Microstructure and Thermal Properties of Atmospheric Plasma-Sprayed Yb<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> Coating

In the present work, Yb₂Si₂O₇ powder was synthesized by solid-state reaction using Yb₂O₃ and SiO₂ powders as starting materials. Atmospheric plasma spray technique was applied to fabricate Yb₂Si₂O₇ coating. The phase composition and microstructure of the coating were characterized. The density, open porosity and Vickers hardness of the coating were investigated. Its thermal stability was evaluated by thermogravimetry and differential thermal analysis (TG-DTA). The thermal diffusivity and thermal conductivity of the coating were measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb₂Si₂O₇ with amorphous phase. The coating had a dense structure containing defects, such as pores, interfaces and microcracks. The TG-DTA results showed that there was almost no mass change from room temperature to 1200 °C, while a sharp exothermic peak appeared at around 1038 °C in DTA curve, which indicated that the amorphous phase crystallized. The thermal conductivity of the coating decreased with rise in temperature up to 600 °C and then followed by an increase at higher temperatures. The minimum value of the thermal conductivity of the Yb₂Si₂O₇ coating was about 0.68 W/(m K).     

Thermally induced crystallization of Fe73.5Cu1Nb3Si15.5B7 amorphous alloy

Thermally induced crystallization of Fe_73.5Cu₁Nb₃Si_15.5B₇ amorphous alloy occurs in two well-separated stages: the first, around 475 °C, corresponds to formation of α-Fe(Si)/Fe₃Si and Fe₂B phases from the amorphous matrix, while the second, around 625 °C, corresponds to formation of Fe₁₆Nb₆Si₇ and Fe₂Si phases out of the already formed α-Fe(Si)/Fe₃Si phase. Mössbauer spectroscopy suggests that the initial crystallization occurs through formation of several intermediate phases leading to the formation of stable α-Fe(Si)/Fe₃Si and Fe₂B phases, as well as formation of smaller amounts of Fe₁₆Nb₆Si₇ phase. X-ray diffraction (XRD) and electron microscopy suggest that the presence of Cu and Nb, as well as relatively high Si content in the as-prepared alloy causes inhibition of crystal growth at annealing temperatures below 625 °C, meaning that coalescence of smaller crystalline grains is the principal mechanism of crystal growth at higher annealing temperatures. The second stage of crystallization, at higher temperatures, is characterized by appearance of Fe₂Si phase and a significant increase in phase content of Fe₁₆Nb₆Si₇ phase. Kinetic and thermodynamic parameters for individual steps of crystallization suggest that the steps which occur in the same temperature region share some similarities in mechanism. This is further supported by investigation of dimensionality of crystal growth of individual phases, using both Matusita–Sakka method of analysis of DSC data and texture analysis using XRD data.     

Fabrication of Y2Si2O7 coating and its oxidation protection for C/SiC composites

Yttrium silicate (Y₂Si₂O₇) coating was fabricated on C/SiC composites through dip-coating with silicone resin + Y₂O₃ powder slurry as raw materials. The synthesis, microstructure and oxidation resistance and the anti-oxidation mechanism of Y₂Si₂O₇ coating were in–estigated. Y₂Si₂O₇ can be synthesized by the pyrolysis of Y₂O₃ powder filled silicone resin at mass ratio of 54.2:45.8 and 800 °C in air and then heat treated at 1400 °C under Ar. The as-fabricated coating shows high density and fa–orable bonding to C/SiC composites. After oxidation in air at 1400, 1500 and 1600 °C for 30 min, the coating-containing composites possess 130%–140% of original flexural strength. The desirable thermal stability and the further densification of coating during oxidation are responsible for the excellent oxidation resistance. In addition, the formation of eutectic Y–Si–Al–O glassy phase between Y₂Si₂O₇ and Al₂O₃ sample bracket at 1500 °C is disco–ered.     

Phase transformations and phase equilibria in a Ti–37% Al–20% Mn alloy

The solid-state phase transformations and phase equilibria in a Ti–37 at% Al–20 at% Mn alloy have been investigated. The alloy was prepared by plasma-arc melting and the microstructures of high-temperature-annealed and water-quenched or furnace-cooled samples were studied. The results show that at 1235°C β-phase and (Mn,Al)₂Ti (Laves phase) are present whereas below 1000°C the phases (γ+α/α₂+(Mn,Al)₂Ti) are formed. A comparison between calculated equilibrium phase compositions and values measured by EDXA shows reasonable agreement for the β, γ and α/α₂ phases over a range of temperatures but agreement for the (Mn,Al)₂Ti phase is less good, particularly at the higher temperatures. The transformation kinetics in this system appear to be sluggish and true equilibrium does not appear to have been achieved in all samples annealed below 1145°C. DTA analysis was also undertaken and the heating thermogram obtained is interpreted with the aid of the calculated phase equilibria.     

Tetragonal-cubic phase boundary in nanocrystalline ZrO2-Y2O3 solid solutions synthesized by gel-combustion

By means of synchrotron X-ray powder diffraction (SXPD) and Raman spectroscopy, we have detected, in a series of nanocrystalline and compositionally homogeneous ZrO₂-Y₂O₃ solid solutions, the presence at room temperature of three different phases depending on Y₂O₃ content, namely two tetragonal forms and the cubic phase. The studied materials, with average crystallite sizes within the range 7-10 nm, were synthesized by a nitrate-citrate gel-combustion process. The crystal structure of these phases was also investigated by SXPD. The results presented here indicate that the studied nanocrystalline ZrO₂-Y₂O₃ solid solutions exhibit the same phases reported in the literature for compositionally homogeneous materials containing larger (micro)crystals. The compositional boundaries between both tetragonal forms and between tetragonal and cubic phases were also determined.     

Mechanical properties and atomistic deformation mechanism of γ-Y2Si2O7 from first-principles investigations

The theoretical mechanical properties and atomistic shear deformation mechanisms of γ-Y₂Si₂O₇, one of the most refractory silicates and potentially useful as a high-temperature structural ceramic, were investigated using first-principles calculations. The material shows low shear moduli to bulk modulus ratios, as well as a low ideal shear strength to tensile strength ratio. The unusual low shear deformation resistance of γ-Y₂Si₂O₇ originates from the inhomogeneous strength of its chemical bonds. The Y–O bond is weaker and readily stretches and shrinks; and Si–O bond is stronger and more rigid. The relative softer YO₆ octahedron positively accommodates shear deformation by structural distortion, while the Si₂O₇ pyrosilicate unit is more resistant to deformation. The reported shear-load-bearing mechanism is quite similar to those found in the “quasi-ductile” LaPO₄ monazite and ternary layered carbides (the so-called MAX phases), and can endow γ-Y₂Si₂O₇ with quasi-ductility and damage tolerance.     

Properties of nickel-cobalt composite silicides by thermal annealing of Ni<Subscript>1−x</Subscript>Co<Subscript>x</Subscript> (x=0.2, 0.5, and 0.8) alloy thin films on silicon and polysilicon substrates

10 nm-Ni_1−xCo_x (x=0.2, 0.5, and 0.8)/p-Si(100)(or poly crystalline Si) was thermally annealed using rapid thermal annealing for 40 s at 600–1100°C. The annealed film structures developed into NiCoSi_x, and the resulting changes in sheet resistance, microstructure, and composition were investigated using a four-point probe, a scanning electron microscope, a field ion beam, an X-ray diffractometer, and an Auger electron spectroscope. The final thickness of NiCoSi_x formed on single-crystal silicon was approximately 12.64nm, and it maintained its sheet resistance below 20 Ω/sq. during the silicidation annealing at 1100°C. The NiCoSi_x formed on polysilicon had a thickness of 35.04nm, and its low resistance was maintained up to 900°C. Additional annealing of silicides at the given RTA temperature for 30 min resulted in a drastic increase in sheet resistance. We identified Ni₃Si₂ and a NiSi phase at 700°C and 1000°C for single-crystal silicon substrates. Moreover, Ni₃Si₂, NiSi, and CoSi₂ phases were stable at 700°C, and then NiSi₂ and Ni₃Si₂ became stable for polycrystalline silicon substrates at 1000°C. When the amount of Co was 80%, only a Ni₃Si₂ phase was confirmed at 700°C and 1000°C in both the single and polycrystalline substrates. With less Co (Co=0.2, 0.5), Ni₃Si₂, NiSi, and CoSi₂ phases were observed at 700, and Ni₃Si₂ and NiSi₂ phases were observed at 1000°C. Cobalt also improved thermal stability of the silicides formed on the polysilicon gate, but this enhancement was lessened due to the silicon mixing during high temperature diffusion. In conclusion, the proposed nickel cobalt composite silicides formed from the nano-thick alloy films may be superior to conventional nickel monosilicides due to improved thermal stability.     

700 °C Isothermal Section of the Al-Ti-Si Ternary Phase Diagram

The 700 °C isothermal section of the Al-Ti-Si ternary phase diagram has been determined experimentally by means of scanning electron microscopy coupled with energy dispersive x-ray spectroscopy and x-ray powder diffraction. Fourteen three-phase regions have been determined experimentally in the isothermal section at 700 °C. The ternary phases τ₁ (I4₁/amd, Zr₃Al₄Si₅-type) and τ₂ (Cmcm, ZrSi₂-type) are confirmed in the system at 700 °C. The compositions of τ₁ and τ₂ are found as Al_6.2-9.3Ti_32.4-34.0Si_57.5-60.9 and Al_10.0-11.6Ti_34.2-34.5Si_53.9-55.6, respectively. The τ₃ and Ti₃Al₅ phases are not found in the section. The Ti-rich corner at 700 °C shows the presence of three three-phase equilibriums, i.e., (TiAl + Ti₃Al + Ti₅Si₃), (α-Ti + Ti₃Si + Ti₅Si₃) and (α-Ti + Ti₃Al + Ti₅Si₃). The maximum solubility of Al in Ti₅Si₃, Ti₃Si and α-Ti is 6.0, 1.5 and 13.9 at.% at 700 °C, respectively. The maximum solubility of Si in L-Al, TiAl₃, TiAl₂, TiAl, Ti₃Al and α-Ti is 24.1, 13.6, 1.5, 0.8, 2.3 and 2.3 at.%, respectively.     

Amorphization by dislocation accumulation in shear bands

Microcrystalline γ-Y₂Si₂O₇ was indented at room temperature and the deformation microstructure was investigated by transmission electron microscopy in the vicinity of the indent. The volume directly beneath the indent comprises nanometer-sized grains delimited by an amorphous phase while dislocations dominate in the periphery either as dense slip bands in the border of the indent or, further away, as individual dislocations. The amorphous layers and the slip bands are a few nanometers thick. They lie along well-defined crystallographic planes. The microstructural organization is consistent with a stress-induced amorphization process whereby, under severe mechanical conditions, the crystal to amorphous transformation is mediated by slip bands containing a high density of dislocations. It is suggested that the damage tolerance of γ-Y₂Si₂O₇, which is exceptional for a ceramic material, benefits from this transformation.     

Phase diagram of the Ba2YCu3O6-Ba2YCu3O7 system below 400°C

X-ray diffraction analysis and transmission electron microscopy have been used to study the low-temperature decomposition of the nonstoichiometric (in oxygen) HTSC compound Ba₂YCu₃O_{7 ? δ}. The phase diagram of the Ba₂YCu₃O₆-Ba₂YCu₃O₇ system below ≤400°C has been constructed. The temperature range corresponding to phase separation has been found to be divided into two portions. At T > 250°C, two orthorhombic phases characterized by different oxygen contents are formed; at the higher temperatures, the phase separation of the compound into a tetragonal and an orthorhombic phase takes place. The separation was also found to observe at T = 100°C; this indicates the possibility of natural aging for the Ba₂YCu₃O_{7 ? δ} compound at room temperature.     

Effect of Convection Gas on the Synthesis of Nanophase Tin Oxides During a Gas Condensation Method

Tin oxide powders of nanometer size have been synthesized by a gas condensation method using helium or a mixture of oxygen and helium as the convection gas. Changes in the average size, morphology, and crystal phases were investigated during heat treatment at temperatures between 350°C and 720°C in the air. Spherical tin oxide powders of 15 nm in average diameter were synthesized in a helium atmosphere, which was composed of Sn, SnO, and Sn₂O₃ phases. After annealing at 720°C, these multiphase particles transformed to a single SnO₂ phase and became an irregular shape of about 50 nm in diameter. This rapid coarsening was attributed to fast mass transfer among particles. The spherical SnO₂ powder of 7 nm in average diameter was directly synthesized using a gas mixture of oxygen and helium. Upon annealing up to 720°C, morphological changes were barely observed in the powder synthesized using a convection atmosphere containing oxygen.     

The oxidation behavior of a novel Ni-free Zr–Cu-based bulk metallic glass composite in the supercooled liquid and crystallization states

The oxidation behavior of a novel Ni-free (Zr₄₈Cu₃₂Al₈Ag₈,Ta₄)Si_0.75 bulk metallic glass composite (BMG-C) in dry air in the supercooled liquid state (SLS at 430 °C and 480 °C) and the crystallization state (CS at 520 °C and 560 °C) for 100 h were studied herein.Test results showed that the oxidation kinetics of the BMG-C in the SLS and CS followed a multi-stage oxidation rate law. The scales forming in both the SLS and CS consist of t-ZrO₂, and m-ZrO₂, CuO and Ag. In the CS, additional Al₂O₃ was observed. In the substrate area, Cu₁₀Zr₇ crystalline likely formed in the amorphous substrate in the SLS. In the CS, more crystallization phases were found in the substrate, including additional CuZr₂ observed at temperatures ≥520 °C; AlCu₂Zr was observed at 560 °C.The Ta-reinforced phase in the BMG-C was more likely to react with Si in the scales forming Ta₂Si at temperatures ≥480 °C, which resulted in cracks in the Ta. Furthermore, channels between Ta precipitates and the matrix might exist, facilitating oxygen diffusivity. As the oxidation temperature (e.g., CS) and test duration were increased, the effects of the cracks and the channels became more significant and were responsible for the fast-growth oxidation in the final stage of the test.     

Evolution of precipitates in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr alloy with fine plate-like 14H-LPSO structures aged at 240 °C

The morphology and crystal structure of the precipitates in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr (wt.%) alloy with fine plate-like 14H-LPSO structures aged at 240 °C were investigated using transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Fine plate-like 14H-LPSO structures precipitate after heat treatment at 500 °C for 2 h, and β-type phases precipitate after the alloy is aged at 240 °C. The long-period atomic stacking sequence of 14H-LPSO structures along the [0001]_α direction is ABABCACACACBABA. After being aged at 240 °C for 2 h, the β-type phases are the ordered solution clusters, zig-zag GP zones, and a small number of β′ phases. The peak hardness is obtained at 240 °C for 18 h with a Brinell hardness of 112, the β-type phases are β’ phases and local RE-rich structures. After being aged at 240 °C for 100 h, the β-type phases are β’, β₁ and β’_F phases. β′ phases nucleate from the zig-zag GP zones directly without β″ phases, and then transform into β₁ phase by β’→β’_F→β₁ transformations. The Zn not only can form LPSO structure, but also is the constituent element of β₁ phases. LPSO structures have a certain hindrance to the coarsening of β’ and β₁ along 〈0001〉_α.     

A study on microstructure development and oxidation phenomenon of arc consolidated Mo-Nb-Si-(Y) alloys

Two alloys of compositions Mo-26Nb-19Si and Mo-26Nb-19Si-0.5Y (at.%) were prepared by non-consumable arc melting. The alloys characterized using BSE, EDS and EBSD techniques confirmed the formation of two phase (Mo, Nb)_SS-(Mo, Nb)₅Si₃ microstructures. Y-rich (Y₂O₃) particles were mainly found to be present at the inter-phase boundaries creating different phase morphologies in Mo-26Nb-19Si-0.5Y alloy. Both the phases showed a preferred orientation indicating a directional growth during solidification. Oxidation tests conducted in air at 1000 and 1300 °C showed a porous oxide scale formation on the alloy surface and loss of material due to evaporation of MoO₃. The detailed SEM-EDS analysis of the oxide scale formed on these alloys in argon environment at 1000 °C, indicated the presence of Nb₂O₅ particles in the silica (SiO₂) matrix.     

In situ synthesis and phase analysis of low density O＇-sialon-based multiphase ceramics

In situ formed low density O'-sialon-based multiphase ceramics were prepared by liquid-phase sintering method at 1400°C with Si3N4, SiO2 and Al2O3 as raw materials.Crystalline phases were identified by X-ray diffraction(XRD).The quantitative phase analysis was finished by matrix-flushing method and the substitution parameter x value of O'-sialon was estimated.The effects of sintering additives on the phase composition of the material were studied.The results show that, when using Y2O3 alone, Al6Si2O13 phase can be formed in the material, but when using Y2O3 and MgO, MgAl2O4 phase can be preferentially formed and the Al6Si2O13 is not observed.The mechanical properties of the material were measured and the relationships between microstructure and mechanical properties were discussed.The sample with Y2O3 and MgO sintering additives, using fused quartz alone as SiO2 source, displays a combination of high bending strength(163 MPa) and good fracture toughness(3.11 MPa·m1/2).Bending strength and fracture toughness of the samples increase with the increase of the content and aspect ratio of elongated grains and decrease with the increase of the porosity.     

Effects of P2O5 and sintering temperature on microstructure and mechanical properties of lithium disilicate glass-ceramics

This paper investigates the effect of P₂O₅ content and sintering temperature on the microstructure of lithium disilicate glass-ceramics and analyzes the relation between microstructure and mechanical properties. Specimens were prepared by hot-pressing technology, adding 1.0–4.0 mol% P₂O₅ (G1P, G2P, G3P and G4P). Lithium disilicate (Li₂Si₂O₅), lithium metasilicate (Li₂SiO₃) and lithium orthophosphate (Li₃PO₄) crystalline phases were detected. The morphology of Li₂Si₂O₅ crystals transformed from rod-shaped to needle-like and the mechanical properties inevitably decreased. Additionally, G1P and G2P were prepared by hot-pressing technology with sintering temperatures ranging 780–840 °C. The results showed that sintering temperature had no significant effect on the crystalline phases and microstructure. The optimal mechanical performances (flexural strength 290 MPa; fracture toughness 3.3 MPa m^1/2) were obtained with 1 mol% P₂O₅ and a sintering temperature of 820 °C.     

Fabrication and performance evaluation of metal bond diamond tools based on aluminothermic reaction

Herein, the fabrication of metal bond diamond tools is proposed by using the Fe₂O₃-Al aluminothermic reaction. Moreover, the influence of sintering temperature and TiH₂- and Si-doping on phase composition and mechanical properties of the reactive sintered bond is investigated. Furthermore, the grinding performance of metal bond diamond tool on ceramic tile is also examined. At 930 °C, the sintered bond is composed of Fe, Al₂O₃, Fe₃O₄ and FeO phases. However, the aluminothermic reaction initiated at 1028.8 °C and resulted in the formation of FeAl₂O₄ and Fe₃Al phases. Moreover, the content of Al₂O₃, Fe₃Al, α-Fe and FeAl₂O₄ phases increased with the increase of sintering temperature. The maximum flexural strength, hardness and relative density are achieved when sintering at 1230 °C. In addition, the dehydrogenation of TiH₂ can impede the formation of FeAl₂O₄ phases and improve the flexural strength, hardness and relative density of the bond. Also, the Si-doping into Fe₂O₃-Al aluminothermic reaction system resulted in Fe₂Al₃Si₃ phase and reduced the content of Al₂O₃ and Fe₃Al phases, leading to higher flexural strength, lower hardness and inferior relative density. In wet grinding, the as-prepared metal bond diamond tool can be used to grind ceramic tiles with lower grinding force and better surface quality than the dry grinding. However, the as-prepared metal bond diamond tool rendered low wear resistance due to the brittle nature of the metal bond.     

Temperature-dependent mechanical properties of alpha-/beta-Nb5Si3 phases from first-principles calculations

The temperature-dependent structural properties and anisotropic thermal expansion coefficients of α-/β-Nb₅Si₃ phases have been determined by minimizing the non-equilibrium Gibbs free energy as functions of crystallographic deformations. The results indicate that the crystal anisotropy of α-Nb₅Si₃ phase is much more temperature dependence than that of β-Nb₅Si₃ phase. The total/partial density of states of α-/β-Nb₅Si₃ phases are discussed in detail to analyze their electronic hybridizations. It is demonstrated that the bonding of the two phases is mainly contributed from the hybridization between Nb-4d and Si-3p electronic states. The temperature-dependent mechanical properties of α-/β-Nb₅Si₃ phases are further investigated via the quasi-harmonic approximation method in coupling with continuum elasticity theory. The calculated single-crystalline and polycrystalline elasticity shows that both phases are mechanically stable and exhibit the intrinsic brittleness. The results also suggest that α-Nb₅Si₃ phase possesses a superior ability of compression resistance but an inferior ability of high-temperature resistance of mechanical properties than those of β-Nb₅Si₃ phase. The bonding features of α-/β-Nb₅Si₃ phases are discussed by means of charge density difference analysis in order to explain the difference of the temperature-dependent mechanical properties between the two phases.     

Phase Equilibria of the Mg-Y-Zn System at 500 °C in the Region of &lt; 50 at.% Mg and &lt; 50 at.% Y

The phase equilibria of the Mg-Y-Zn system at 500 °C in the region of?<?50 at.% Mg and?<?50 at.% Y were investigated with heat-treated alloys, by means of the electron probe microanalysis and x-ray diffraction. Seven ternary phases, denoted τ₁ to τ₇, were found to exist in this region, and an additional ternary phase, τ₈, in the more Mg-rich region. The homogeneity ranges of the ternary phases have been well measured. The ternary phases τ₁, τ₂, τ₆, τ₇ and τ₈ are considered to be identical to the previously reported Z-Y₇Mg₂₈Zn₆₅, I-YMg₃Zn₆, W-Y₂Mg₃Zn₃, YMgZn and 10H-Y₄Mg₂₃Zn₃, respectively. The ternary phases τ₃, τ₄ and τ₅ have been found for the first time in the present work. The solubility of Mg in YZn₅ was measured to be up to 25.1 at.% Mg. A partial isothermal section at 500 °C was constructed for the Zn-Mg₂Zn-YMg-Zn region.     

Particle growth mechanism of nanocrystalline zirconia powder during high temperature heat treatment

Agglomerated nanocrystalline ZrO2-8%Y2O3 powder prepared by spray drying was heat-treated in air at temperatures from 1 200 ℃ to 1 400 ℃ for 2 h. Scanning electron microscopy was used to examine the changes of particle size and morphology, and X-ray diffraction was used to analyze the change of constituent phases before and after the high temperature heat treatment. Nano-particle growth behavior was also investigated. The results show that the major constituent phase of the agglomerated nanocrystalline powder is tetragonal, and non-uniform growth of the nano-particles occurs while the heat treatment temperature reaches 1 300 ℃. This non-uniform growth phenomenon is related with the inhomogeneous distribution of Y2O3 in ZrO2. Nano-particles grow into micron particles through the mechanisms of gradual merging of nano-particles in some areas and sudden merging ofnano-particles in other areas.