Compound Mg2Si Research Articles

A literature review on Al-Si alloy matrix based in situ Al-Mg2Si FG-composites: Synthesis, microstructure features, and mechanical characteristics

The current study examined a comprehensive review of the literature on functionally graded composites, particularly Al-Mg2Si in situ metal matrix composites. Functionally graded Al-Si-Mg2Si in situ metal matrix composites are potential materials for meeting a variety of property demands in various components of automotive engines. The in situ formed intermetallic compound Mg2Si in Al-Si matrix exhibits a high melting temperature, low density, high hardness, a low thermal expansion coefficient, and a reasonably high elastic modulus. Due to all of aforementioned characteristics, Mg2Si is an attractive form of reinforcement that can be created in situ using a simple melt reaction approach. However, the coarse size of primary Mg2Si reinforcement in cast composites is a demerit which reduces the strength and ductility of the ultimate composite. Hence several researches have been attempted to refine the primary Mg2Si particles as well as to change the morphology of the eutectic structure. Apart from the monolithic in situ cast composites, attempts have been made to develop functionally graded (FG) composites in which the volume% of segregation of reinforcements are intentionally varied from one surface to another. These types of FG composites are economically developed by centrifugal casting technique in which Mg2Si reinforcements are segregated to inner the surface due to lower relative density with respect to the molten Al matrix. This type of graded microstructure results in higher strength, hardness and wears resistance at the inner surface of the tubular products. The as-cast microstructure and properties are significantly improved by solution treatment and aging. In the last, some of the most recent manufacturing techniques for FG-composites are addressed, along with their benefits, drawbacks, and applications. In general, all cutting-edge and detailed surveys of the present state of knowledge are included in this article.

Machine-learned model Hamiltonian and strength of spin–orbit interaction in strained Mg2X (X = Si, Ge, Sn, Pb)

Machine-learned multi-orbital tight-binding (MMTB) Hamiltonian models have been developed to describe the electronic characteristics of intermetallic compounds Mg2Si, Mg2Ge, Mg2Sn, and Mg2Pb subject to strain. The MMTB models incorporate spin–orbital mediated interactions and they are calibrated to the electronic band structures calculated via density functional theory by a massively parallelized multi-dimensional Monte-Carlo search algorithm. The results show that a machine-learned five-band tight-binding (TB) model reproduces the key aspects of the valence band structures in the entire Brillouin zone. The five-band model reveals that compressive strain localizes the contribution of the 3s orbital of Mg to the conduction bands and the outer shell p orbitals of X (X = Si, Ge, Sn, Pb) to the valence bands. In contrast, tensile strain has a reversed effect as it weakens the contribution of the 3s orbital of Mg and the outer shell p orbitals of X to the conduction bands and valence bands, respectively. The π bonding in the Mg2X compounds is negligible compared to the σ bonding components, which follow the hierarchy , and the largest variation against strain belongs to σ pp . The five-band model allows for estimating the strength of spin–orbit coupling (SOC) in Mg2X and obtaining its dependence on the atomic number of X and strain. Further, the band structure calculations demonstrate a significant band gap tuning and band splitting due to strain. A compressive strain of can open a band gap at the Γ point in metallic Mg2Pb, whereas a tensile strain of closes the semiconducting band gap of Mg2Si. A tensile strain of removes the three-fold degeneracy of valence bands at the Γ point in semiconducting Mg2Ge. The presented MMTB models can be extended for various materials and simulations (band structure, transport, classical molecular dynamics), and the obtained results can help in designing devices made of Mg2X.

Enhancement of thermoelectric performance of Mg2Si via co-doping Sb and C by simultaneous tuning of electronic and thermal transport properties

Thermoelectric power generation using distributed waste heat energy has received attention as a long-life, environmentally friendly power supply. The intermetallic compound Mg2Si is a lightweight, mid-temperature thermoelectric material that contains no toxic elements, and its thermoelectric performance has been enhanced by various methods such as impurity doping, nanostructuring, and alloying. In this study, we examined the influence of the influence of co-doping with Sb and dilute amounts of the isoelectronic impurity C on the thermoelectric properties of Mg2Si. We fabricated dense polycrystalline specimens of Mg2CxSbySi using the melting process and subsequent plasma-activated sintering. Doping Mg2Si with Sb increased the electrical conductivity ~102 times. Further co-doping with isoelectronic C did not significantly change the electrical conductivity; however, it did reduce the thermal conductivity independently of the electrical properties. Consequently, the specimens co-doped with Sb and C achieved higher thermoelectric performance than specimens of Mg2Si single-doped with Sb. The dimensionless figure of merit ZT of the co-doped specimens reached 0.79 at 873 K over the temperature range 323–873 K.

Analysis of some mechanical properties of friction stir welded joints of AA6101 and AA6351 aluminum alloys under T6 condition

Abstract Friction In the present work, friction stir welding has been carried out with AA 6101-T6 and AA 6351-T6 dissimilar aluminium alloys (plate thickness being 6 mm). The different the process parameters are tool rotational speed, weld speed and axial force and a cylindrical threaded profile tool pin have been used. For the fabrication of the joints the experiments have been carried out with a tool shoulder diameter 18 mm and different tool rotational speeds (900, 1100, 1300 and1500 rpm) with constant welding speed 60 mm/min. Visual inspections confirm the absence of external macro defects. The microstructure studies of weld zone reveal mechanically mixing pattern of both the aluminum alloys around the tool pin. X-ray diffraction (XRD) analysis confirms the presence of intermetallic compound Mg2Si in base metals and FSW joints too. Tensile, hardness, bending tests have been conducted. It is found that the rotational speed of 1300 rpm produces better mechanical and metallurgical properties joints compared to joints fabricated by other rotational speeds. Thus mechanical properties of weld joints depends on combined effect of both microstructure of alloys and processing welding parameters. The fracture surface of tensile test samples presents ductile and dimple fibrous fracture.

Metallurgical Characterization and Hardness Evaluation of Dissimilar Friction Stir Welded Al Alloy Flat Plates

Friction Stir Welding (FSW) is commonly developed for welding of metals of the same kind like aluminium copper, steel and titanium. However, FSW also has the potential to produce high-quality joints between dissimilar metals which are differ in physical and mechanical properties. In this work dissimilar aluminium alloys plates of AA 6101-T6 and AA 6351-T6 with 6 mm thick were friction stir butt welded under the influence of different process parameters. The influence of various parameters, particularly the rotational speed on microstructures and hardness properties of the joints has been addressed. The microstructure of weld joints has been studied using an optical microscope and distinct zones are also identified. Fine-equiaxed and refined grains are observed at nugget zone of dissimilar joints due to effective stirring action of cylindrical profile tool pin at higher tool rotational speeds. The intermetallic compounds that form in the joints has been examined by the X-ray diffraction (XRD) and the presence of intermetallic compound Mg2Si in base metals and FSW joints was detected. Therefore, it can be understood that the intermetallic compound affects the hardness of butt joints. The highest and lowest SZ hardness are obtained with the welds fabricated with 1300rpm, 60 m/min, 4kN and with 900 rpm, 60 mm/min and 8kN respectively.

Room-Temperature Solution Synthesis of Mesoporous Silicon for Lithium Ion Battery Anodes.

As an important optoelectronic and energy-storage material, porous silicon (PSi) has attracted great interest in various fields. The preparation of PSi, however, usually suffers from low yields and/or complicated syntheses. Herein, we report a facile solution method to prepare PSi with controllable high specific surface area. Commercial Zintl compound Mg2Si readily reacts with HSiCl3 in the presence of amines at room temperature to produce amorphous PSi in high yields, where in situ formed salt byproducts serve as templates to generate uniform mesopores of ca. 3.8 nm in diameter. After crystallization treatment at 700 °C in flow Ar gas for 40 min, the obtained crystalline PSi coated with carbon layers shows excellent electrochemical performance when served as lithium ion battery anodes. The reversible specific capacity is about 2250 mA h g-1 at 0.1 A g-1 and the capacity retention is maintained at 90% after cycling at high current density of 2 A g-1 for 320 times. This simple, facile preparation method is very promising and paves the way for massive production of porous Si as high-performance anodes in Li-ion battery industry or for other applications, such as drug delivery systems and catalysis.

The Unique Electronic Structure of Mg2Si: Shaping the Conduction Bands of Semiconductors with Multicenter Bonding

AbstractThe electronic structures of the antifluorite‐type compound Mg2Si is described in which a sublattice of short cation–cation contacts creates a very low conduction band minimum. Since Mg2Si shows n‐type conductivity without intentional carrier doping, the present result indicates that the cage defined by the cations plays critical roles in carrier transport similar to those of inorganic electrides, such as 12 CaO⋅7 Al2O3:e− and Ca2N. A distinct difference in the location of conduction band minimum between Mg2Si and the isostructural phase Na2S is explained in terms of factors such as the differing interaction strengths of the Si/S 3s orbitals with the cation levels, with the more core‐like character of the S 3s leading to a relatively low conduction band energy at the Γ point. Based on these results and previous research on electrides, approaches can be devised to control the energy levels of cation sublattices in semiconductors.

The Unique Electronic Structure of Mg2 Si: Shaping the Conduction Bands of Semiconductors with Multicenter Bonding.

The electronic structures of the antifluorite-type compound Mg2 Si is described in which a sublattice of short cation-cation contacts creates a very low conduction band minimum. Since Mg2 Si shows n-type conductivity without intentional carrier doping, the present result indicates that the cage defined by the cations plays critical roles in carrier transport similar to those of inorganic electrides, such as 12 CaO⋅7 Al2 O3 :e- and Ca2 N. A distinct difference in the location of conduction band minimum between Mg2 Si and the isostructural phase Na2 S is explained in terms of factors such as the differing interaction strengths of the Si/S 3s orbitals with the cation levels, with the more core-like character of the S 3s leading to a relatively low conduction band energy at the Γ point. Based on these results and previous research on electrides, approaches can be devised to control the energy levels of cation sublattices in semiconductors.

Thermoelectric Properties of Mg2Si1-x-yGexSby Prepared by Spark Plasma Sintering

ABSTRACTThe magnesium compound Mg2Si and its solid solutions are expected asn-type thermoelectric (TE) material because they are non-toxic, have a large Clarke number, and are light weight. In this study, we improved TE performance by doping Ge into Sb-doped Mg2Si to cause phonon scattering and increase carrier concentration. A bulk of Sb-doped Si-Ge alloy as the raw material was fabricated using an arc-melting method. A high-purity Mg2Si was synthesized from metal Mg and Sb-doped Si-Ge alloy using spark plasma sintering equipment. For the samples with the same Sb concentration, the electrical conductivity was equivalent. On the other hand, the Seebeck coefficient was dependent on Ge concentration. Due to phonon scattering, thermal conductivity decreased by a small amount of Ge doping andκphdominated for thermal conduction. The minimum thermal conductivity of Mg2Si0.90Ge0.10was 2.25 W/mK (κph: 2.06 W/mK,κel: 0.19 W/mK). The dimensionless figure of merit (ZT) for the Mg2Si0.945Ge0.05Sb0.005sample was higher than that of the others due to reducing thermal conductivity and increasing carrier concentration. The maximumZTwas 0.47 at 713 K.

Identification and description of intermetallic compounds in Mg–Si–Sr cast and heat-treated alloys

The ternary Mg–Si–Sr system is experimentally investigated in this work. Twelve compositions located in the Mg rich corner of the ternary system are processed. The microstructure and phase content of the different alloys are analysed in the as-cast condition and after additional heat treatments.Apart from the Mg matrix, various intermetallic phases are observed. The binary compounds Mg2Si and Mg17Sr2 are identified, as well as, two ternary phases MgSiSr and MgSi2Sr.The morphology and distribution of the intermetallic phases is discussed with particular interest on the ternary intermetallics MgSiSr and MgSi2Sr for which only scarce information is available in literature. The phase stability and the stability of the intermetallic phases in these alloys are discussed.The Mg-rich side of the Mg–Si–Sr phase diagram is constructed based on descriptions of the binary phase diagrams Mg–Si, Mg–Ca and Ca–Si from literature and assuming complete solubility (i.e. a line compound) between the ternary phase MgSiSr and the binary phase Sr2Si. It is also assumed that MgSi2Sr is a stoichiometric compound. These assumptions were made based on the chemical similarities between Ca and Sr and between Si and Sn, and the similarity in crystal structures between the two studied ternary compounds of the Mg–Si–Sr diagram and the ternary compounds present in the Ca–Mg–Si and Mg–Sn–Sr system. A good agreement is found between the constructed phase diagram and the experimental results.

Comparative XRD and Microstructure Analysis on Biodegradable Mg-Si-Ca Alloys

Biodegradable materials are considered to be the third generation of biomaterials. The aim of these biodegradable alloys is to provide healing and tissue regeneration of the bone. In recent years new types of BM, such as Mg-Si-Ca based alloys were developed and are considered promising candidates for implant materials during a limited time. The effect of Si and Ca addition on pure magnesium alloy has been investigated. The results showed the formation of an intermetallic compound Mg2Si. Morphology of this compound changed to polyhedral shape with increasing Si addition over 3wt%. The purpose of this paper is to investigate morphological and XRD aspects of casted pure Mg and Mg-XSi-YCa (X – 1.5wt%, 5wt%, 10wt% / Y-0.5wt%, 1wt%, 2wt%) by using optical microscopy, scanning electron microscopy and X-ray diffraction.

MICROSTRUCTURE EVOLUTION AND WEAR RESISTANCE OF IN SITU Mg2Si/Al COMPOSITES UNDER DIFFERENT PREPARATION CONDITIONS

Hypereutectic Al-Si alloys with high Mg content are in fact an in situ aluminium matrix composites containing a large amount of hard particles of Mg2Si, and the Mg2Si/Al composite has a potential as automobile brake disc material because the intermetallic compound Mg2Si exhibits high melting temperature, low density, high hardness, low thermal expansion coefficient(TEC) and reasonably high elastic modulus. However, the primary Mg2Si particles in normal Mg2Si/Al composites are usually very coarse and thus lead to room temperature brittleness and deficient wear resistance. Therefore, the composite with coarse primary Mg2Si particles need to be modified to obtain adequate mechanical strength and wear resistance. Numerous experiments have shown that development of a semi-solid microstructure in which dendritic characteristic is absent can lead to significant enhancement of the mechanical properties in the composite. The semi-solid forming has been recognized as a technique offering several potential advantages over casting or solid state forming, such as producing high quality components capable of full heat treatment to maximize properties, and reducing macrosegregation, solidification shrinkage and forming temperature. The key feature that permits the shaping of alloys in the semi-solid state is the absence of dendritic characteristics from the morphology of the solid phase. In the present work, in situ Mg2Si/Al composites were fabricated by using gravity casting, squeeze casting and semi-solid extrusion. The microstructure evolution and wear resistance of Mg2Si/Al composites were investigated. Mg2Si/Al semi-solid composites were fabricated by isothermal heat treatment technology, forming spherical reinforced phase and matrix structure. The effects of holding time on the microstructure and grain sizes of the composite were investigated. The results show that with P modification, Mg2Si particle in the as-cast microstructure of the composites is evolved from coarse dendrite into fine block structure with grain size of 35 μm. Furthermore, reinforcement Mg2Si with fine size and uniformly distribution exhibits regular spherical structure and a-Al grains exhibit spherical or ellipsoidal structure. The size of a-Al changes from 60 to 115 μm with increasing the holding time from 50 to 160 min. It is calculated that the cubic coarsening rate constants K of a-Al is 1.78×10-16m3/s according to the statistical data. In addition, the hardness of squeeze casting and semi-solid extrusion composites enhanced 23.5% and 39% in comparison with casting composite, respectively. The wear test results show that, the wear resistance of Mg2Si/Al composite fabricated by using semi-solid extrusion is higher than those of composites fabricated by using gravity casting and squeeze casting under same load and wear particle size.

Effects of Different Filler Metals on the Mechanical Behaviors of GTA Welded AA7A52(T6)

ER4043, ER5356, and AA7A52 on behalf of the Al-Si, Al-Mg, and Al-Zn-Mg-based welding material, respectively, were chosen as the filler metal to weld AA7A52(T6) plates by GTAW. The variance in mechanical performances of the joints caused by the various filler materials was investigated with reference to the SEM and EDS test results for the weld seam and the fracture surface. Failure was found in the seam for all the welded joints. With regard to the joint obtained with ER4043 welding wire, the total elongation was limited by the brittle intergranular compound Mg2Si of which Mg was introduced by convection mass transfer. As for the other two welds, the content ratio of Zn and Mg was found to play the dominant role in deciding the mechanical properties of the intergranular Mg-Zn compounds which were responsible for the tensile behavior of the joints. The content ratio (wt.%) of beyond 2:1 gave birth to the strengthening phase MgZn2 leading to a ductile fracture. Cr in the seam obtained with AA7A52 filler metal was found to enhance the strength of the joint through isolated particles.

Effect of Duplex Ageing on the Properties of A356 Alloy Containing Rare Earth Elements

Reduced weight of automobiles for the purpose of fuel economy has encouraged the use of light metals especially aluminium alloys. A356 Al alloy containing 7% Si and 0.3% Mg is widely used in automobile and aircraft industries due to excellent castability, good corrosion resistance and good pressure tightness. A356 is age hardenable alloy and there is appreciable improvement in strength and hardness achievable due to precipitation of intermetallic compound Mg2Si. In the present investigation, aluminium alloy A356 with and without rare earth (RE) addition (0.5 wt%) was subjected to single ageing as well as double aging treatment. The results were compared for mechanical properties like hardness and ultimate tensile strength with the material not containing RE additions.

Ion beam synthesis of Mg2Si

The semiconducting metal silicides and Mg2Si in particular have been extensively studied in the last few years because of the following reasons: the materials have the potential to be successfully integrated in the optoelectronic silicon technology, the materials are nontoxic and the constituting elements are found in a large amount in nature, and lastly the formation of nanocrystals in a perticular matrix offers the possibility of semiconducting structures with new properties. The Mg-Si phase diagram [1] shows that Mg2Si is the only silicide in this system and it melts congruently at 1085 ◦C. Therefore, crystal growth from the melt should be possible, but one has to be careful about the considerable evaporation of the Mg component as the boiling point of pure Mg is very close to the compound melting temperature. Growth by chemical vapor transport is hardly possible because of the lack of stable magnesium halides and the growth of Mg2Si bulk material has been accomplished exclusively from a melt [2–5]. The compound Mg2Si has, similar to Si, a cubic structure with a larger lattice parameter. The lattice mismatch for the Mg2Si (111) face on the Si (111) face is 1.9%. Due to the comparatively small lattice mismatch high quality epitaxial growth of Mg2Si onto monocrystalline silicon appears to be possible [6]. However thin film formation is difficult because of the low condensation and high vapor pressure of Mg and that is why thin film preparation and studies are scarce. The first reports on Mg2Si thin film formation [7, 8] note that the silicidation proceeds rather rapidly in Mg/Si thin film substrates at relatively low temperatures and silicide columns matching the monocrystalline silicon substrate are formed. There has been an attempt to fabricate Mg2Si by laser melting of magnesium film deposited on a Si substrate [9]. The synthesized layers contain both silicides and Si crystallites. Mahan et al. [10] have prepared Mg2Si films, textured in the (111) direction by molecular-beam epitaxy. They find that intended reactive deposition of magnesium onto silicon substrate at temperatures from 200 to 500 ◦C results in no accumulation of magnesium. However, codeposition of magnesium and silicon at 200 ◦C, using a magnesium-rich flux ratio, gives stoichiometric Mg2Si films. A solidphase growth technique with preliminary formation of templates was used by Galkin et al. [11] to prepare continuous Mg2Si films with a thickness of the order of 20 nm on Si (111) substrates. The very low condensation coefficient of magnesium as well as the limited Mg2Si films thickness due to the barrier behavior of Mg2Si can be overcome using the ion-beam synthesis (IBS) method, reported in this paper. The samples were prepared by IBS, followed by rapid thermal annealing (RTA). The implantation of 24Mg+ ions was performed using an ion accelerator type ILU-4, allowing a high current density. As an ion plasma source 4N pure Mg, heated at 500 ◦C, was used. The vapor pressure of Mg at this temperature is about 10−2–10−1 Torr. During the implantation, in order to avoid amorphization [12], the substrates were heated to a temperature of about 230 ◦C by means of the incident ion beam (current density 10–12 μA · cm−2). The mass separated 24Mg+ ions were implantated into n-type Si wafers with (100) orientation and resistivity 4.3–4.5 ·cm. Four types of samples were prepared. The first two types were fabricated by two-step ion implantation with two different doses—5 × 1016 and 1 × 1017 cm−2, with energies 15 and 40 keV respectively. The other two types of samples were prepared by one-step implantation of two different higher doses—2 × 1017 and 4 × 017 cm−2 with energy 40 keV. The Mg implantation profiles were simulated by SRIM (Stopping and Range of Ions in Matter) and the initial profiles of the implanted Mg ions were estimated as:

Mill setting and microstructural evolution during mechanical alloying of Mg2Si

The mechanical alloying behaviour of magnesium and silicon to form the intermetallic compound Mg2Si and the optimum setting of a planetary ball mill for this task, were examined. For the ductile–brittle magnesium–silicon system it was found that the efficiency of the mill is mostly influenced by the ratio of the angular velocity of the planetary wheel to that of the system wheel and the amount of load. The examination of the kinetics inside the planetary ball mill for different mill settings showed that a ratio of angular velocities of at least 3 is necessary to compensate the reduction of efficiency due to slip. The optimum powder load for the 500 ml vial was found to be 10–20 g. The milling process starts with elemental magnesium and silicon bulk particles. During the milling, the silicon pieces are rapidly diminished and together with the constantly forming Mg2Si they act as an emery powder for the magnesium bulk pieces. Simultaneous to the diminution of the magnesium, alloying occurs.

High pressure and high temperature investigations on intermetallic compounds using energy-dispersive X-ray powder diffraction

The intermetallic compounds Mg2Si, MoSi2, WSi2, CrSi2, NbSi2, Ti5Si3, TiSi2, ZrSi2 and Al3Nb were investigated in a pressure range up to 70 kbar and a temperature range up to 1500°C in order to determine previously unknown elastic constants and possible phase transformations. The experiments were carried out using the multi-anvil-X-ray apparatus MAX-80 and synchrotron radiation at HASYLAB. A complete phase transition of cubic Mg2Si to a hexagonal phase was observed at 70 kbar and 450°C. The high-pressure/high-temperature phase was quenched and recovered for further investigations. All the other compounds remained stable in the mentioned pressure and temperature range. The bulk moduli covered the range from 436 (10) (Mg2Si) to 3350 (190) (WSi2) kbar.

５０５６合金の被削性に及ぼす鉄，けい素量の影響

Dry cutting tests of 5056 alloys which were added Fe (0.014-0.19wt%) and Si (0.006-0.07wt%) were made using high speed steel (SKH4) and sintered carbide tool (K10). Effect of these elements on the machinability was studied from the viewpoints of cutting resistance, built up edge, cut surface roughness, tool wear and chip disposal.(1) Under a low cutting speed, machined surface of all specimens were rough and scaly for forming of built up edge. However, under a high cutting speed, feed marks on machined surface appeared clearly and its roughness value was low. Especially the finished surface of alloy which contains less Fe and Si approximated to the theoretical value.(2) Following the increase of Si content which formed metallic compound Mg2Si in this alloy, the tool wear rates on flank and rake face were rapid. In increasing Fe content which formed metallic compound Al-Fe, the rake face wear progressed rapidly. But, it was less affected with the flank wear rates.

５０８３合金の高温焼なましによる組織および機械的性質の変化

Yield strength and hardness of the 5083 sheet annealed at 520°C especially for 2000min or more are considerably lowered. The surface and interior portion of the sheets annealed for 4000 and 14400min have substantially different hardness. The fact is attributed to difference in Mg concentration resulted from vaporization of Mg. An intermetallic compound Mg2Si remarkably decreases during prolonged annealing for 4000min or more. Another intermetallic compounds composed of Al, Fe, Mn, Cr and Si relatively increase with respect to decrease of Mg2Si. Initial Mg content 4.59wt% reduces. down to 0.26wt% by annealing for 14400min.

Microhardness and Other Properties of Magnesium Compounds with the Calcium Fluoride Structure

RELATIONS have previously been proposed1,2 between hardness and latent heat of fusion, L f, for pure metals. This communication shows that a relation also holds for the series of intermetallic compounds Mg2Si, Mg2Sn and Mg2Pb, and by using the results for these compounds, L f for Mg2Ge is obtained by interpolation.