New TiNiSi-type RMgAg and RMgPd with R=Ca, Yb, and MgZn2-type Ca(Mg,Ag)2 compounds

The phases CaMgAg, YbMgAg, CaMgPd, and YbMgPd were synthesized by melting the constituent metals in sealed tantalum crucibles and by annealing at 1023 K. All the samples were homogeneous, and the crystallographic analysis, which was performed by powder and singlecrystal techniques, shows that the four compounds are isotypic and belong to the orthorhombic TiNiSi type. Magnetic measurements showed that YbMgAg and YbMgPd behave like Pauli paramagnets, according to the divalency of Yb in both phases. Within the Ca-Mg-Ag system, the existence range of the MgZn₂-type phases in the Mg-rich CaMg_2−x Ag _x pseudobinary system goes from CaMg₂ to CaMg_1.6Ag_0.4. Another stability region of the MgZn₂ structure occurs around the Ag-rich composition (Ca_0.94Mg_0.06)(Ag_1.60Mg_0.40), where magnesium replaces both the Ca and Ag atoms.     

Hydrogenation of CaLi2−xMgx (0 ≤ x ≤ 2) with C14 Laves phase structure

CaLi_2−xMg_x (0 ≤ x ≤ 2) which has the C14-type Laves phase structure has been successfully synthesized and hydrogenated. The C14-type Laves phase structure was kept after hydrogenation of CaLi_2−xMg_x (x = 0.2, 0.5, 1). After hydrogenation of CaLi₂ and CaMg₂, the Laves phase disappeared. The CaH₂ and LiH phases were formed from CaLi₂ and the CaH₂ and Mg phases from CaMg₂, respectively. CaLi_2−xMg_x (0 < x < 2) ternary alloys formed stable hydride phases with the C14-type Laves phase structure in contrast to CaLi₂ and CaMg₂ binary alloys.     

Diffusion kinetics,mechanical properties,and crystallographic characterization of intermetallic compounds in the Mg–Zn binary system

Pure Mg was diffusion bonded to pure Zn at 315 °C for 168 h to produce equilibrium intermetallic compounds of the Mg–Zn system. All equilibrium phases at 315 °C, Mg₂₁Zn₂₅, Mg₄Zn₇, MgZn₂, Mg₂Zn₁₁, were observed to develop. Concentration profiles by electron probe microanalysis, electron diffraction patterns by transmission electron microscopy, and load–displacement curves by nano-indentation were examined to characterize the phase constituents, crystal structure, diffusion kinetics, and mechanical properties. Mg₂₁Zn₂₅ with trigonal, Mg₄Zn₇ with monoclinic, and Mg₂Zn₁₁ with cubic structures were found and their lattice parameters were reported herein. Mg₄Zn₇ and Mg₂Zn₁₁ were observed to have a range of solubility of approximately 2.4 at% and 1.6 at%, respectively. Interdiffusion in MgZn₂ occurred most rapidly, was an order of magnitude slower in Mg₄Zn₇ and Mg₂Zn₁₁, and was the slowest in Mg₂₁Zn₂₅. Composition-dependence of interdiffusion within each intermetallic phase was negligible. The intermetallic phases exhibited insignificant creep, but evidence of discontinuous yielding was observed. The average hardness and reduced moduli were similar for Mg₂₁Zn₂₅, Mg₄Zn₇, and MgZn₂ phases, ∼5 GPa and ∼90 GPa, respectively. However, the Mg₂Zn₁₁ phase had lower hardness of 3.76 GPa and higher modulus of 108.9 GPa. The mechanical properties in the characterized intermetallic phases, exclusive of Mg₂₁Zn₂₅, were strongly concentration-dependent.     

Preparation, structure and electrical conductivity of the pyrochlore-type phase Sm2Zr2O7 codoped with bivalent magnesium and trivalent dysprosium cations

The pyrochlore-type phases with the compositions of SmDy_1−xMg_xZr₂O_7−x/2 (0 ≤ x ≤ 0.20) have been prepared by pressureless-sintering method for the first time as possible solid electrolytes. The structure and electrical conductivity of SmDy_1−xMg_xZr₂O_7−x/2 ceramics have been studied by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy measurements. SmDy_1−xMg_xZr₂O_7−x/2 (x = 0, 0.05, 0.10) ceramics exhibit a single phase of pyrochlore-type structure, and SmDy_1−xMg_xZr₂O_7−x/2 (x = 0.15, 0.20) ceramics consist of pyrochlore phase and a small amount of the second phase magnesia. The total conductivity of SmDy_1−xMg_xZr₂O_7−x/2 ceramics obeys the Arrhenius relation, and the total conductivity of each composition increases with increasing temperature from 673 to 1173 K. SmDy_1−xMg_xZr₂O_7−x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest total conductivity value is about 8 × 10⁻³ S cm⁻¹ at 1173 K for SmDy_1−xMg_xZr₂O_7−x/2 ceramics.     

Thermodynamic calculation of high zinc-containing Al-Zn-Mg-Cu alloy

Phase fraction and solidification path of high Zn-containing Al-Zn-Mg-Cu series aluminum alloy were calculated by calculation of phase diagram (CALPHAD) method. Microstructure and phases of Al-9.2Zn-1.7Mg-2.3Cu alloy were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The calculation results show that η(MgZn₂) phase is influenced by Zn and Mg. Mass fractions of η(MgZn₂) in Al-xZn-1.7Mg-2.3Cu are 10.0%, 9.8% and 9.2% for x=9.6, 9.4, 8.8 (mass fraction, %), respectively. The intervals of Mg composition were achieved for θ(Al₂Cu)+η(MgZn₂), S(Al₂CuMg)+η(MgZn₂) and θ(Al₂Cu)+S(Al₂CuMg)+η(MgZn₂) phase regions. Al₃Zr, α(Al), Al₁₃Fe₄, η(MgZn₂), α-AlFeSi, Al₇Cu₂Fe, θ(Al₂Cu), Al₅Cu₂Mg₈Si₆ precipitate in sequence by no-equilibrium calculation. The SEM and XRD analyses reveal that α(Al), η(MgZn₂), Mg(Al,Cu,Zn)₂, θ(Al₂Cu) and Al₇Cu₂Fe phases are discovered in Al-9.2Zn-1.7Mg-2.3Cu alloy. The thermodynamic calculation can be used to predict the major phases present in experiment.     

Ca–Mg–Zn–(Ag) bulk metallic glasses prepared by unidirectional quenching

We fabricated ternary Ca–Mg–Zn and quaternary Ca–Mg–Zn–Ag bulk metallic glasses with diameter of 7 mm by unidirectional quenching into water-cooled Ga–In–Sn liquid alloys. It was suggested that the electromagnetic stirring caused by induction eddy current facilitated the glass formation. Glass forming ability of Ca_62.5Mg_17.5Zn_20?xAg_x (x = 0, 1, 3, 5, 7, 9) system was dependent on Ag content. It was found that in the system the in-situ formed crystalline phases enhanced the second-stage crystallization, whereas there seemed no contribution to the third-stage crystallization.     

The isothermal section of the La–Ag–Mg phase diagram at 400 °C

The whole isothermal section of the La–Ag–Mg phase diagram at 400 °C was constructed by means of phase identification and analysis on about eighty annealed ternary alloys. The tendency of Ag and Mg to reciprocally substitute in binary Ag–Mg phases reflects in the formation of several La–Ag–Mg ternary phases including solid solutions based on boundary binary phases, which show wide homogeneity regions extending at a constant La-content.Among the solid solutions, La(Ag_xMg_1-x) (0 ≤ x ≤ 1, cP2-CsCl), La(Ag_xMg_1-x)₃ (0 ≤ x ≤ 0.67, cF16-BiF₃) and La₂(Ag_xMg_1-x)₁₇ (0 ≤ x ≤ 0.30, hP42–3.64-CeMg_10.3) are the most extended. The crystal structure of La₂(Ag_xMg_1-x)₁₇ was determined from X-ray diffraction data refinement on two single crystals corresponding to x = 0 and x ≈ 0.2: a hexagonal disordered structure based on the CeMg_10.3 type structure is proposed for this solid solution (for La₂Mg₁₇: P6₃/mmc, hP42–3.64, a = 1.0387(1), c = 1.0263(2) nm, wR₂ = 0.0727, 408 F² values, 29 variables).Among the ternary compounds, the crystal structures of τ₁-LaAg_1+xMg_1-x (0 ≤ x ≤ 0.15, hP9-ZrNiAl), τ₂-La₄Ag_10+xMg_3-x (?0.15 ≤ x ≤ 0.3, oS68-Ca₄Au₁₀In₃) and τ₅-La₄Ag_10.3+xMg_12-x (?0.4 ≤ x ≤ 0.70, oS116–10.33-La₄Ag_10.3Mg₁₂) were confirmed and their homogeneity ranges established. The crystal structures of τ₄-LaAg_4+xMg_2-x (?0.15 ≤ x ≤ 0.88, tI14-YbAl₄Mo₂), τ₄’-LaAg_5+xMg_1-x (?0.1 ≤ x ≤ 0.05, tP14-LaAg₅Mg) and τ₆-La₃Ag_4+xMg_12-x (?1.6 ≤ x ≤ 1.0, hP38-Gd₃Ru₄Al₁₂) were solved and refined by X-ray powder diffraction data analysis. The phases τ₄ and τ₄’ crystallize in two strictly correlated tetragonal structures, body centered and primitive respectively; these two structures were interpreted as structural modifications of the same phase. Two more new ternary phases, τ₃-LaAg_2+xMg_2-x (0 ≤ x ≤ 0.45, oS20-LaAg₂Mg₂) and τ₇-La(Ag_xMg_1-x)₁₂ (0.11 ≤ x ≤ 0.21, tI208-La(Ag_xMg_1-x)₁₂), were also detected in this system.     

New dielectric material system of La(Mg1/2Ti1/2)O3–Ca0.6La0.8/3TiO3 for microwave applications

The microstructure and microwave dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics system with ZnO additions (0.5 wt.%) investigated by the conventional solid-state route have been studied. Doping with ZnO (0.5 wt.%) can effectively promote the densification and the dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics. 0.6La(Mg_1/2Ti_1/2)O₃–0.4Ca_0.6La_0.8/3TiO₃ ceramics with 0.5 wt.% ZnO addition possess a dielectric constant (_r) of 43.6, a Q × f value of 48,000 (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −1 ppm/°C sintering at 1475 °C. As the content of La(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 62,900 (GHz) for x = 0.8 is achieved at the sintering temperature 1475 °C. A parallel-coupled line band-pass filter is designed and simulated using the proposed dielectric to study its performance.     

Structure and Stoichiometry of Mg<Subscript>x</Subscript>Zn<Subscript>y</Subscript> in Hot-Dipped Zn–Mg–Al Coating Layer on Interstitial-Free Steel

Correlations of stoichiometry and phase structure of Mg_xZn_y in hot-dipped Zn–Mg–Al coating layer which were modified by additive element have been established on the bases of diffraction and phase transformation principles. X-ray diffraction (XRD) results showed that Mg_xZn_y in the Zn–Mg–Al coating layers consist of Mg₂Zn₁₁ and MgZn₂. The additive elements had a significant effect on the phase fraction of Mg₂Zn₁₁ while the Mg/Al ratio had a negligible effect. Transmission electron microscope (TEM) assisted selected area electron diffraction (SAED) results of small areas Mg_xZn_y were indexed dominantly as MgZn₂ which have different Mg/Zn stoichiometry between 0.10 and 0.18. It is assumed that the Mg_xZn_y have deviated stoichiometry of the phase structure with additive element. The deviated Mg₂Zn₁₁ phase structure was interpreted as base-centered orthorhombic by applying two theoretical validity: a structure factor rule explained why the base-centered orthorhombic Mg₂Zn₁₁ has less reciprocal lattice reflections in the SAED compared to hexagonal MgZn₂, and a phase transformation model elucidated its reasonable lattice point sharing of the corresponding unit cell during hexagonal MgZn₂ (a, b?=?0.5252 nm, c?=?0.8577 nm) transform to intermediate tetragonal and final base-centered orthorhombic Mg₂Zn₁₁ (a?=?0.8575 nm, b?=?0.8874 nm, c?=?0.8771 nm) in the equilibrium state.     

zA62镁合金中AB_2型金属间化合物的结构稳定性与弹性性能的第一原理计算

采用基于密度泛函理论Castep和Dmol程序软件包, 计算了ZA62镁合金中AB₂型金属间化合物 MgZn₂, Mg₂Sn和MgCu₂的结构稳定性、弹性性能与电子结构. 合金形成热和结合能的计算结果显示:  Mg₂Sn具有最强的合金化形成能力, 而MgCu₂结构最稳定; 体模量(B)、弹性各向异性系数(A)、Young's模量(E)、剪切模量(G)和Poisson比(ν)的计算结果表明: MgZn₂和MgCu₂为延性相, 而Mg₂Sn为脆性相, MgZn₂的塑性最好; 采用弹性常数、体模量和结合能的经验公式计算金属间化合物的熔点, 实验值均在采用弹性常数(±300 K)和体模量(±500 K)计算熔点预测的范围内, 采用弹性常数比采用体模量和结合能预测熔点的平均相对误差小, 其中采用弹性常数计算Mg₂Sn的熔点与对应的实验值十分接近, 相对误差仅为0.31%. 不同温度下热力学性质的计算结果表明, 在298-573 K温度范围内, MgCu₂的Gibbs自由能始终最小, 其结构热稳定性最好, 结构稳定性的强弱顺序并不随温度的升高而消失; 而对MgZn₂和Mg₂Sn, 以475 K为临界, 结构稳定性的强弱顺序随温度的升高发生了变化; 态密度和Mulliken电子占据数的计算结果表明: MgCu₂ 结构最稳定的原因主要在于体系在Fermi能级以下区域成键电子存在强烈的离子键作用.     

Monoclinic phases and stress-relief conditions in (1 − x)Pb(Mg1/3Nb2/3)TiO3–xPbTiO3 solid solutions

A comparative study on heterophase states in perovskite-type solid solutions of (1 − x)Pb(Mg_1/3Nb_2/3)TiO₃–xPbTiO₃ is carried out for compositions near the morphotropic phase boundary. The conditions for mechanical stress relief at elastic matching of phases are analysed at x = const in a wide temperature range. The heterophase states concerned with the presence of the intermediate monoclinic phase are interpreted using the domain state–interface diagrams calculated for x = 0.28, 0.32 and 0.34. It is shown that optimum volume fraction parameters of the domains in the monoclinic phase of the B type are varied in relatively wide ranges and promote complete stress relief with cubic–monoclinic phase coexistence. Two scenarios of stress relief at x = 0.32 are considered in connection with different heterophase states (either tetragonal–monoclinic of the B type or tetragonal–monoclinic of the C type) in a wide temperature range. Possibilities of elastic matching of two polydomain phases (tetragonal–monoclinic of the B type) with almost equal relative widths of the domains in these phases are shown for x = 0.34. The active role of domains of the monoclinic phases in stress relief and forming the planar unstrained interfaces is discussed.     

Thermoelectric properties and electronic structure of p-type Mg2Si and Mg2Si0.6Ge0.4 compounds doped with Ga

Mg₂Si:Ga_x and Mg₂Si_0.6Ge_0.4:Ga_x (x = 0.4% and 0.8%) solid solutions have been synthesized by direct melting in tantalum crucibles and hot pressing. The effect of Ga doping on the thermoelectric properties has also been investigated by measurements of thermopower, electrical resistivity, Hall coefficient and thermal conductivity in temperature range from 300 to 850 K. All samples exhibit a p-type conductivity evidenced by positive sign of both thermopower and Hall coefficient in the investigated temperatures. The maximum value of the dimensionless figure of merit ZT was reached for the Mg₂Si_0.6Ge_0.4:Ga(0.8%) compound at 625 K (ZT ∼ 0.36). The p-type character of thermoelectric behaviours of Ga-doped Mg₂Si and Mg₂Si_0.6Ge_0.4 compounds well corroborates with the results of electronic structure calculations performed by the Korringa-Kohn-Rostoker method and the coherent potential approximation (KKR-CPA), since Ga diluted in Mg₂Si and Mg₂Si_0.6Ge_0.4 (on Si/Ge site) behaves as hole donor due to the Fermi level shifted to the valence band edge. The onset of large peak of DOS from Ga impurity at the valence band edge, well corroborates with high Seebeck coefficient measured in Ga-doped samples.     

Phase relationships in the systems RE2O3-Al2O3-SiO2 (RE = rare earth element, Y, and Sc)

A literature survey and recent results on phase relationships in the quasi-ternary systems RE₂O₃-Al₂O₃-SiO₂ are given. The investigated systems exhibit extended ternary solid solutions, RE_9.33+2x(Si_{1_x}Al_xO₄)₆O₂ (withx up to ~0.33) and/or RE₄Al_{2(1_X)}Si_2xO_9+x (withx up to ~0.3), which are based on the quasi-binary phases RE_9.33(SiO₄)₆O₂ and RE₄A1₂O₉, respectively. The former is encountered only in systems with laige RE³⁺ ions (e.g., La³⁺), whereas the latter is found in systems with small RE³⁺ ions (e.g., Yb³⁺); in systems with medium-sized KE³⁺ ions (e.g., Gd³⁺) both types exist Quasi-ternary compounds are known only in the La, Ce, and Sc systems. Severe discrepancies in reported ternary eutectic temperatures led to a need for their accurate redeteimination.     

An investigation on hydrogen storage kinetics of nanocrystalline and amorphous Mg2Ni1−xCox (x = 0-0.4) alloy prepared by melt spinning

In order to improve the hydrogen storage kinetics of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Co, and melt-spinning technology was used for the preparation of the Mg₂Ni_1−xCo_x (x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys. The structures of the as-cast and spun alloys are characterized by XRD, SEM and TEM. The hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage kinetics of the as-spun alloys is tested by an automatic galvanostatic system. The hydrogen diffusion coefficients in the alloys are calculated by virtue of potential-step method. The electrochemical impedance spectrums (EIS) and the Tafel polarization curves are plotted by an electrochemical workstation. The results show that the substitution of Co for Ni notably enhances the glass forming ability of the Mg₂Ni-type alloy. Furthermore, the substitution of Co for Ni, instead of changing major phase Mg₂Ni, leads to forming secondary phases MgCo₂ and Mg. Both the melt spinning treatment and Co substitution significantly improve the hydrogen absorption and desorption kinetics. The high rate discharge ability, the hydrogen diffusion coefficient and the limiting current density of the alloys significantly increase with raising both the spinning rate and the amount of Co substitution.     

Hot tearing behaviors and in-situ thermal analysis of Mg-7Zn-xCu-0.6Zr alloys

Thermal analysis was used to investigate the microstructural evolution of Mg-7Zn-xCu-0.6Zr alloys during solidification. The effect of Cu content (0, 1, 2 and 3, mass fraction, %) on the hot tearing behavior of the Mg-7Zn-xCu-0.6Zr alloys was investigated with a constrained rod casting (CRC) apparatus, equipped with a load sensor and a data acquisition system. The thermal analysis results of Mg-7Zn-xCu-0.6Zr alloy revealed that the alloy consisted of two distinct phases: α-Mg and MgZn₂. Three distinct peaks were observed in the alloys with Cu addition, which were identified as α-Mg, MgZnCu and MgZn₂. In addition, the reaction temperature of α-Mg decreased and the reaction temperatures of MgZn₂ and MgZnCu increased as the Cu content increased. The experimental results of hot tearing demonstrated that the addition of Cu significantly reduced the hot tearing susceptibility (HTS) of Mg-7Zn-xCu-0.6Zr alloys due to the higher eutectic temperature and the shorter solidification temperature region.     

Solidification pathways and hot tearing susceptibility of MgZnxY4Zr0.5 alloys

Hot tearing is known as one of the most serious solidification defects commonly encountered during solidification. It is very important to study the solidification path of alloys. In the work, thermal analysis with cooling curve was used for the investigation of microstructure evolution with different Zn contents during solidification process of MgZn_xY_4Zr_(0.5) alloys. Thermal analysis results of MgY_4Zr_(0.5) alloys revealed one distinct phase precipitation: α-Mg. Three different phase peaks were detected in the Zn-containing alloys: α-Mg, Z-phase(Mg_(12)YZn) and W-phase(Mg_3 Y_2Zn_3). In addition, for the present MgZn_xY_4Zr_(0.5) alloys, the freezing ranges of these alloys from large to small were: MgZn_(1.5)Y_4Zr_(0.5)MgZn)(3.0) Y)4Zr_(0.5)MgZn0.5 Y4 Zr0.5MgY_4Zr_(0.5). The effect of different contents of Zn(0, 0.5, 1.5, 3.0 wt.%) on hot tearing behavior of MgY_4Zr_(0.5) alloy was investigated using a constrained rod casting(CRC) apparatus equipped with a load cell and data acquisition system. The experimental results show that the addition of Zn element significantly increases hot tearing susceptibility(HTS) of the MgY_4Zr_(0.5) alloy due to its extended freezing range. Some free dendrite-like bumps and ruptured liquid films on the fracture surfaces were observed in all the fracture surfaces. These phenomena proved the fact that the hot tearing formation was caused by interdendritic separation due to lack of feeding at the end of solidification.     

Growth and annealing properties of Mg0.4Al2.4O4 crystal

Mg_0.4Al_2.4O₄ single crystals with good optical quality were successfully grown by the Czochralski method. The transmission spectrum indicated that the absorption edge of the crystal was at 220 nm, while no apparent absorption peaks were found. The X-ray diffraction and DSC curve analysis showed that Mg_0.4Al_2.4O₄ crystal was stable at room temperature. While after annealing in the air and hydrogen atmosphere at about 1200 °C, Mg_0.4Al_2.4O₄ decomposed into Al₂O₃ and (MgO)_0.4(Al₂O₃)_x (0.4 < x < 1.2). The reaction mainly occurred on the crystal surface, barely inside.     

Thermoelectric performance of Mg2−xCaxSi compounds

Thermoelectric materials Mg_2−xCa_xSi (x = 0, 0.01, 0.03, 0.05, 0.07, 0.1) compounds have been prepared by vacuum melting followed by hot-pressing. Effects of the substitution of Ca for Mg on phase structures and the thermoelectric properties of the hot-pressed compounds were investigated. It was found that the alloying of Ca in Mg₂Si based compounds increases the electrical conductivity and decreases the Seebeck coefficient of the compounds, due to the electronegativity difference between Ca and Mg. The dimensionless figures of merit of Mg₂Si and Mg_1.99Ca_0.01Si reach, respectively, 0.41 and 0.34 at 660 K.