSA) with Al and Al2O3 in order to improve its wear and corrosion resistance. However, this treatment was successful only in improving wear resistance but not corrosion resistance due to the presence of micro−cracks in the coated layer, which has been discussed in an earlier paper. The LSA coated Mg alloy has been further subjected to plasma electrolytic oxidation (PEO) treatment in alkaline silicate electrolyte in order to cover those micro−cracks and improve corrosion resistance, which is discussed in the present manuscript. For comparison, the PEO coating has also been applied on the as−cast MRI 230D Mg alloy. The microstructural characterization of coatings and corroded surfaces was carried out by scanning electron microscope and X−ray diffraction. Electrochemical corrosion tests were conducted in 3.5 wt% NaCl solution having neutral pH to investigate the corrosion behavior. The LSA coatings consisted mainly of β (Mg17Al12) phase, the coatings produced by PEO treatment on MRI 230D Mg alloy consisted mainly of Mg2SiO4 phase, and hybrid coatings of PEO on LSA consisted of Mg2SiO4 and MgAl2O4 phases in the PEO layer. Scanning electron micrographs of the cross−section revealed that the PEO treatment covered the micro−cracks present in the LSA and corrosion tests revealed that it improved the corrosion resistance, though not to the extent of the corrosion resistance of the PEO coated MRI 230D Mg alloy. All the samples exhibited localized form of corrosion.

Synthesis and Characterization of Ni/Mg/Al Mixed Oxides Obtained by Co-precipitation

ABSTRACTNi/Mg/Al mixed oxides were obtained from hydrotalcite-like precursors by thermal decomposition at 400º, 600º and 800ºC using co-precipitation method at pH 7. The mixed oxides were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Energy-dispersive X-ray spectroscopy (EDS) techniques. XRD analysis revealed the formation of NixMg1-xO (x = 0 - 1), α-Al2O3 and traces of MgAl2O4 (NiAl2O4) phases. The diffraction peak positions for MgO or NiO oxides were shifted towards 2θ values higher than simulated for pure bulk oxides that can be attributed to the possible presence of lattice vacancies or surface compressive stress. The evaluated grain size by XRD technique was about 8 -10 nm. It was also detected the presence of microstrains that can be associated with the presence of extended defects in the grains. SEM observations showed that the particles of oxides are formed as agglomerates with the particle sizes of 50 nm up to 200 nm. EDS detected the presence of Mg2+, Ni2+, and Al3+ cations and oxygen in all particles independently on their size. The obtained results revealed the presence of mixture of Ni/Mg/Al oxides in each particle obtained. The oxides calcined at temperatures of 400º and 600ºC were unstable and under air storage they revert to the precursor. The incorporation of Ni2+ in Mg-Al mixed oxides leads to stability of the compounds calcined at 800ºC.

The Characteristic of the Coating Prepared on Magnesium Alloy AZ91D by Micro-Arc Oxidization

Micro-arc oxidation coating on AZ91D magnesium alloy is prepared in a NaAlO2-NaOH-montmorillonite-acacia gum electrolyte solution with adaptive electric parameters. The morphologies, composition, phase component of the coatings are analyzed by SEM, EDS and XRD. And CHI600B electrochemistry workstation is employed to investigate the corrosion resistance of the coatings. The results show that the MAO coating has the relatively uniform in thickness. The coating is mainly composed of MgO and MgAl2O4 phases. It provides corrosion protection by acting as a barrier. The MAO coating enhances the corrosion resistance of the magnesium alloy AZ91D to some extent.

Synthesis and Characterization of Al–Mg–SiO2 Particulate Composite Using Amorphous SiO2 from Rice Husk Ash

This paper discusses the effect of the dispersion of amorphous nano size (35–55 nm) SiO2 particles in Al–Mg (5%) alloy. Amorphous SiO2 (>95% SiO2) extracted from rice husk was added to the Al–Mg (5%) alloy in the proportion of 5 wt% of the Al–Mg alloy. The work aims to study the evolution of different phases like MgAl2O4, Mg2Si, and MgO in Al–Mg–SiO2 composite using amorphous nano SiO2. Experimental results of the synthesized composite show presence of MgAl2O4 (Spinel structure) and other phases like MgO and Mg2Si which impart hardness of 126.82 HV10g to the composite. The Al–Mg (5%) SiO2 composite microstructure appeared as a typical lamellar structure. The XRD and energy dispersive spectroscopy analysis display the presence of Mg2Si formed along the grain boundary.

Effect of electromagnetic field on the slag resistance of MgO-C refractories

The recent years have witnessed an unprecedented growth in the development and deployment of electromagnetic field(EMF) used in steel metallurgy. This paper investigates the effect of EMF on the slag resistance of MgO-C refractories. Using MgO-C refractories containing 14% carbon and the slag with a basicity(CaO/SiO2) of around 0.8, the experiments of melting slag resistance of MgO-C refractories were carried out in an induction furnace and a resistance furnace. The results show that in induction furnace having EMF, the refractory sample corroded by the slag has an apparent penetration layer. MgFe2O4 phase is formed at a low temperature by MgO dissolved in Fe2O3. The low melting phases are monticellite (CaMgSiO4, CMS) and forsterite (Mg2SiO4, MS). Under the condition of resistance furnace without EMF, scaling MgO dissolves into slag and reacts with Al2O3 to generate MgAl2O4 phase. The low melting phases are gehlenite (Ca2Al2SiO7, C2A2S) and akermanite (Ca2MgSi2O, C2MS2). As EMF exists at a high temperature, it could enhance the diffusion and penetration of Fe2+/3+ ions to form the MgFe2O4 phase. At the same time, some MgAl2O4 spinels exist. However, only MgAl2O4 phase can be generated without EMF.

Mg-Al水滑石“记忆效应”及其对Cr(VI)阴离子吸附性能研究

Mg-Al hydrotalcite (MAH) with high crystallinity was prepared using urea method. The memory effect and the chromium(VI) adsorption property of Mg-Al Hydrotalcites were studied. The MAH, recon- structed MAH (RMAH), calcined MAH (MAO) and calcined RMAH (RMAO) were characterized by XRD, FT-IR, SEM and DSC as well as self-deconvolution analyses. The results showed that the MAH and RMAH possessed a layered structure of hydrotalcites with high crystallinity, and there was little difference in crystal structures and charge density of brucite-like sheets between MAH and RMAH. The brucite-like sheets were still held in MAO, and a little remained in RMAO with more MgAl2O4. The adsorbility of Mg-A1 LDOs(MAO and RMAO) to Cr(VI) anions was substantially higher than that of Mg-A1 LDHs(MAH and RMAH), mainly because of the structure memory effect of hydrotalcites. The adsorbility of the MAO was higher than that of the RMAO. The adsorbility decrease of the RMAO might be due to the increase of the content of spinel MgAl2O4 phase and the decline of brucite-like sheets in RMAO that resulted in the descent of the memory effect reconstruction property. This newly discovery for anionic clays is useful for the de- contamination of waste waters. The Mg-Al hydrotalcite can be considered as a potential material for sorption of anions in wastewater treatment systems.