The influence of the cooling rate on bulk metallic glass formation in Mg<Subscript>80</Subscript>Cu<Subscript>15</Subscript>Y<Subscript>5</Subscript> and Mg<Subscript>80</Subscript>Cu<Subscript>10</Subscript>Y<Subscript>10</Subscript>

The Mg–Cu–Y system is known to be one of the best glass formers among the various existing magnesium alloys. The compositions chosen for the current study were Mg₈₀Cu₁₅Y₅ and Mg₈₀Cu₁₀Y₁₀. Different casting processes yielded four different microstructures that were analyzed by means of X-ray diffraction, scanning electron microscopy, high resolution scanning electron microscopy, and energy-dispersive X-ray spectroscopy chemical analysis. The different casting procedures were gravity castings of 3 mm diameter specimens into a copper mold held at different temperatures (cooled to −195 °C with the aid of liquid nitrogen, held at room temperature and heated up to 300 °C) and melt-spinning. Detailed microstructure study was then performed on the melt-spun specimen using transmission electron microscopy and high resolution transmission electron microscopy. The above-mentioned investigation revealed a crystalline rather than amorphous structure. The observed microstructure could not be explained on the basis of current models referring to the frequency of nucleation events.     

Local amorphization of atomic structure in Mg<Subscript>0.54</Subscript>Zn<Subscript>0.46</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> ferrites

The structure of Mg_0.54Zn_0.46Fe₂O₄ ferrites has been studied using x-ray diffraction techniques. It is established that high-temperature annealing (T = 1280°C, τ = 0.5–8.0 h) leads to a change in the crystal structure of samples, which is accompanied by their local amorphization.     

Crystallization and thermally induced surface relief effects in a Mg<Subscript>65</Subscript>Cu<Subscript>25</Subscript>Y<Subscript>10</Subscript> bulk metallic glass

The crystallization behaviour of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass (BMG) under different reheating conditions was investigated. X-ray diffraction spectrometery (XRD), differential scanning calorimetery (DSC), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to examine the crystallization of different samples and the surface relief generated on as-polished surfaces during heat treatment. Different phase constituents were found in samples that experienced different reheating stages. It is proposed that both the reheating temperature and holding time have a significant effect on the phase constituents. The BMG was found to generate surface corrugations of amplitude 1–2 μm during annealing above its crystallization temperature. Such thermally induced surface relief effects are probably a result of the development of surface stresses generated by volumetric changes associated with crystallization of the residual amorphous phase.     

Dielectric responses in Mg<Subscript>1/3</Subscript>Ta<Subscript>2/3</Subscript>-replaced Pb[(Zn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>),Ti]O<Subscript>3</Subscript> ceramics

Octahedral lattice sites of Pb[(Zn_1/3Nb_2/3),Ti]O₃ were replaced by 20 at.% Mg_1/3Ta_2/3 complex to enhance perovskite development, especially at Pb(Zn_1/3Nb_2/3)O₃-rich compositions. Resultant changes in the perovskite formation and associated dielectric responses were investigated. A perovskite structure was identified at Pb(Zn_1/3Nb_2/3)O₃-rich compositions by X-ray diffraction, although the development was rather incomplete. Phase transition modes in the dielectric constant spectra changed from diffuse to sharp ones, regardless of the introduction of Mg_1/3Ta_2/3. Dielectric maximum temperatures of the ceramics shifted linearly with the compositional change.     

Elastic properties of Li<Subscript>2</Subscript>Zn<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The complete elastic modulus matrix of Li₂Zn₂(MoO₄)₃ single crystals has been measured for the first time. The sound velocity has been measured in different directions of the crystals by a pulse-phase method. The measurement results have been used to calculate elastic moduli. The sound velocity has been calculated in the three main crystallographic planes of the crystals.     

Far infrared reflectance of sintered Zn<Subscript>2</Subscript>TiO<Subscript>4</Subscript>

Zinc-titanate ceramics were obtained by initial mechanical activation in a high-energy planetary mill for 15 min followed by sintering at temperatures 900–1100 °C for 2 h. Room temperature far infrared reflectivity spectra for all samples were measured in the range 100–1200 cm⁻¹. The same ionic oscillators were present in the measured spectra, but their intensities increased with the sintering temperature in correlation with the increase in sample density and microstructure changes. Optical parameters were determined for seven oscillators belonging to the spinel structure using the four-parameter model of coupled oscillators. Born effective charges were calculated from the transversal/longitudinal splitting.     

Sintering behavior and thermal property of Mg<Subscript>2</Subscript>SnO<Subscript>4</Subscript>

The sintering behavior of Magnesium Orthostannate (Mg₂SnO₄) by Magnesium oxide (MgO) and tin oxide (SnO₂) was investigated. Mg₂SnO₄ compound was formed by traditional solid state reaction (SSR) at elevated temperatures over a range of 600 C–1300 C. X-ray studies have revealed that the resulting, as-fired, compound is polycrystalline composed of an inverse spinel-structure grains separated partially by porosity. However it failed to detect any phase change as a result of annealing. Scanning electron microscope (SEM) was employed to follow-up any change in the morphology throughout the sintering and reduction processes. Thermal property Differential Scanning Calorimetry (DSC) revealed that reduction takes place at a temperature between 400 and 600 C, depending on the concentration of H₂ in the atmosphere in accordance with the X-ray studies. The technique employed has also demonstrated the stability of reduced species in typical atmospheres and working conditions.     

Phase developments in Pb(Mg<Subscript>1/2</Subscript>W<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> and Pb(Zn<Subscript>1/2</Subscript>W<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> via B-site precursor route

Phase formation stages of MgWO₄ and ZnWO₄ (precursor compositions for following steps) were investigated by monitoring the reactions of oxide chemicals at various temperatures. Developed phases were examined by using X-ray diffraction (XRD). Successive attempts were also conducted for Pb(Mg_1/2W_1/2)O₃ (PMW) and Pb(Zn_1/2W_1/2)O₃ (PZW) by reacting PbO with the precursor compounds. Stages of phase development in the two compositions were also analyzed. The results are compared with those of another tungsten-containing perovskite Pb(Fe_2/3W_1/3)O₃ (PFW) and its B-site precursor Fe₂WO₆. After PbO addition to the precursor powders, a perovskite phase formed directly (i.e., without any intermediate phases) in the case of PMW. For PbO + ½ZnWO₄, in contrast, the decomposition of ZnWO₄ and preferential reaction with PbO resulted in Pb₂WO₅ and ZnO, instead of the perovskite PZW.     

High-temperature deformation and crystallization behavior of a Cu<Subscript>36</Subscript>Zr<Subscript>48</Subscript>Al<Subscript>8</Subscript>Ag<Subscript>8</Subscript> bulk metallic glass

The influence of annealing on the crystallization behavior of a Cu₃₆Zr₄₈Al₈Ag₈ (at.%) bulk metallic glass (BMG) was investigated. In both isochronal and isothermal annealing processes, the effective activation energies of the primary crystallizations were obtained as 295.8 ± 13.4 and 302.7 ± 14.5 kJ/mol by applying the Kissinger and Ozawa methods, respectively. Using the isothermal transformation kinetics described by the Johnson–Mehl–Avrami model, the Avrami exponent n was found to range between 2.56 and 3.25, which indicates that the primary crystallization behavior was three-dimensional diffusion-controlled growth with an increasing nucleation rate. The high-temperature deformation behavior of a Cu₃₆Zr₄₈Al₈Ag₈ BMG was then investigated by performing a series of compression tests after rapid heating within a supercooled liquid region. It was found that at least 14–17 dense randomly packed atoms are necessary to produce a unit local flow when the present BMG is subjected to non-Newtonian homogeneous deformation, as described by the transition state equation. Deformation and processing maps were also constructed based on the dynamic materials model to predict optimum bulk formability in a Cu₃₆Zr₄₈Al₈Ag₈ BMG taking warm deformation-induced crystallization within a supercooled liquid into account.     

Hydriding of Mg<Subscript>2</Subscript>Ni in ammonia

The chemical interaction between the intermetallic compound Mg₂Ni and ammonia in the presence of NH₄Cl as an activator is investigated at temperatures from 100 to 450 °C, and the reaction scheme is presented. The results demonstrate that the use of ammonia for hydriding/nitriding the intermetallic compound makes it possible to prepare various magnesium compounds (Mg₂NiH₄, Mg₃N₂, and Mg (NH₂)₂) in a highly dispersed state.     

Crystal structure,phonon characteristic,and intrinsic properties of Sm(Mg<Subscript>1/2</Subscript>Sn<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> double perovskite ceramic

Sm(Mg_1/2Sn_1/2)O₃(SMS) microwave dielectric ceramic was prepared by the traditional solid state synthesis method. The samples were tested by X-ray diffraction (XRD), vibrational spectra (Raman scattering and far-infrared reflectance spectroscopy). The Sm(Mg_1/2Sn_1/2)O₃ perovskite is monoclinic with the space group P2₁/n proved by XRD. Raman vibrational modes were fitted and assigned by Lorentz function. To calculate the intrinsic properties, far-infrared spectra with seven active modes were fitted using a four-parameter semi-quantitative (FPSQ) model. The dielectric constant calculated according to FPSQ is similar to that obtained from the molecular polarization and the Clausius equation. The intrinsic loss has been determined by the relationship between the damping factor and the spectral center frequency, which is slightly larger than the value obtained in the four-parameter fitting, and indicates that A_1g(Sm) Raman mode has a enormous effect on the dielectric loss. F_3u ⁽³⁾ and F_4u ⁽⁴⁾ have the greatest contribution to the dielectric constant and loss. Finally, the real and imaginary parts of permittivity were analyzed with Kramers-Krönig transformation.     

Synthesis of Mg<Subscript>2</Subscript>Ni nanoparticles in a KCl-NaCl-MgCl<Subscript>2</Subscript> melt

The reaction between magnesium and nickel powders in a KCl-NaCl-MgCl₂ ionic melt at 970 K (reaction time, 5 h) has been studied by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray microanalysis, and chemical analysis. According to scanning electron microscopy data, the synthesized Mg₂Ni powder consists of particles 70–75 nm in size, in reasonable agreement with the equivalent particle diameter, ≃ 64 nm, determined from the specific surface area of the Mg₂Ni powder and with the crystallite size, D _hkl ≃ 65 nm, evaluated from X-ray diffraction data. The hydrogen sorption properties of the Mg₂Ni obtained in a KCl-NaCl-MgCl₂ ionic melt are identical to those of Mg₂Ni powder prepared by a standard method, but the former reacts with hydrogen far more rapidly.     

XPS study of the initial oxidation of the bulk metallic glass Zr<Subscript>46.75</Subscript>Ti<Subscript>8.25</Subscript>Cu<Subscript>7.5</Subscript>Ni<Subscript>10</Subscript>Be<Subscript>27.5</Subscript>

The surface oxidation behaviour of the bulk metallic glass Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 was investigated in situ by using X-ray photoelectron spectroscopy (XPS). The initial stages of oxidation at room temperature were studied by exposing the clean alloy specimen surface to varying doses of pure oxygen (up to 1,000 L) in an UHV chamber. Progressive oxidation of Zr, Be and Ti was observed with increasing doses, the major species in the oxide layer being Zr(IV) and Be(II) possibly existing as ZrO₂, BeO, while Cu and Ni remained in their elemental forms. High temperature in situ oxidation in the temperature range 423–653 K for a fixed oxygen dose of 300 L was also investigated. Oxidation of Be was observed at all temperatures, while a sharp decrease in the oxidation of Zr and Ti was observed for temperatures at 573 K and above. The results show a preferential oxidation of Be and Zr at room temperature, while at higher temperatures oxidation is controlled by the reduction of oxides of Zr and Ti and the diffusion of oxygen into the alloy bulk. The role of the dissolved carbon impurity in the reduction of the oxides is discussed.     

Magnesiothermic Reduction of Mg<Subscript>4</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> in the Combustion Regime

We have studied the magnesium reduction of the Mg₄Ta₂O₉ tantalate in the combustion regime and assessed the influence of starting mixture parameters on the combustion speed and temperature, the degree of reduction, and characteristics of the resultant tantalum powders. The use of Mg₄Ta₂O₉ as a precursor for the reduction process has made it possible to increase the degree of reduction and the specific surface area of tantalum powders. We have obtained powders with a specific surface area of up to 21 m²/g, which is a factor of 4–5 larger than in the case of the reduction of Ta₂O₅ under the same conditions.     

Chemical interaction of the Ti<Subscript>90</Subscript>Mg<Subscript>10</Subscript> alloy with ammonia

We have studied the chemical interaction of the Ti₉₀Mg₁₀ alloy with ammonia in the presence of NH₄Cl at temperatures from 150 to 500°C and identified conditions for the formation of fine-particle hydrides and nanocrystalline nitrides of titanium and magnesium.     

Synthesis and conductivity of Zn<Subscript>2</Subscript>SnO<Subscript>4</Subscript>-based cermet material

SHS synthesis of Zn₂SnO₄-based cermet material from Zn + NiO + SnO₂ compacted powder mixtures has been studied. The final product is obtained as a monolithic cylindrical block composed of a ZnO-based outer layer and Zn₂SnO₄-based central part, in which the metal phase is distributed. The phase composition and microstructure of combustion products have been studied by x-ray phase analysis (XPA), electron microscopy, and microprobe techniques. It is established that the structure of the obtained cermet material has a dramatic effect on their conductivity.     

Fabrication and mechanical properties of WC particulate reinforced Cu<Subscript>47</Subscript>Ti<Subscript>33</Subscript>Zr<Subscript>11</Subscript>Ni<Subscript>6</Subscript>Sn<Subscript>2</Subscript>Si<Subscript>1</Subscript> bulk metallic glass matrix composites

The mechanical behavior of the WC particulate (WC_p) reinforced Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁ bulk metallic glass (BMG) matrix composites has been examined. The mechanical properties are improved with increasing WC_p content up to 20 wt%. The ultimate compression strength and plastic strain of the composite containing 20 wt% WC_p are 2.4 GPa and 2.4%, while those of the monolithic BMG are 1.6 GPa and ∼0%, respectively. The multiple shear band formation and crack deflections through WC particles have been identified as the main mechanism for the improved toughness.