Catalytic effect in the hydrogenation of Mg and Mg compounds: Surface analysis of MgMg2Ni and Mg2Ni

By means of XPS and AES we have analysed the chemical states and the concentrations of Mg and Ni at the surface of the hydrogen storage materials MgMg₂Ni eutectic alloy and Mg₂Ni intermetallic compound. Mg₂Ni decomposes at the surface into Mg oxide and metallic Ni. Molecular hydrogen can be dissociated at the metallic Ni precipitations or at the metallic subsurface layer. Thus the proposed model describes the catalytic effect of Mg₂Ni on the hydrogenation [1] of Mg by surface decomposition and the ability to supply atomic hydrogen. With a similar model we described already successfully catalytic effects at the surface of LaNi₅ and FeTi [2]. It is likely that it is applicable to other Mg compounds, too.     

Investigation of micro-structural transition through disproportionation and recombination during hydrogenation and dehydrogenation in Mg/Cu super-laminates

Micro/nano-structures and hydrogen storage properties of Mg/Cu super-laminates were investigated. Mg/Cu super-laminates showed reversible hydrogenation and dehydrogenation at 473 K. In order to clarify the process of hydrogenation and dehydrogenation at 473 K, we performed TEM observations of micro/nano-structures of the Mg/Cu super-laminates and Mg₂Cu powder prepared by conventional casting method. TEM observations revealed that the as-rolled Mg/Cu super-laminates had laminated structures in size of sub-micrometer thickness composed of Mg and Cu layers with dense lattice defects. The super-laminates after initial activation kept laminated structures and had uniformly distributed pores with a sub-micrometer diameter. On the other hand, the cast Mg₂Cu powder after initial activation had pores only beneath the surface oxide layers. It is considered that these micro/nano-structures of Mg/Cu super-laminates lead to lower dehydrogenation temperature and better kinetics, which would contribute to achieve high-performance hydrogen storage materials.     

Effect of surface oxidation on the hydriding and dehydriding of Mg<Subscript>2</Subscript>Ni alloy produced by hydriding combustion synthesis

Hydriding combustion synthesis (HCS) has been regarded as an innovative process for the preparation of high active magnesium-based hydrogen storage alloys. For the purpose of understanding the interrelation of the unique hydrogen storage properties and the surface characteristics of the HCS product, the samples of Mg₂Ni alloy/hydride with and without exposure to air were prepared from the HCS product of Mg₂NiH₄. The hydriding and dehydriding properties were compared and the surface compositions were analyzed by means of X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES). It was shown that the air exposure considerably decreases the hydriding activity of Mg₂Ni. Absorbing of 3.0 wt.% of hydrogen under the conditions of 603 K and 3.0 MPa after the air exposure takes 1500 s, which is six times longer than for the unexposed alloy. The hydrogen desorption of the hydride are also impeded by the air-exposure, which results in the increase of dehydriding temperature from 450 K to 540 K. XPS and AES analyses indicated that Mg segregates and exists in the form of hydroxide on the surface of the air-exposed sample, which is responsible for the degradation of the hydriding and dehydriding properties. It was confirmed that the fresh surfaces generated during the dehydriding process of the as-synthesized hydride product contributes to the high activity of the HCS product in the first cycle of the hydriding determination.     

Invariant three- and four-phase equilibria in the magnesium-rich corner of the Mg-Cu-Sn ternary system

Invariant three- and four-phase equilibria in the magnesium-rich corner of the Mg-Cu-Sn ternary system have been studied by differential thermal analysis, optical microscopy, electron probe microanalysis and X-ray diffraction. The L(Mg) + Mg₂ Cu + Mg₂Sn ternary eutectic reaction was found to be at 467 °C and at Mg–13.5 at % Cu–4.4 at% Sn. The LMg₂Cu + Mg₂Sn pseudo-binary eutectic reaction is tentatively located at 522°C and at Mg–26.0 at % Cu–7.7 at % Sn.     

Comparative study on characteristics of hybrid laser-TIG welded AZ61/Q235 lap joints with and without interlayers

AZ61 Mg alloy to Q235 mild steel were lap joined using hybrid laser-TIG welding technique. At the joint interface and fusion zone (FZ), microstructure was revealed by scanning electron microscopy equipped with energy dispersive X-ray spectroscopy; element distribution was analyzed by electron probe micro-analyzer; intermediate phases were identified using X-ray diffraction test. Comparing with interlayer-free joints, the new intermediate phases Mg₂Ni and Mg₂Cu were generated in the FZ and at the Mg alloy/interlayer interface, and the solid solution of Ni or Cu in Fe was found along the edge of weld pool on steel side. It was found that direct joint without any interlayer was mechanical bonding, while Ni- and Cu-added joints were semi-metallurgical bonding. The joint shear strength was not only related to the penetration depth, but also related to the bonding mode. The strengthening effect on Cu-added joint was higher than that on Ni-added joint.     

Synthesis of Mg2Cu and MgCu2 nanoparticles in a KCl-NaCl-MgCl2 melt

The reaction between magnesium and nickel powders in a KCl-NaCl-MgCl₂ ionic melt at 973 K (reaction time, 5 h) has been studied by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray microanalysis, and chemical analysis. According to scanning electron microscopy data, the synthesized Mg₂Cu and MgCu₂ powders consist of particles ? 100 and ? 95 nm in size, respectively, in reasonable agreement with the equivalent particle diameters, ? 98 and ? 87 nm, determined from the specific surface area of the Mg₂Cu and MgCu₂ powders and with the crystallite sizes evaluated from X-ray diffraction data: D _hkl ? 90 and 84 nm, respectively. The Mg₂Cu synthesized in the ionic melt reacts with hydrogen under milder conditions than do Mg₂Cu samples prepared through standard melting of magnesium and copper in an electric arc or vacuum induction furnace.     

Size control of in situ formed reinforcement in metal melts-theoretical treatment and application to in situ (AlN + Mg2Si)/Mg composites

An approach of selecting alloying elements to control the size of in situ formed reinforcement in metal melts is proposed. This approach is based on the effect of alloying addition on nucleation of reinforcement predicted by the extended Miedema model and Wilson equation and on growth of reinforcement predicted by diffusion coefficient model. Using this approach, the effect of alloying element addition on the size of in situ formed Mg₂Si in (AlN + Mg₂Si)/Mg composites is evaluated. The results show that the addition of Ti, V, Mo, Cr, Fe and Mn can delay the nucleation and promote the growth of Mg₂Si, while the addition of Ge, Sn, Cu and Zn can promote the nucleation and hinder the growth of Mg₂Si. The microstructure of (AlN + Mg₂Si)/Mg with the addition of Ti has been investigated based on the theoretical prediction.     

Influences of Si and Mg contents on microstructures of Al–xSi–yMg functionally gradient composites reinforced with in situ primary Si and Mg2Si particles by centrifugal casting

Hypereutectic Al–xSi–yMg functionally gradient composites reinforced with primary Si and Mg₂Si particles were fabricated by centrifugal casting. The influence of Si and Mg contents on microstructures of the Al–xSi–yMg functionally gradient composites was investigated. Calculations of the volume fractions and the sizes of primary Si and Mg₂Si particles in the cross section of each tube along the radial direction from the inner-to-outer surface revealed that this type of gradient composite tube can be fabricated by centrifugal casting when the contents of Si and Mg are more than or equal to 19 and 4%, respectively. The tubes consist of an inner layer, the middle layer, and the outer layer measured in the radial direction on the cross section. The inner layer segregates blocky primary Si and Mg₂Si particles, the middle layer contains no primary Si and Mg₂Si particles, and the outer layer contains few primary Si and Mg₂Si particles. We compared resulting mean volume fractions and average sizes of produced primary Si and Mg₂Si particles with initial Si and Mg contents to determine their relationship in this process. The morphology of the Mg₂Si phase is another key factor in the formation of the composites. It was found that the blocky primary Mg₂Si particles have greater centroclinal velocity than that of primary Si particles due to the lower density of the Mg₂Si particles. The primary Si particles are pushed by blocky primary Mg₂Si particles. The two kinds of particles move toward the inner wall of the tube together during the solidification. A model of particles movement has been established according to the experimental results.     

Preparation and characterisation of MgSiN2 powders

The powder preparation of MgSiN₂ was studied using several starting mixtures (Mg₃N₂/Si₃N₄, Mg/Si₃N₄ and Mg/Si) in the temperature range 800–1500°C in N₂ or N₂/H₂ atmospheres. The phase formation was followed with TGA/DTA and powder X-ray diffraction (XRD). At 1250°C Mg/Si mixtures did not yield single phase MgSiN₂ whereas for Mg/Si₃N₄ and Mg₃N₂/Si₃N₄ mixtures nearly single-phase powders were obtained. The Mg/Si₃N₄ mixtures yielded MgSiN₂ at the lowest processing temperature but the Mg₃N₂/Si₃N₄ mixtures yielded the most pure MgSiN₂ powder with respect to secondary phases. The main secondary phase detectable with XRD was MgO when starting from Mg₃N₂/Si₃N₄ or MgO and metallic Si when starting from Mg/Si₃N₄ mixtures. When the processing starting from Mg₃N₂/Si₃N₄ mixtures was optimised MgSiN₂ powders containing only 0.1 wt% O could be prepared. Using XRD the solubility of oxygen in the MgSiN₂ lattice was estimated to be at maximum 0.6 wt%. The MgSiN₂ powder was oxidation resistant in air till 830°C. The morphology and particle size were studied with the scanning electron microscope (SEM) and the sedimentation method. Two different kinds of morphology were observed determined by the morphology of the Si₃N₄ starting material.     

Effect of copper addition on wear and corrosion behaviours of Mg2Si particle reinforced composites

The purpose of this study is to investigate the effect adding Cu has on the wear and corrosion properties of “in situ” Mg₂Si particle reinforced Al–12Si–20Mg matrix composites, produced with help of the nucleation and growth of the reinforcement from the source matrix, in order to overcome the disadvantages of composites produced by externally reinforcing ceramic particles. Composites known as Al–12Si–20Mg–XCu were produced by adding Cu, at the rate of 1%, 2%, and 4%, to the Al–12Si–20Mg alloy in order to achieve this purpose. The microstructural characterisation, hardness, wear and corrosion properties of composites, produced using the casting method, were analysed. Dry environment wear experiments for investigated alloys were conducted using a pin-on-disc type wear device under different loads and at different sliding distances. The change in weight loss of the solution containing 30 g/l NaCl + 10 ml/l HCl, and the tafel extrapolation method were used to analyse corrosion behaviour. Results of microstructural characterisation concluded that as the amount of Cu added to the Al–12Si–20Mg alloy increased, the size and volume of the Mg₂Si particle, formed within the matrix, decreased, and CuAl₂ intermetallics formed within the matrix. Results of wear experiments concluded that adding Cu developed wear resistance under small loads; however, reduced wear resistance under high loads. According to results of corrosion experiment, corrosion resistance increased with the addition of Cu.     

Optimizing the tensile properties of Al–Si–Cu–Mg 319-type alloys: Role of solution heat treatment

The work presented in this study was carried out on Al–Si–Cu–Mg 319-type alloys to investigate the role of solution heat treatment on the dissolution of copper-containing phases (CuAl₂ and Al₅Mg₈Cu₂Si₆) in 319-type alloys containing different Mg levels, to determine the optimum solution heat treatment with respect to the occurrence of incipient melting, in relation to the alloy properties. Two series of alloys were investigated: a series of experimental Al–7 wt% Si–3.5 wt% Cu alloys containing 0, 0.3, and 0.6 wt% Mg levels. The second series was based on industrial B319 alloy. The present results show that optimum combination of Mg and Sr in this study is 0.3 wt% Mg with 150 ppm Sr, viz. for the Y4S alloy. The corresponding tensile properties in the as-cast condition are 260 MPa (YS), 326 MPa (UTS), and 1.50% (%El), compared to 145 MPa (YS), 232 MPa (UTS), and 2.4% (%El) for the base alloy with no Mg. At 520 °C solution temperature, incipient melting of Al₅Mg₈Cu₂Si₆ phase and undissolved block-like Al₂Cu takes place. At the same time, the Si particles become rounder. Therefore, the tensile properties of Mg-containing alloys are controlled by the combined effects of dissolution of Al₂Cu, incipient melting of Al₅Mg₈Cu₂Si₆ phase and Al₂Cu phase, as well as the Si particle characteristics.     

Microstructure evolution and elemental diffusion of SiCp/Al–Cu–Mg composites prepared from elemental powder during hot pressing

15vol%SiCp/Al–Cu–Mg composites were fabricated by hot pressing method using pure elemental powders. Microstructure evolution and elemental diffusion of Cu and Mg were studied. The microstructure of as-hot pressed composites and the elemental distribution of the composites before and after solution treatment were also investigated. The results showed that there were two types of eutectic liquid phases with different compositions after the compact was heated to 580 °C. After the compact was held at 580 °C for 60 min, the eutectic liquid was absorbed into the Al matrix and some equilibrium liquid phases formed in the boundaries of the initial Al particles. Meanwhile, Cu was homogeneously distributed in the Al particles while Mg tended to be distributed near the boundaries of the initial Al particles and in the SiC clusters. The presence of Al₂Cu, Mg₂Si, and some oxides of Mg was identified in the as-hot pressed composite. After solution treatment, Al₂Cu dissolved into the Al matrix, however, some Mg-rich compounds (silicide and oxide of Mg) did not dissolve into the matrix completely.     

Metallurgical parameters controlling the microstructure and hardness of Al–Si–Cu–Mg base alloys

Castings were prepared from both experimental and industrial 319 alloy melts containing 0–0.6 wt% Mg. Test bars were cast in two different cooling rate molds, a star-like permanent mold and an L-shaped permanent mold, with DASs of 24 μm and 50 μm, respectively. The bars were tempered at 180 °C (T6 treatment) and 220 °C (T7 treatment) for 2–48 h. The results showed that Mg content, aging conditions, and cooling rate have a significant effect on the microstructure of both experimental and industrial alloys and, consequently, on the hardness. The addition of Mg resulted in the precipitation of the β-Mg₂Si, Q-Al₅Mg₈Cu₂Si₆, π-Al₈Mg₃FeSi₆ and of the block-like θ-Al₂Cu phases. The Mg and Cu, as well as the higher cooling rates improved the hardness values, especially in the T6 heat-treated condition, whereas the addition of Sr decreased these values.     

Effect of Mg content and solution treatment on the microstructure of Al-Si-Cu-Mg alloys

Three different quaternary alloys Al-6Si-3Cu-xMg (x = 0.59, 3.80 and 6.78 wt.%) were produced using conventional ingot casting metallurgy. The study was focused to investigate the effect of magnesium and solution heat treatment on the microstructure. Results shown variations in composition and morphology for the silicon-rich phases as well as a change of the predominant copper-rich phase Al₂Cu (θ) to Al₅Cu₂Mg₈Si₆ (Q phase) when magnesium content is increased. The amount of Mg in solid solution was constant for the three different cast-alloys, increasing considerably after solution heat treatment to 2.7 at.% for the alloy with higher Mg content This fact allowed to obtain Cu:Mg ratios (in at.%) in solid solution lower than 1.0 for the alloys with 3.80 and 6.78 Mg wt.%, impossible to reach for the alloy with low Mg content. During dissolution process, Al₂Cu phase was observed to be more suitable to dissolve than Q phase. Fragmentation, spheroidization and coarsening of Q and silicon-rich phases were observed. Solution time required for these processes occurrence was longer for Q phase. Solution heat treatments at 480°C for 12 h were found to be appropriate for the studied alloys.     

钠钙玻璃上Mg2Si薄膜的制备及其电学性质

采用磁控溅射技术和退火工艺在钠钙玻璃衬底上制备了Mg_2Si半导体薄膜,研究了Mg膜厚度对Mg_2Si薄膜结构及其电学性质的影响。在钠钙玻璃上分别溅射两组相同厚度(175nm)的P-Si和N-Si膜,然后在其上溅射不同厚度Mg膜(240nm、256nm、272nm、288nm、304nm),低真空退火4h制备一系列Mg_2Si半导体薄膜。通过X射线衍射仪(XRD)、扫描电子显微镜(SEM)、霍尔效应测试仪对Mg_2Si薄膜的晶体结构、表面形貌、电学性质进行表征与分析。结果表明:采用磁控溅射技术在钠钙玻璃衬底上成功制备出以Mg_2Si(220)为主的Mg_2Si薄膜。随着沉积Mg膜厚度的增加,Mg_2Si衍射峰逐渐增强,薄膜表面更连续,电阻率逐渐减小,霍尔迁移率逐渐降低,载流子浓度逐渐增加。此外,Si膜导电类型和Mg膜厚度共同影响Mg_2Si薄膜的导电类型。溅射N-Si膜时,Mg_2Si薄膜的导电类型随着Mg膜厚度的增加由P型转化为N型;溅射P-Si膜时,Mg_2Si薄膜的导电类型为P型。可以控制制备的Mg_2Si半导体薄膜的导电类型,这对Mg_2Si薄膜的器件开发有着重要的指导意义。     

Possibility of Mg- and Ca-based intermetallic compounds as new biodegradable implant materials

Mg- or Ca-based intermetallic compounds of Mg₂Ca, Mg₂Si, Ca₂Si and CaMgSi are investigated as possible new candidates for biodegradable implant materials, attempting to improve the degradation behavior compared to Mg and Ca alloys. The reactivity of Ca can be indeed reduced by the formation of compounds with Mg and Si, but its reactivity is still high for applications as an implant material. In contrast, Mg₂Si shows a higher corrosion resistance than conventional Mg alloys while retaining biodegradability. In cytotoxicity tests under the severe condition conducted in this study, both pure Mg and Mg₂Si showed relatively high cytotoxicity on preosteoblast MC3T3-E1. However, the cell viability cultured in the Mg₂Si extract medium was confirmed to be better than that in a pure Mg extract medium in all the conditions investigated with the exception of the 10% extract medium, because of the lower corrosion rate of Mg₂Si. The cytotoxicity derived from the Si ion was not significantly detected in the Mg₂Si extract medium in the concentration level of ~ 70 mg/l measured in the present study. For aiming the practical application of Mg₂Si as an implant material, however, its brittle nature must be improved.     

Formaton of nanocrystalline Mg2Si and Mg2Si dispersion strengthened Mg-Al alloy by mechanical alloying

Formation of Mg₂Si via mechanical alloying of elemental Mg and Si powders has been investigated. The formation of Mg₂Si occurs after 10 hours of mechanical alloying. Nanocrystalline structure of Mg₂Si with grain size of 22 nm obtained after 50 hours of milling was found to be stable upon heating to about 390 °C. Sudden increase in crystalline size to 157 nm after annealing at 520 °C was observed. Although the reaction between Mg and Si could be completed after about 50 hours of mechanical alloying, thermal assisted reaction starting at as low as 190 °C could promote the formation of Mg₂Si at a short milling duration and hence reduce Fe contamination. Mg-Al alloy reinforced by Mg₂Si was prepared by milling Mg, Si and Al powders. Intermediate phase of Al₁₂Mg₁₇ has been detected after 5 hours of mechanical alloying. This intermediate phase was observed to disappear to form equilibrium solid solution of Mg-Al alloy after annealing at 300 °C.     

High-resolution elemental maps for three directions of Mg2Si phase in Al-Mg-Si alloy

Elemental maps of the Mg and Si sub-lattices of the Mg₂Si phase in an Al-1.0mass% Mg₂Si alloy were produced using an energy-filtering transmission electron microscope (EFTEM). Low magnification elemental maps were obtained using both low and high energy loss edges, and the intensities of the high energy loss edges were sufficiently high to allow the Mg₂Si phase to be observed at high magnification. High-resolution core-loss images of Mg and Si-K edges were taken parallel to [001], [111] and [110] of the Mg₂Si phase. In the [110] direction, Mg and Si atoms were successfully identified as sub-lattices. The Mg atoms formed a 0.39 nm diamond network, whereas the Si atoms formed a 0.32 nm by 0.22 nm rectangular network. This result is in good agreement with the projected potential of the Mg₂Si phase in the [110] direction. This is the first report of magnesium and silicon atoms in the Mg₂Si phase being successfully identified at the atomic level by EFTEM.     

Experimental study of the Ca–Mg–Zn system using diffusion couples and key alloys

Abstract

Nine diffusion couples and 32 key samples were prepared to map the phase diagram of the Ca–Mg–Zn system. Phase relations and solubility limits were determined for binary and ternary compounds using scanning electron microscopy, electron probe microanalysis and x-ray diffraction (XRD). The crystal structure of the ternary compounds was studied by XRD and electron backscatter diffraction. Four ternary intermetallic (IM) compounds were identified in this system: Ca₃Mg_xZn_15?x (4.6x12 at 335 °C, IM1), Ca_14.5Mg_15.8Zn_69.7 (IM2), Ca₂Mg₅Zn₁₃ (IM3) and Ca_1.5Mg_55.3Zn_43.2 (IM4). Three binary compounds were found to have extended solid solubility into ternary systems: CaZn₁₁, CaZn₁₃ and Mg₂Ca form substitutional solid solutions where Mg substitutes for Zn atoms in the first two compounds, and Zn substitutes for both Ca and Mg atoms in Mg₂Ca. The isothermal section of the Ca–Mg–Zn phase diagram at 335 °C was constructed on the basis of the obtained experimental results. The morphologies of the diffusion couples in the Ca–Mg–Zn phase diagram at 335 °C were studied. Depending on the terminal compositions of the diffusion couples, the two-phase regions in the diffusion zone have either a tooth-like morphology or contain a matrix phase with isolated and/or dendritic precipitates.     

A Stable Solid Electrolyte Interphase for Magnesium Metal Anode Evolved from a Bulky Anion Lithium Salt

Rechargeable magnesium (Mg) metal batteries are a promising candidate for “post-Li-ion batteries” due to their high capacity, high abundance, and most importantly, highly reversible and dendrite-free Mg metal anode. However, the formation of passivating surface film rather than Mg²⁺-conducting solid electrolyte interphase (SEI) on Mg anode surface has always restricted the development of rechargeable Mg batteries. A stable SEI is constructed on the surface of Mg metal anode by the partial decomposition of a pristine Li electrolyte in the electrochemical process. This Li electrolyte is easily prepared by dissolving lithium tetrakis(hexafluoroisopropyloxy)borate (Li[B(hfip)₄]) in dimethoxyethane. It is noteworthy that Mg²⁺ can be directly introduced into this Li electrolyte during the initial electrochemical cycles for in situ forming a hybrid Mg²⁺/Li⁺ electrolyte, and then the cycled electrolyte can conduct Mg-ion smoothly. The existence of this as-formed SEI blocks the further parasitic reaction of Mg metal anode with electrolyte and enables this electrolyte enduring long-term electrochemical cycles stably. This approach of constructing superior SEI on Mg anode surface and exploiting novel Mg electrolyte provides a new avenue for practical application of high-performance rechargeable Mg batteries.