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Abstract: A graded composite of Mg2Si/Al has successfully been fabricated by an electric arc remelting process. The results show that the graded microstructure of the composite consists of a large amount of Mg2Si primary particles surrounded by clouds of α-Al particles. The primary Mg2Si particles gradually increase in size, while there is a decrease in length of the dendrites with distance from the bottom. In addition, the primary Mg2Si dendritic crystals have certain orientations tending to be identical to the direction from top to bottom, and the dendritic crystals in the same array are almost parallel. The morphology of eutectic Mg2Si is identified as fine and short fibrous like. The microhardness of graded material along the graded microstructure gradual increased with increasing distance from the top part. [Copyright &y& Elsevier]

Abstract: The thermoelectric properties of Bi-doped Mg2Si (Mg2Si:Bi=1:x) fabricated by spark plasma sintering process have been characterized by Hall effect measurements at 300K and by measurements of electrical resistivity , Seebeck coefficient (S), and thermal conductivity between 300 and 900K. Bi-doped Mg2Si samples are n-type in the measured temperature range. The electron concentration of Bi-doped Mg2Si at 300K ranges from 1.8×1019 cm−3 for the Bi concentration to 1.1×1020 cm−3 for . The solubility limit of Bi in Mg2Si is estimated to be about 1.3at% and first-principles calculation revealed that Bi atoms are expected to be primarily located at the Si sites in Mg2Si. The electrical resistivity, Seebeck coefficient, and thermal conductivity are strongly affected by the Bi concentration. The sample of shows a maximum value of the figure of merit, ZT, is 0.86 at 862K. [Copyright &y& Elsevier]

In situ three-layer surface composites of Zn-27Al-9.8Mg-5.2Si and Zn 27A1-6.3Mg 3.7Si alloys were obtained by centrifugal casting with preheated metal mould, which possesses the microstructures of accumulated lots of blocky primary Mg2Si and a litter blocky primary Si in inner layer, accumulated primary Mg2Si and primary Si in outer layer, and without primary Mg2Si and primary Si in middle layer. The effects of composition, mold temperature, rotating rate and modification on microstructures of centrifugally cast Zn-27Al-Mg-Si alloys were investigated. The results show that the inner and outer layers of Zn-27Al-9.8Mg-5.2Si alloy are respectively thicker than those of Zn-27Al-6.3Mg-3.7Si alloy and have more primary Mg2Si. During the centrifugal casting of Zn-27Al-6.3Mg-3.7Si alloy, with rotating rate increasing, the thickness of the inner layer increases and the outer layer becomes thinner and the addition of Na salt can refine grain and primary Mg2Si, and make primary Mg2Si distribute more evenly and make primary Si nodular. Moreover, the forming process of composites was also analyzed. [ABSTRACT FROM AUTHOR]

This article describes the effect of extrusion ratio in direct extrusion process on mechanical behaviours of alloy AA-6101. The alloy is used mainly as feedstock in manufacturing of AAAC type aluminium alloy conductors and extrusions for compression joints and connectors in electrical industry. Extrusion ratio effect was examined by extruding the billets at different billet temperatures. The billet structures have been prepared as homogenised-rapid quenched (HRQ) and unhomogenised-slow cooled (UHSC). Keeping up constant the forward speed of the main ram and the exit temperature of the extrusions, AA-6101 billets were extruded directly by using three different dies with diameters 10, 30 and 50 mm. Mechanical measurements of the specimens were made after T-1 and T-6 treatments. Extrusion ratio effect on tensile strength, elongation and hardness of extrusions from the AA-6101 billets was appreciated on the graphics. [Copyright &y& Elsevier]

Abstract: Effects of composition, mold temperature, rotating rate and modification on microstructure of centrifugally cast Zn–27Al–xMg–ySi alloys have been investigated. In situ composites of Zn–27Al–6.3Mg–3.7Si and Zn–27Al–9.8Mg–5.2Si alloys were fabricated by centrifugal casting using heated permanent mold. These composites consist of three layers: inner layer segregates lots of blocky primary Mg2Si and a litter blocky primary Si, middle layer contains without primary Mg2Si and primary Si, outer layer contains primary Mg2Si and primary Si. The position, quantity and distribution of primary Mg2Si and primary Si in the composites are determined jointly by alloy composition, solidification velocity under the effect of centrifugal force and their floating velocity inward. Na salt modifier can refine grain and primary Mg2Si and make primary Mg2Si distribute more evenly and make primary Si nodular. For centrifugally cast Zn–27Al–3.2Mg–1.8Si alloy, the microstructures of inner layer, middle layer and outer layer are almost similar, single layer materials without primary Mg2Si and primary Si are obtained, and their grain sizes increased with the mold temperature increasing. [Copyright &y& Elsevier]

Abstract: The modification of in situ formed Mg2Si in Mg–Si alloys with yttrium (Y) was investigated. As-cast microstructure analysis of Mg–5wt.% Si alloys indicates that all the primary Mg2Si crystals are surrounded by a layer of α-Mg dendritic halos, which is again surrounded by the eutectic Mg2Si+Mg structures. When the Y content is 0.1 or 0.4wt.%, the sizes of primary and eutectic Mg2Si do not significantly reduce in Mg–5wt.% Si alloys. When the Y content is 0.8wt.%, the mean size of primary Mg2Si decreases from more than 100μm to about 30μm or less; furthermore, the Chinese script type Mg2Si particle also exhibits modified morphology as a fine fiber. When the Y content is further increased to 1.2wt.%, the primary Mg2Si becomes coarser again and even larger than that in the unmodified alloy; however, the eutectic Mg2Si still exhibits modified morphology as a fine size. The size and amount of Mg–Y point compounds are increased with the increasing of Y content in the alloys. In addition to distributing in α-Mg phase, the Mg–Y point compounds are also present in primary Mg2Si dendritic crystal. In the unmodified alloy, the coarse primary Mg2Si is formed by the preferred growth that occurs at the tips of branches; however, when the alloy is modified with 0.8wt.% Y, the preferred growth manner of the primary Mg2Si was depressed, while another type of growth, isotropic growth manner, is enhanced in the modified alloy. [Copyright &y& Elsevier]

Abstract: The modification effect of KBF4 on the primary and eutectic Mg2Si in Mg–Si alloy is better than that of K2TiF6. This may be the reason of the presence of B in melts. However, the modification effect of KBF4 is greatly weakened by the simultaneous addition of K2TiF6 due to the decreasing of B content in the melts by the formation of TiB and TiB2. [Copyright &y& Elsevier]

Mg2Si were grown by the vertical Bridgman (VB) method in crucibles made of chemical vapor deposition (CVD) pyrolytic graphite (PG) in order to minimize the reaction and sticking of molten Mg–Si during growth. Congruent crystallization was derived from a stoichiometric melt of Mg2Si, and incongruent crystallization was derived from nonstoichiometric melts having Mg/Si ratios of 85:15, 70:30 and 60:40. Grown samples were characterized by X-ray diffraction and electron-probe microanalysis, and their power factors were calculated from the Seebeck coefficients and electrical conductivities measured from room temperature to 773 K. The grown crystals were single-crystal-like and had high Seebeck coefficients at the temperatures from 500 to 773 K. A sample derived from a stoichiometric melt had a Seebeck coefficient of -470 μV/K and the highest power factor, 7.8×10-6 W/cm K2 at 373 K, was calculated for the sample derived from a melt with an Mg/Si ratio of 70:30. [Copyright &y& Elsevier]

The effect of strontium on the crystallization of Mg2Si phase in Al–11.6%Si–0.4%Mg alloys was investigated with optical microscope and SEM. In the partially modified alloys, the Mg2Si phase grows as network or bamboo-shoot shape. However, very few and fine Mg2Si particles are isolated at the boundaries of the eutectic cells in the fully modified alloys. Addition of strontium restrains the crystallization of Mg2Si phase. The Mg2Si phase in Al–Si–Mg casting alloy is thought to nucleate on the surfaces of the eutectic silicon flakes. This hypothesis was analyzed from the point of view of the lattice match between the Mg2Si and Si crystals. The restraining effect of strontium is thought to be related to the combination of two effects of strontium, the increase of the amount of dendritic α phase and the change of the surface characteristic of silicon crystal caused by the addition of strontium. [Copyright &y& Elsevier]

A hexagonal plate-shaped β-phase was discovered in Al–0.70at.%Mg–0.36at.%Si–0.21at.%Cu alloy. This phase has the same crystal lattice as β-Mg2Si, commonly formed in Cu-free Al–Mg–Si alloys, but a different habit plane, and contains less than 2.0at.%Cu. [Copyright &y& Elsevier]

Mg2Si was synthesized by mechanically activated annealing and evaluated as a negative electrode material. A maximum discharge capacity of 830 mAh/g was observed by cycling over a wide voltage window of 5–650 mV versus Li, but capacity fade was rapid. Cycling over the range 50–225 mV versus Li produced a stable discharge capacity of approximately 100 mAh/g. X-ray diffraction (XRD) experiments showed that lithium insertion converts Mg2Si into Li2MgSi after lithium intercalation into Mg2Si. Electrochemical evidence of Li–Si reactions indicated that the Li2MgSi structure can be converted to binary lithium alloys with extensive charging. [Copyright &y& Elsevier]

Ab initio calculations were used to study the mechanisms of the processes of Mg–Si solid state intermixing and silicide formation. An evolutionary algorithm was used to search for Mg α Si (1-α) structures with lowest formation energies, E f ; the composition parameter α was varied over the range 0–1. New structures with negative formation energies have been found for α > 0.67. Tentatively, the obtained structure Mg 3 Si, has been observed in experiments. Besides the minimum at α = 0.67 corresponding to Mg 2 Si, the dependence of E f (α) has two positive maxima: near low and high α . This dependence indicates that certain energy barriers prevent the initiation of Mg–Si intermixing. High temperatures and abundance of both Mg and Si atoms at the interface are required for initiation of the intermixing and silicide formation processes. Practical implications of this conclusion for routine and new methods of Mg 2 Si growth in vacuum are analyzed. Some characteristic experimental data are explained. Rational methods of fabrication of cheap and environmentally-friendly Mg 2 Si and related compounds for applications in solar cells and thermoelectric devices are indicated. • Calculations with USPEX code were used to study the reaction at Mg/Si interface. • The shape of the Mg α Si 1-α alloy formation energy dependence on α was analyzed. • Some energy barriers block the initiation of Mg/Si intermixing. • High T and abundance of both Mg and Si are necessary for intermixing initiation. • The calculated structure Mg 3 Si (tentatively) has been observed in experiments. [ABSTRACT FROM AUTHOR]

In current study; the effect of various Gadolinium (Gd) additions on the microstructure and sliding wear behaviour of Al-15%Mg2Si composite before and after the hot extrusion process was examined. Optical microscopy (OM), scanning electron microscopy (SEM) equipped with EDX facility and X-ray diffraction (XRD) were used to characterize the microstructure. The results showed that with addition of 1.0 wt.% Gd to Al-15%Mg2Si composite, the primary Mg2Si particles size reduced from 44 µm to 23 µm and its morphology altered from dendritic to polygonal shape. Further refinement of primary Mg2Si particles was achieved after conducting hot extrusion which resulted in a decrease in its size to 19 µm with a transfer to near-spherical morphology. The Vickers hardness value increased from 55.6 HV in the as-cast and unmodified composite to 72.9 HV in the extruded 1.0% Gd modified composite. The wear test results revealed that composites treated with Gd possess higher wear resistance in comparison with those of without Gd. The highest wear resistance obtained with the lowest wear rates of 0.19 mm3/km and 0.14 mm3/km in the Al-15%Mg2Si-1.0% Gd before and after the hot extrusion, respectively. The high wear resistance of extruded Gd-modified Al-15%Mg2Si composite is due to the refinement of primary Mg2Si particles with uniform distribution in the composite matrix along with fragmentation of Gd intermetallic compounds. [ABSTRACT FROM AUTHOR]

The new in situ fabrication process for Mg-Mg2Si composites composed of interpenetrating metal/intermetallic phases via powder metallurgy was characterized. To obtain the designed composite microstructure, variable nanosilicon ((n)Si) (i.e., 2, 4, and 6 vol.% (n)Si) concentrations were mixed with magnesium powders. The mixture was ordered using a sonic method. The powder mixture morphologies were characterized using scanning electron microscopy (SEM), and heating and cooling-induced thermal effects were characterized using differential scanning calorimetry (DSC). Composite sinters were fabricated by hot-pressing the powders under a vacuum of 2.8 Pa. Shifts in the sintering temperature resulted in two observable microstructures: (1) the presence of Mg2Si and MgO intermetallic phases in α-Mg (580 °C); and (2) Mg2Si intermetallic phases in the α-Mg matrix enriched with bands of refined MgO (640 °C). Materials were characterized by light microscopy (LM) with quantitative metallography, X-ray diffraction (XRD), open porosity measurements, hardness testing, microhardness testing, and nanoindentation. The results revealed that (n)Si in applied sintering conditions ensured the formation of globular and very fine Mg2Si particles. The particles bonded with each other to form an intermetallic network. The volume fraction of this network increased with (n)Si concentration but was dependent on sintering temperature. Increasing sintering temperature intensified magnesium vaporization, affecting the composite formation mechanism and increasing the volume fraction of silicide. [ABSTRACT FROM AUTHOR]

The wide application of Al–Mg–Si alloys in automobile industry requires good formability, high strength and high bake-hardening response. Herein, this study presents a strategy to improve the comprehensive mechanical properties of Al–Mg-xSi-Cu-Zn alloys assisted by the designed multi-scale Si-containing phases. The results show that with increasing Si content from 0.55 to 1.08 wt%, not only the transformation rate of β-Al 5 FeSi to α-Al 12 (Fe,Mn) 3 Si during the homogeneous treatment can be increased, but also the number density and homogenization distribution level of the multi-scale Si-containing phases (i.e., β-Al 5 FeSi, α-Al 12 (Fe,Mn) 3 Si, Mg 2 Si, Q and Si phases) can be improved by the optimized thermomechanical processing route. These improvements further result in the fine-grained microstructure and enhanced average r, elongation and strengths. The positive effects change greatly with the changes of type, morphology, number density and distribution of the multi-scale particles. The related mechanisms of forming multi-scale particles and high formability have been discussed in this paper. • The multi-scale Si-containing phases were formed by controlling thermomechanical processing. • The formation mechanism of multi-scale Si-containing phases was deeply studied. • The dispersion multi-scale Si-containing phases could give a positive effect on the evolution of microstructure and texture. • The multi-scale Si-containing phases could assist the improvement of mechanical properties. [ABSTRACT FROM AUTHOR]

• High Zn containing Mg-Si alloys were developed. • Mg-Si-Zn alloys (>2 wt% Zn) can generate fine fibrous Mg 2 Si under fast solidification. • Superior tensile properties were attained due to the fineness of the phases and solubility of Zn in Mg. In this study it was found that Mg-0.6Si-Zn alloys with a high Zinc (Zn) content (>2 wt%) can generate fine fibrous Mg 2 Si particles, as long as the solidification cooling rates could be supervised. Due to the solubility of Zn in Mg and the morphology of Mg 2 Si fibers, improved tensile properties were obtained for samples having refined dendritic arrangements. These results open the perspective of development of Mg-Si alloys with higher Zn contents, with greater strength and ductility due to microstructural control. Although there is a considerable difference in Zn alloying between them, both the Mg-0.6wt.%Si-2.0wt.%Zn and Mg-0.6wt.%Si-3.0wt.%Zn alloys have reached quite close optimum levels of tensile properties of the order of 200 MPa and 9%. [ABSTRACT FROM AUTHOR]

In order to improve the corrosion resistance of Al 10Si coating, Al-10Si- x Zn- y Mg (x = 5, 10 wt%; y = 0, 1, 2, 3 wt%) alloy was studied by phase diagram calculation, microstructure observation and polarization curve test. According to the phase diagram and microstructure, Al-10Si- x Zn- y Mg (x = 5, 10 wt%; y = 0, 1, 2, 3 wt%) alloy is composed of α-Al, Si and Mg 2 Si. In addition, the cooling rate has no effect on the phase type of the alloy. In the joint efforts of the solid solution strengthening of Zn and the increase of Mg 2 Si reinforcing phase, the hardness of Al 10Si alloy increases with the addition of Zn and Mg. With the cathodic protection of Mg 2 Si, the corrosion resistance of the Al-10Si-10Zn-2 Mg alloy is the best. • Phase diagram was used to study the improvement effect of Mg 2 Si precipitation on the corrosion resistance of Al-Si-Zn-Mg alloy. • By adding an appropriate amount of Mg, the alloy with low content of Zn has good corrosion resistance. • The addition of Zn and Mg can enhance the sacrificial protection of Al 10Si alloy coating. [ABSTRACT FROM AUTHOR]

The bonding strength of a Gr/Mg2Si interface was calculated by first principles. Graphene can form a stable, completely coherent interface with Mg2Si. When the (0001) Gr/(001) Mg2Si crystal plane is combined, the mismatch degree is 5.394%, which conforms to the two-dimensional lattice mismatch theory. At the interface between Gr/Mg2Si, chemical bonds were not formed, there was only a strong van der Waals force; the interfaces composed of three low index surfaces (001), (011) and (111) of Mg2Si and Gr (0001) have smaller interfacial adhesion work and larger interfacial energy, the interfacial energy of Gr/Mg2Si is much larger than that of α-Al/Al melt and Gr/Al interfacial (0.15 J/m2, 0.16 J/m2), and the interface distance of a stable interface is larger than the bond length of a chemical bond. The interface charge density difference diagram and density of states curve show that there is only strong van der Waals force in a Gr/Mg2Si interface. Therefore, when the Gr/AlSi10Mg composite is stressed and deformed, the Gr/Mg2Si interface in the composite is easy to separate and become the crack propagation source. The Gr/Mg2Si interface should be avoided in the preparation of Gr/AlSi10Mg composite. [ABSTRACT FROM AUTHOR]

The mechanical and electronic properties of β phase (Mg 2 Si) doping with rare earth (RE) atoms (Sc, La, Ce, Yb) were investigated by first-principles based on density functional theory. The results showed that both Mg 8 Si 3 RE and Mg 7 Si 4 RE can exist stably by doping from the analysis of formation energy and cohesive energy. We found that RE atoms doping into the Mg 2 Si lattice tends to occupy Mg position preferentially. Doping Yb which has the largest H v can strength the stiffness of compound, meanwhile, Mg 7 Si 4 Ce has the smallest stiffness. The ductility of Mg 2 Si increases after doping with La (Ce). Doping RE atoms cause strong orbital hybridization between Al and RE, so the bonding strength and stability of Mg 2 Si can be significantly changed. • The mechanical properties of rare earth elements doped on Mg 2 Si compound are explored by first principle calculations. • The doping RE atoms can improve structural stability of Mg 2 Si, and further change its toughness and plasticity properties. • Doping RE atoms cause strong Al-RE orbital hybridization and change the bonding strength and stability of Mg 2 Si. [ABSTRACT FROM AUTHOR]

The influence of AlFeSi and Mg2Si phases on corrosion behaviour of the cast 6061 aluminium alloy was investigated. Scanning Kelvin probe force microscopy (SKPFM), electron probe microanalysis (EPMA), and in situ observations by confocal laser scanning microscopy (CLSM) were used. It was found that Mg2Si phases were anodic relative to the matrix and dissolved preferentially without significantly affecting corrosion propagation. The AlFeSi phases' influence on 6061 aluminium alloy local corrosion was greater than that of the Mg2Si phases. The corroded region width reached five times that of the AlFeSi phase, and the accelerating effect was terminated as the AlFeSi dissolved. [ABSTRACT FROM AUTHOR]

Magnesium silicide (Mg2Si) is a promising as an environmental friendly thermoelectric material. It is difficult to synthesize high-purity Mg2Si because of the boiling of Mg and the oxidation of Mg. In order to avoid the disagreement of stoichiometric ratio and the oxidation, we have performed high-pressure synthesis. And also the high-pressure X-ray diffraction by the synchrotron radiation source was carried out to find synthetic condition under pressure, and we have found that Mg2Si was synthesized at 1.0 GPa beyond 573 K. Because high synthetic temperature causes the oxidation of Mg, the high-pressure synthesis by means of a piston cylinder apparatus performed at 673 K. The synthesized compounds were evaluated by XRD analysis and thermoelectric measurement. Because the size of compounds synthesized under pressure is small, we also developed and evaluated the new equipment of the thermoelectric measurement for such as small sample. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We have studied the effect of thermal annealing periods on the optoelectrical properties of Mg2Si pn -junction diodes fabricated by a conventional thermal diffusion process of Ag acceptor. The Au/Ag/ n -Mg2Si (carrier concentration = 2 × 1015cm-3, resistivity = 5-10 Ωcm) substrate was annealed at 550 °C in the Ar-atmosphere using a rapid thermal annealing (RTA) furnace. The annealing periods were varied between 2 and 10 minutes. Clear rectifying behavior of J - V characteristics was observed for the diodes annealed for more than 2 minutes. Spectral responsivity from the photon energy threshold of approximately 0.6 eV was demonstrated with the Mg2Si pn -junction diodes under the zero bias condition at room temperature. The maximum intensity of spectral responsivity was increased with decrease the annealing period. The spectral responsivity of the diode with the RTA periods of 4 minutes was approximately 4 times higher than that of 10 minutes annealed one. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Mg2Si is expected as a candidate for environmental-friendly material of a thermoelectric device. Nano-crystallization is considered to be one of the most effective approaches to improve thermoelectric property. If grain grows up during sintering, we are afraid not to obtain nano-effect. We have, therefore, investigated grain growth during sintering. We have found that grain growth is not observed if particle size distribution of starting powder is relatively uniform. On the contrary, grain growth is observed if it is relatively not uniform. We have concluded this may be due to un-uniformly distributed current during sintering. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract: We have studied metal-semiconductor contact of -type Mg2Si single crystal grown by the Bridgman method. We found that a contact of Ag/n-Mg2Si showed ohmic property whereas a contact of Au/n-Mg2Si performed Schottky property. Mesa structure at the Au/n-Mg2Si contact, which was formed by etching with diluted HF (2%) solution and Au-metal mask, successfully improved the leakage current in reverse bias region. The Shottky barrier height was obtained from the J-V characteristics of the mesa structured Au/n-Mg2Si/Ag contacts. [Copyright &y& Elsevier]

• Al-Si/Mg 2 Si in-situ composite were fabricated by FSP at solid state. • Modification of Al-Si cast structure with uniform distribution of Mg 2 Si particles. • Formation of Mg 2 Si at the expense of Si particles can increase the Hardness. Al/Mg 2 Si in - situ cast composites are very popular in Al casting industries, but they suffer low toughness due to very coarse Mg 2 Si particles. The friction stir processing (FSP) method can produce composites with very fine and uniform distribution of the reinforcement phase. Hence, in the present research, for the first time, FSP was used to fabricate Al/Mg 2 Si in - situ composites. For this purpose, pure Mg was added to Al-Si cast eutectic alloy by FSP. The formation of Mg 2 Si was studied by a variety of analysis techniques. The results prove the applicability of FSP for producing of Al/Mg 2 Si composite. The hardness of the prepared composite shows moderate improvement (~15%) compared to the FSPed base alloy. [ABSTRACT FROM AUTHOR]

Knowledge of the mechanical properties of thermoelectric materials at high temperature is required for the modelling and optimization of a thermoelectric generator under application conditions. We present room and high temperature characterization of Young's modulus for Mg 2 Si, Mg 2 Sn and their solid solutions establishing a linear dependence, both on composition and on temperature. Through the comparison of Resonant Ultrasound Spectroscopy and free vibration techniques, we measure the elastic moduli with high precision. The comparison of our results (E = 110 GPa for Mg 2 Si and 78 GPa for Mg 2 Sn) to first principles calculations and experimentally reported data shows good agreement. Additionally, by estimating the Poisson ratio, we calculate the temperature-dependent shear modulus G and finally provide a simple bilinear function for Young's and shear moduli as a function of temperature and composition. Image 1 • Resonant characterization methods provide a fast and accurate measurement of elastic properties. • Temperature and composition elastic behavior of solid solutions Mg 2 Si–Mg 2 Sn can be described by a bi linear equation. • Temperature dependent elastic constants can further increase the precision of thermomechanical model. [ABSTRACT FROM AUTHOR]

The current study focusses on the phase composition, solidification path, and microstructure evaluation of gravity cast Al-4Mg-0.5Si-xLa aluminum alloy, where x = 0, 0.1, 0.25, 0.5, 0.75, and 1 wt.% La. A computational CalPhaD approach implemented in Thermo-Calc software and scanning electron microscopy technique equipped with electron microprobe analysis (EMPA) was employed to assess its above-mentioned characteristics. The thermodynamic analysis showed that the equilibrium solidification path of La-containing Al-Mg-Si alloys consists of only binary phases LaSi2 and Mg2Si precipitation along with α-Al from the liquid and further solid-state transformation of this mixture into α-Al + Al11La3 + Mg2Si + Al3Mg2 composition. Scheil–Gulliver simulation showed a similar solidification pathway but was accompanied by an increase in the solidification range (from ~55 °C to 210 °C). Furthermore, microstructural observations were congruent with the calculated fraction of phases at 560 °C and related to α-Al + LaSi2 + Mg2Si three-phase region in terms of formation of La-rich phase having both eliminating effect on the eutectic Mg2Si phase. Quantitative EMPA analysis and elemental mapping revealed that the La-rich phase included Al, La, and Si and may be described as Al2LaSi2 phase. This phase shows a visible modifying effect on the eutectic Mg2Si phase, likely due to absorbing on the liquid/solid interface. [ABSTRACT FROM AUTHOR]

In this paper, the microstructure and mechanical properties of two magnesium matrix composites—a hypoeutectic with 1.9 wt% Mg2Si phase and a hypereutectic with 19 wt% Mg2Si compound—were analyzed. The investigated materials were prepared using the gravity casting method. Microstructure analyses of the fabricated composites were carried out by XRD and light microscopy. The tensile and compression strength as well as yield strength of the composites were examined in both uniaxial tensile and compression tests. The microstructure of the hypoeutectic composite was in agreement with the phase diagram and composed of primary Mg dendrites and an Mg–Mg2Si eutectic mixture. For the hypereutectic composite, besides the primary Mg2Si phase and eutectic mixture, additional magnesium dendrites surrounding the Mg2Si compound were observed due to nonequilibrium solidification conditions. The composites exhibited a rise in the examined mechanical properties with an increase in the Mg2Si weight fraction and also a higher tensile and compression strength in comparison to the pure magnesium matrix (cast in the same conditions). Additionally, analyses of fracture surfaces of the composites carried out using scanning electron microscopy (SEM + EDX) are presented. [ABSTRACT FROM AUTHOR]

Mg2Si is a promising eco-friendly thermoelectric material, and Ni is suited for electrical contact on it. In this study, Bi-doped Mg2Si ingots with Ni contacts were fabricated by co-sintering, and thermal stability was investigated by long-time (500 h, 500 cycles) temperature cycling from 25 °C to a peak temperature (Th = 400 and 450 °C) in N2. The as-sintered Ni/Mg2Si interfacial region is a multilayer consisting of Mg3Bi2, a series of MgxSiyNiz ternary compounds (ω, ν, ζ, and η-phases), and MgNi2. In the complex microstructure, the MgNi2 / η-phase interface was vulnerable to stress-induced voiding at Th = 450 °C, which arises from the mismatch of the thermal expansion coefficients. Interfacial voiding was avoided by adding 10 mol% Ag in Ni, which is probably due to the suppression of vacancy migration by the Ag-containing 2nd phase formation at the MgNi2/η-phase interface. [ABSTRACT FROM AUTHOR]

The microstructure and fracture characteristics of 105 mm thick 5083 aluminum alloy hot rolled plate are investigated by X-ray diffractometer, metallurgical microscope, scanning electron microscope, transmission electron microscope, energy dispersive spectrometer and tensile testing machine in this paper. The results show that the plate is mainly composed of α-Al, Al 21 (Fe, Mn) 4 Si, Al 6 (Fe, Mn) and Mg 2 Si. The second phase distribution is inhomogeneous along the thickness direction, resulting in an U shape change of elongation. The minimum elongation in Layer 5 is related to the dependence of Mg 2 Si on Fe-containing intermetallic compounds, state of dislocations and grooves. Al 6 (Fe, Mn) presents acicular in the surface layer and fragmented in the center, and its fracture mode can be used to roughly judge the stress. Under SEM, the common black phases in high Mg-containing aluminum alloys may be not Mg 2 Si, but holes formed by their shedding. Mg 2 Si tends to adhere to the surface of Fe-containing intermetallic compounds and it is more easily separated from Al matrix. This indicates that Mg 2 Si may be the source of stress concentration initially causing materials fracture. A comprehensive summary has been made to clarify the influence of structures on fracture characteristics of thick plate. • The second phase distribution is inhomogeneous along the thickness direction, resulting in an U shape change of elongation. • Al 6 (Fe, Mn) presents acicular in the surface layer and fragmented in the center. • The common black phases in high Mg-containing aluminum alloys may be not Mg 2 Si, but holes formed by their shedding. • Mg 2 Si may be the source of stress concentration initially causing materials fracture. • A comprehensive summary has been made to clarify the influence of structures on fracture characteristics of thick plate. [ABSTRACT FROM AUTHOR]

• In the Al-Al 3 Ni coupling system, the intermetallic Al 3 Ni phase acts as cathode. • The polarity conversion of Al and primary Mg 2 Si lags behind that of Al and eutectic Mg 2 Si. • The corrosion pits around the Al 3 Ni and Mg 2 Si are connected to form long corrosion channels. Corrosion behaviors of Al-15%Mg 2 Si-1%Ni alloy in 3.5% NaCl solution were studied by characterizing the corrosion morphologies of Al 3 Ni, Mg 2 Si, and Al phases. The results show that there are two galvanic coupling systems (Al/Al 3 Ni and Al/Mg 2 Si) in Al-15%Mg 2 Si-1%Ni alloy. In the Al/Al 3 Ni coupling system, the intermetallic Al 3 Ni phase (nobler than the Al matrix) acts as the cathode throughout the corrosion process. For the Al/Mg 2 Si coupling system, electrochemical polarity conversion occurs between the Al matrix and the Mg 2 Si phase. The corrosion pits around the Al 3 Ni and Mg 2 Si are connected to form long corrosion channels, which accelerate corrosion of the Al-Mg 2 Si-Ni alloys. [ABSTRACT FROM AUTHOR]