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Segregation can seriously damage the mechanical properties of the aluminum alloys. 6061 aluminum alloy wheel spokes were prepared by squeeze casting. To investigate the formation mechanism of segregation, the microstructure of the alloy was observed using scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction and electron microprobe analysis methods. The Gibbs energy of each phase during solidification was calculated by JMatPro. Results show that the segregation phases in the R-joint of the wheel spokes are mainly composed of Mg2Si, β-AlFeSi and Al5Cu2Mg8Si6 intermetallics. During the solidification of the 6061 aluminum alloy wheels, Mg2Si and α-AlFeSi phases precipitate in the mushy zone at first. With the decrease of temperature, α-AlFeSi transforms into β-AlFeSi, while Al5Cu2Mg8Si6 precipitates from the solid-state aluminum alloy after solidification. Segregation at the R-joint of wheel spokes is mainly caused by insufficient cooling, so the cooling during alloy solidification should be enhanced to avoid segregation.

The semi-solid processed Mg2Si/A356 composites were fabricated using a sloping plate, and the phase and morphology evolution of the semi-solid Mg2Si/A356 slurry during the remelting process was investigated. Results indicate that compared to as-cast microstructure, the size of primary α-Al phase and Mg2Si phase of semi-solid microstructure fabricated by using a sloping plate decreases and the morphology of α-Al phase becomes fine and globular. With increasing the reheating temperature and prolonging the holding time, the primary α-Al phase spheroidizes, the liquid fraction in semi-solid microstructure increases, and the Chinese script Mg2Si phase embedded in the primary α-Al phase cannot be observed. The Chinese script Mg2Si phase is distributed in the secondary α-Al phase and becomes smaller. The net-shaped eutectic phase also becomes finer after reheating. The optimum remelting parameters suitable for thixoforming in this study are remelting at 580 °C for 30 min.

In the present study, complex modification of primary and eutectic Mg2Si particles in Al–15%Mg2Si, composite was examined by simultaneous addition of gadolinium (Gd) and antimony (Sb). It was realized that combined addition of 1.5 wt. % Gd–Sb was efficient for modifying Mg2Si, in which coarse dendritic primary Mg2Si particles were changed into a perfect truncated octahedral morphology with a decrease in average size from 40 µm to 12 µm, an increase in density of Mg2Si per area from 495 to 1435 particle/mm2, and a reduction in the aspect ratio from 1.34 to 1.17. Furthermore, eutectic Mg2Si particles transformed from plate-like to fiber-like structure with an average eutectic Mg2Si cell decreasing from 132 µm to 51.22 µm. Thermal analysis revealed that nucleation and growth temperatures of primary Mg2Si increased with addition of Gd–Sb. However, it had an opposite effect for eutectic Mg2Si. It was proposed that refinement/modification of primary Mg2Si particles was mainly derived from formation of Mg3Sb2 compounds as heterogeneous nuclei for primary Mg2Si. Meanwhile, the preferred growth of primary Mg2Si crystals along direction can be obviously suppressed by the absorption and poisoning of Gd and Sb. The tensile results presented that addition of 1.5 wt. % Gd–Sb to the composite enhanced the UTS and %El from 204.79 MPa and 2.65% in base condition to 242.73 MPa and 3.9% in the modified composite, respectively. This study demonstrated that combined addition of Gd–Sb can provide a better effect on the modification and tensile properties of Al–15%Mg2Si composite than addition of Gd or Sb separately.

This study aims to investigate the microstructural alterations, mechanical properties, sliding wear behavior, and corrosion properties of Al-15%Mg2Si composites with different contents of yttria-stabilized zirconia (YSZ). Al-15%Mg2Si composites with the different contents of YSZ (0, 3, 6, and 9 wt.%) were fabricated using the stir-casting technique. The fabricated composites were characterized by means of optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS), Vickers hardness tester, linear reciprocating tribometer (LRT), and electrochemical test. The results showed that with the introduction of YSZ particles, the average size of the primary Mg2Si particles in the base composite was 137.78 µm, which was reduced to 88.36 µm after adding 9 wt.% YSZ. The aspect ratio of Mg2Si particles also decreased from 3, for the base composite, to 1.27 in the composite containing 9 wt.% YSZ. Moreover, the hardness value displays an incremental trend from 102.72 HV, as recorded for the base in situ composite, to 126.44 HV in the composite with 9 wt.% YSZ. On top of that, the Al-15%Mg2Si-9%YSZ demonstrates exceptional wear resistance, with the lowest wear rate of 0.46 mm3/km under a 25 N applied load. Its average coefficient of friction (COF) was recorded at 0.42, which is lower than both the 3 and 6 wt.% of YSZ-containing composites. The smoother worn surface in Al-15%Mg2Si-9%YSZ hybrid composite implies the abrasion phenomenon, as dominant wear behavior is milder than the other fabricated composites. On top of that, the Al-15%Mg2Si-9%YSZ also possesses optimum corrosion resistance. The corrosion rate is 0.080 mmpy, comparable to the 0.164 mmpy rate obtained in the in situ composite.

Mg−Li alloy possesses a high hydrogen capacity. However, the hydrogenation and dehydrogenation performances are still far from practical application. In this work, Mg2Si (MS) and graphene (G) were employed together to synergistically improve the hydrogen storage properties of Mg−Li alloy. The structures of the samples were studied by XRD and SEM methods. The hydrogen storage performances of the samples were studied by nonisothermal and isothermal hydrogenation and dehydrogenation, thermal analysis, respectively. It is shown that the onset dehydrogenation temperature of Mg−Li alloy was synergistically reduced from 360°C to 310°C after co-addition of Mg2Si and graphene. At a constant temperature of 325°C, the Mg−Li−MS−G composite can release 2.7 wt.% of hydrogen within 2 h, while only 0.2 wt.% of hydrogen is released for the undoped Mg−Li alloy. The hydrogenation activation energy of the Mg−Li−MS−G composite was calculated to be 86.5 kJ mol−1. Microstructure and hydrogen storage properties studies show that graphene can act as a grinding aid during the ball milling process, which leads to a smaller particle size for the composites. This work demonstrates that coaddition of Mg2Si and graphene can synergistically improve the hydrogen storage properties of Mg−Si alloy and offers an insight into the role of graphene in the Mg−Li−MS−G composite.

WOS: 000487287600015 Mg2X (X=Si, Sn, and Ge) based systems have attracted widespread attention owing to their various benefits in thermoelectric applications. in particular, to date, ternary Mg2X based solid solutions have become one of the most widely investigated thermoelectric systems. However, the investigation of temperature varied thermoelectric properties of Mg2X based quaternary systems is rather limited both theoretically and experimentally. Therefore, here, we report a rigorous theoretical work of thermoelectric properties for n-type Mg2Si0.55-zSn0.4Ge0.05Biz quaternary solid solutions (z=0.02, 0.025, 0.03, and 0.035) from 300 K to 850 K. By using nearly-free-electron model together with Fermi-Dirac statistics we define Fermi level both in extrinsic and intrinsic regimes as a function of temperature. We follow Hicks and Dresselhaus' approach to calculate electronic transport properties. By performing the Debye's isotropic continuum model a detailed theoretical investigation of lattice thermal conductivity is presented among with various phonon relaxation rates. From our theoretical analysis the highest ZT is attained for Mg2Si0.53Sn0.4Ge0.05Bi0.02 solid solution as 1.14 at 850 K. (C) 2019 Published by Elsevier B.V. on behalf of Chongqing University. TUBITAK (Scientific and Technical Research Council of Turkey)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [115F387] Ovgu Ceyda Yelgel wishes to acknowledge financial support from TUBITAK (Scientific and Technical Research Council of Turkey) (Project number: 115F387).

The problem of preparing a ternary powder mixture, which was meant to fabricate sintered heterophase composite, and consisted of micro- and two nanosized powders, was analyzed. The microsized powder was a pure magnesium, and as nanocomponents, a silicon powder (nSi) and carbon nanotubes (CNTs) with 2% and 1% volume fractions, respectively, were applied. The powder mixtures were prepared using ultrasonic and mechanical mixing in technological fluid, and four mixing variants were applied. The morphology of the powder mixtures was characterized with scanning electron microscopy (SEM), and then, composite sinters were fabricated in a vacuum with hot temperature pressing at 580 °C under 15 MPa pressure, using a Degussa press. The reaction between the nSi and the Mg matrix, which caused the creation of the Mg2Si phase in the fabricated Mg-Mg2Si-CNT composite, was confirmed with X-ray diffraction (XRD). The porosity and hardness of the composite sinters were examined, and optical microscopy (OM) and quantitative image analyses were carried out to characterize the microstructure of the composites. In the manufacturing process of the Mg-nSi-CNT mixtures, the best results were the following: first separate de-agglomeration of nanocomponents, then their common mixing, and finally, the deposition of nanocomponents at the surface of the microsized magnesium powder. The applied procedure ensured the uniform layer formation of de-agglomerated nanocomponents on the Mg powder, without re-agglomerated nSi and CNTs. Moreover, this type of powder mixture morphology allows to obtain sinters with lower porosity and higher hardness, which is accompanied by precipitation of a finer Mg2Si phase. In the Mg-Mg2Si-CNT composite, the carbon phase was present, and it was located in the magnesium matrix and in silicide.

In the present study, the behaviour of coarse constituent Mg2Si particles during alkaline etching and desmutting of AA5052 aluminium alloy is investigated by monitoring the morphological and compositional evolution of the particles using quasi in-situ electron microscopy. An etching product film, consisting of Mg(OH)2, and SiO2·xH2O sublayers, is formed on particle surface during alkaline etching, which renders the particles inert. During desmutting in nitric acid, the etching product film is dissolved and dealloying of Mg occurs rapidly, resulting in Si-rich remnant. However, the population density of Si-rich remnant on alloy surface is similar to that of the initial Mg2Si particles.

In this paper, the microstructure and mechanical properties of two magnesium matrix composites&mdash, a hypoeutectic with 1.9 wt% Mg2Si phase and a hypereutectic with 19 wt% Mg2Si compound&mdash, 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&ndash, 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.

We perform hybrid functional and quasi-particle band structure calculations with spin-orbit interaction to investigate the band structures of Mg2Si, Mg2Ge, and Mg2Sn. For all Mg2X materials, where X = Si, Ge, and Sn, the characteristics of band edge states, i.e., band and valley degeneracies, and orbital characters, are found to be conserved, independent of the computational schemes such as density functional generalized gradient approximation, hybrid functionals, or quasi-particle calculations. However, the magnitude of the calculated band gap varies significantly with the computational schemes. Within density-functional calculations, the one-particle band gaps of Mg2Si, Mg2Ge, and Mg2Sn are 0.191, 0.090, and -0.346 eV, respectively, and thus severely underestimated compared to the experimental gaps, due to the band gap error in the density functional theory and the significant relativistic effect on the low-energy band structures. By employing hybrid-functional calculations with a 35% fraction of the exact Hartree-Fock exchange energy (HSE-35%), we overcame the negative band gap issue in Mg2Sn. Finally, in quasi-particle calculations on top of the HSE-35% Hamiltonians, we obtained band gaps of 0.835, 0.759, and 0.244 eV for Mg2Si, Mg2Ge, and Mg2Sn, respectively, consistent with the experimental band gaps of 0.77, 0.74, and 0.36 eV, respectively., Comment: 23 pages, including 84 references, 5 tables, 3 figures

The Al-Mg 2 Si pseudo-binary phase diagram is one of the most widely used tool in the aluminum industry for temperature determination of solubility and homogenization in several semi-manufactured products of aluminum 6xxx alloys series. This diagram has been historically used in the redraw rod manufacturing process for electrical purposes to determine the rod coils solubility temperature of AA6101 and AA6201 alloys. The industrial process suggests a modification in the solvus curves established in this diagram. This work established the solubility temperature of commercial AA6201 alloy by means of electrical resistance measurements and mechanical testing of heat treated samples at different temperatures. Findings suggest that the solvus limit of the AA6201 alloy is at a higher temperature than the solvus limit of the commonly accepted diagram of Al-Mg 2 Si.

In the present study, four Al-Mg-Mn 5xxx alloys with different Mg levels (2-5 wt.%) were investigated for better understanding the evolution of intermetallic phases formed during solidification. Optical and scanning electron microscopes, electron backscattered diffraction and differential scanning calorimetry analyses in combination with thermodynamic calculation were used to identify various intermetallic phases. Results showed that the most dominant intermetallic phases are Al6(Mn,Fe), α-Al(Fe,Mn)Si, Al3Fe, Alm(Mn,Fe) and Mg2Si in experimental Al-Mg-Mn alloys, which is greatly dependant on the Mg levels. It is found that Chinese script α-Al(Fe,Mn)Si is the dominant iron-rich intermetallic phase for the alloys containing 2-3 wt.% Mg, while blocky Al6(Mn,Fe) and needle-like Al3(Mn,Fe) become the major phases for the alloy containing 4 wt.% Mg. Further increasing Mg content to 5 wt. %, the dominant phase transfers to blocky Al6(Mn,Fe) intermetallic. Meanwhile, the morphology of primary Mg2Si is changed from well-branched to plate-like with increasing Mg contents. In addition, β-Al3Mg2 and τ-Al6CuMg4 eutectic phases have been observed in the alloys with 3-5 wt. % Mg. A comparison on various intermetallic phases from the Scheil simulation and the actual as-cast microstructure is provided.

AM50/Mg2Si composites containing 5.7 wt. % and 9.9 wt. %. of Mg2Si reinforcing phase were prepared successfully by casting method. The microstructure of the cast AM50/Mg2Si magnesium matrix composites was investigated by light microscopy and X-ray diffractometry (XRD). The microstructure of these composites was characterized by the presence of α-phase (a solid solution of aluminium in magnesium), Mg17Al12 (γ-phase), Al8Mn5 and Mg2Si. It was demonstrated that the Mg2Si phase was formed mainly as primary dendrites and eutectic.

The formation energy, structure relaxation and electronic structure of Y-doped Mg2Si are investigated by the density functional methods. Y atoms are expected to be primarily located at the Mg sites in Y-doped Mg2Si. The Si atoms around the impurity outward relax relative to the Y. Doping makes non-degenerate semiconductor Mg2Si transfer to degenerate semiconductor Mg7Si4Y. The electronic structure of Mg2Si is complicated after doping, such increased complexity by Y doping can improve the Seebeck coefficient of Mg2Si significantly.

Al-Si-Mg alloys are investigated to determine the relationship between changes in the thermal diffusivity and precipitation behavior of the Mg2Si phase with various contents of Mg2Si and aging treatment conditions. The samples were solid solution-treated and then quenched with water (80 &deg, C). Aging treatments were implemented at temperatures ranging from 180 to 240 &deg, C for 5 h. The precipitation behavior of Mg2Si was observed using a heat flow curve using differential scanning calorimetry analysis. The thermal diffusivity of Al-Mg2Si alloy was affected by the precipitation of the Mg2Si phase, particularly in the meta-stable &beta, phase. In the temperature range of precipitation occurrence, the thermal diffusivity of the alloy increased with the temperature when the precipitation of the meta-stable &beta, phase of the sample was incomplete. However, at the same temperature, the samples in which precipitation had completed did not have any increased thermal diffusivity. The thermal diffusivity of the samples decreased when the meta-stable Mg2Si phase had dissolved in the matrix. The precipitation and dissolution of Mg2Si mainly affected the variation of thermal diffusivity in Al-Si-Mg. In contrast, the stable Mg2Si phase was not affected by changes in thermal diffusivity at a high temperature.

We have studied metal-semiconductor contact of n -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 ϕ B = 0.59 eV was obtained from the J-V characteristics of the mesa structured Au/n-Mg2Si/Ag contacts.

We present a comprehensive electron transport model to analyze thermoelectric properties of both n- and p-type bulk Mg2Si1-xSnx (0