Thermal properties of W–Cu composites manufactured by copper infiltration into tungsten fiber matrix

W–Cu composites were produced by the technique of copper infiltration into tungsten fiber preforms (CITFP) under vacuum circumstance. Fibrous structure preforms with various volume fraction of tungsten fiber were fabricated by the process of mold pressing and sintering. The molten copper was infiltrated into the open pores of the preforms under vacuum at 1473 K to 1573 K for 1 h to produce W–Cu composites with compositions of 10–30 wt.% copper balanced with tungsten. The microstructure, relative densities, and thermal properties of the composites were investigated and measured. The relative as-sintered density was enhanced with the increase of the sintering temperature. The thermal conductivity of the W–Cu30 composite with 28.2 wt.% Cu was 241 W/(m · K) at 298 K, 10% higher than that of the W–Cu alloy with similar copper content produced by conventional powder metallurgy process. The thermal expansion of the composites was decreased with the increase of tungsten content, keeping the same tendency as the prediction by the rule of weighted average of volume ratio of compositions.     

Fabrication and mechanical properties of SiCw/MoSi2–SiC composites by liquid Si infiltration of pyrolyzed rice husk preforms with Mo additions

Dense SiC ceramic matrix composites containing SiC whiskers (SiC_w) and MoSi₂ phase (SiC_w/MoSi₂–SiC) are fabricated by a liquid Si infiltration (LSI) method. Pyrolyzed rice husks (RHs) containing SiC whiskers, particles and amorphous carbon are mixed with different amounts of Mo powder to form preforms for the infiltration. Microstructure and mechanical properties of the composites are studied. Fracture mode of the composites is discussed. Results show that the SiC whiskers and fine particles in the pyrolyzed RHs were preserved in the composites after the LSI process. The amorphous carbon and Mo powder in the preforms reacted with molten Si, forming SiC and MoSi₂ in the composites. The presence of MoSi₂ in the composite increases the elastic modulus but lowers the flexure strength. Content of MoSi₂ of ca. 20 wt.% provides an enhanced fracture toughness of 4.1 MPa m^1/2 for the composite. But too large amount of MoSi₂ caused crack formation in the composite. The compressive residual stress introduced by the formation of MoSi₂ and SiC, and the de-bonding of the fine SiC particles and SiC whiskers from the residual Si phase are considered to favor the fracture toughness of the composites.     

Formation,thermal stability and corrosion behavior of glassy Ti45Zr5Cu45Ni5 alloy

Ti-rich Ti₄₅Zr₅Cu₄₅Ni₅ bulk metallic glass with critical diameter reaching 3 mm and supercooled liquid region of 42 K was prepared by copper mold casting. The glass transition temperature and onset temperature of crystallization are 673 K and 715 K, respectively. The glassy Ti–Zr–Cu–Ni alloy is passive in 3 mass% NaCl, 1 N HCl and 1 N H₂SO₄ solutions, although pitting corrosion occurred by anodic polarization at higher potential in the Cl⁻-containing solutions. Corrosion rates of the glassy Ti₄₅Zr₅Cu₄₅Ni₅ alloy are of the order of 10⁻³ mm/year in the NaCl and H₂SO₄ solutions and about 1 × 10⁻² mm/year in the HCl acid.     

Titanium diboride composite with improved sintering characteristics

Titanium diboride (TiB₂) and its ceramic composites were prepared by hot pressing process. The sintering process, phase evolution, microstructure and mechanical properties of TiB₂ ceramics prepared by using different milling media materials: tungsten carbide (WC/Co) or SiAlON was studied. It was found that the inclusion of WC/Co significantly improved the sinterability of the TiB₂ ceramics. A core/rim structure with pure TiB₂ as the core and W-rich TiB₂, i.e. (Ti,W)B₂ as the rim was identified. Microstructure analysis revealed that this core/rim structure was formed through a dissolution and re-precipitation process. In addition, silicon carbide (SiC) was also introduced to form TiB₂–SiC composites. The addition of SiC as the secondary phase not only improved the sinterability but also led to greatly enhanced fracture toughness. The optimum mechanical properties with Vickers hardness ~ 22 GPa, and fracture toughness ~ 6 MPa m^1/2 were obtained on TiB₂–SiC composites milled with WC/Co.     

Effect of impingement angle and WC content on high temperature erosion behavior of SiC-WC composites

Hot pressed dense SiC-(0, 10, 30 or 50 wt%)WC composites were subjected to erosion against SiC particles at 800 °C. Effects of WC content and angle of impingement (30°, 60° or 90°) on the erosion performance of composites were evaluated. Erosion rate ranged from 2.1 × 10² mm³/kg to 7.7 × 10² mm³/kg with varying WC content or angle of impingement. The erosion rate of the composites increased with increasing the impingement angle from 30° to 90°, and decreased with WC content up to 30 wt%. Minimum and maximum erosion wear rates were obtained for SiC-30 wt% WC composites at 30° and for SiC-50 wt% WC composites at normal impact, respectively. Grain fracture and pull-out were observed as major mechanisms of material removal for the composites. Decreased angle of impingement led to reduced grain fracture and pull-out, and hence reduction in material removal. Owing to increased fracture toughness with incorporation of WC particles, the composites showed less fracture and removal of WC particles up to 30 wt% reinforcement.     

The evaluation of W/ZrC composite fabricated through reaction sintering of two precursors: Conventional ZrO2/WC and novel ZrSiO4/WC

In this study the W-ZrC composites fabricated by in situ reaction sintering of two precursors were compared, 1-The conventional WC and ZrO₂ which are ball milled with established molar ratio of 3–1 for 12 hours, gelcasted to form a green body and then undergo a pressure less sintering cycle, 2-A new and innovative way in which for the first time ZrSiO₄ was used instead of ZrO₂, and by testing different molar ratio between WC and ZrSiO₄ it was understood that the optimum ratio is 3–1 once again. Furthermore the starting ZrO₂ and ZrSiO₄ powder were selected in nano size and it was understood that by using nano powders the amount of unreacted and unwanted phase reduce, the reaction progress and the mechanical proprieties improve. Although the reaction sintered WC/ZrO₂ possess better properties, regarding the cost considerations, reaction sintering of WC/ZrSiO₄ is a much cheaper process.     

Optimized interface and mechanical properties of W fiber/Zr-based bulk metallic glass composites by minor Nb addition

W fiber/Zr-based bulk metallic glass composites were prepared by melt infiltration casting and the interface reaction in composites was studied in detail. It was found that minor Nb addition to the matrix can suppress the interface peritectic reaction and optimize the interface structure in W fiber/Zr-based bulk metallic glass composites. Taking the interface characteristics of composites and glass forming ability of the matrix into account, an optimized alloy Zr₄₇Ti₁₃Cu₁₁Ni₁₀Be₁₆Nb₃ was selected as a new metallic glass matrix. The interface in the W fiber/Zr₄₇Ti₁₃Cu₁₁Ni₁₀Be₁₆Nb₃ bulk metallic glass composite has excellent interface bonding, and the compressive strength of 70% W fiber/Zr₄₇Ti₁₃Cu₁₁Ni₁₀Be₁₆Nb₃ metallic glass composite is 2.6 GPa, 58% higher than the unreinfored matrix. Multiple shear band formation in the matrix, which results from the interaction between W fiber and the matrix, can answer for the high ultimate fraction strength and excellent plastic deformation of the composite.     

Fe–W–C thermodynamics and in situ preparation of tungsten carbide-reinforced iron-based surface composites by solid-phase diffusion

Tungsten carbide (WC)-reinforced Fe-based surface composites were prepared by in situ solid-phase diffusion at 1423 K for 4, 6, and 8 h. The thermodynamics, phase composition, microstructure, microhardness, and wear-resistance of the Fe–W–C ternary system of the samples were examined by X-ray diffraction, scanning electron microscopy, Vickers hardness test, and wear test, respectively. Thermodynamic calculations showed that the thermodynamically favored products of the Fe–W–C system were W₂C, WC, and Fe₃C. W also exhibited a stronger carbide-forming tendency than Fe. The Gibbs free-energies of W₂C and WC, which were stable carbides, significantly decreased with increased temperature. The main phases of the composite were WC, γ-Fe, Fe₃C, graphite, and η-carbide (M₆C) with fishbone-like morphology. The longitudinal section of the composite could be easily divided into three reaction zones, namely, WC layer, “no graphite area,” and M₆C-reinforced area. WC particles in the WC layer were irregularly shaped with 0.3–12 μm particle size, with volume fraction of up to > 80%. The average microhardness value of the dense ceramic layer was 2152 HV_0.1. The maximum relative wear-resistance, which was 230.4 times higher than that of gray cast iron, was obtained at 20 N. The high wear-resistance of the composite was due to the in situ formation of dense and hard WC particulates that acted as a reinforcement phase.     

Hydrogen permeation and structural features of melt-spun Ni–Nb–Zr amorphous alloys

Amorphous (Ni_0.6Nb_0.4)_100−xZr_x (x = 0, 20, 30, 40 and 50 at.%) alloys were prepared by the melt-spinning technique, and the hydrogen permeation through those alloy membranes was examined. The local atomic structure in these alloys was also investigated by radial distribution function (RDF) analysis. Moreover, hydrogen solubility and diffusivity were also measured in order to discuss the mechanism for hydrogen permeation. The permeability of the Ni–Nb–Zr amorphous alloys increases with Zr content and temperature. The maximum hydrogen permeability is 1.59 × 10⁻⁸ mol m⁻¹ s⁻¹ Pa^−1/2 at 673 K for the (Ni_0.6Nb_0.4)₅₀Zr₅₀ amorphous alloy. The (Ni_0.6Nb_0.4)₅₀Zr₅₀ amorphous alloy showed larger hydrogen solubility and diffusivity than the (Ni_0.6Nb_0.4)₇₀Zr₃₀ amorphous alloy. As the result, the (Ni_0.6Nb_0.4)₅₀Zr₅₀ amorphous alloy showed higher hydrogen permeability than the (Ni_0.6Nb_0.4)₇₀Zr₃₀ amorphous alloy at 673 K. The RDF analysis shows that the atomic distance between the Zr atoms increases by hydrogenation. The chemical ordering such that the number of Zr coordinates is much higher than that of Ni and Nb coordinates was found in the (Ni_0.6Nb_0.4)₇₀Zr₃₀ and (Ni_0.6Nb_0.4)₅₀Zr₅₀ amorphous alloys. The relation between the amorphous local structure and the permeation was discussed in detail.     

In situ synthesis,microstructure, mechanical properties and thermal shock resistance of (ZrB2 + SiC)/Zr2[Al(Si)]4C5 composites

Dense (ZrB₂ + SiC)/Zr₂[Al(Si)]₄C₅ composites with adjustable content of (ZrB₂ + SiC) reinforcements (0–30 vol.%) were prepared by in situ hot-pressing. The microstructure, room and high temperature mechanical and thermal physical properties, as well as thermal shock resistance of the composites were investigated and compared with monolithic Zr₂[Al(Si)]₄C₅ ceramic. ZrB₂ and SiC incorporated by in situ reaction significantly improve the mechanical properties of Z₂[Al(Si)]₄C₅ by the synergistic action of many mechanisms including particulate reinforcement, crack deflection, branching, bridging, “self-reinforced” microstructure and grain-refinement. With (ZrB₂ + SiC) content increasing, the flexural strength, toughness and Vickers hardness show a nearly linear increase from 353 to 621 MPa, 3.88 to 7.85 MPa·m^1/2, and 11.7 to 16.7 GPa, respectively. Especially, the 30 vol.% (ZrB₂ + SiC)/Zr₂[Al(Si)]₄C₅ composite retains a high modulus up to 1511 °C (357 GPa, 86% of that at 25 °C) and superior strength (404 MPa) at 1300 °C in air. The composite shows higher thermal conductivity (25–1200 °C) and excellent thermal shock resistance at ΔT up to 550 °C. Superior properties render the composites a promising prospect as ultra-high-temperature ceramics.     

Nanocrystallization of Zr61Al7.5Cu17.5Ni10Si4 metallic glass

The primary nanocrystallization behavior and microstructural evolution of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy during annealing were investigated by isothermal differential scanning calorimetry, X-ray diffractometry and transmission electron microscopy. During continuous heating of the 4Si and the base (contains no Si) amorphous alloys at a heating rate of 10 K/min, the saturation point of nucleation for the 4Si amorphous alloy occurs at a crystallization fraction of 78%, which is significantly higher than 65% for the base alloy, implying that these metalloid atoms would extend the nucleation stage and refine crystalline particles. The sequence of crystallization phase from the amorphous matrix for the isothermally annealed 4Si amorphous alloy at 703 K is observed to be Zr₂Cu and Zr₂Ni at the early stage, Zr₃Al at an intermediate stage, and Zr₂Si at the final stage. Moreover, enrichment of Si atoms at the interface between Zr₂Cu crystal and the amorphous matrix is detected. This may result in increasing the thermal stability of the remaining amorphous phase and retardation of the crystal growth of Zr₂Cu particles.     

Development and characterization of cube-textured Ni–Cu–Co substrates for YBCO-coated conductors

Ni–Cu–Co alloys were studied for the development of textured substrates for YBCO-coated conductor application. Three compositions were obtained by adding a fixed amount of 3 at.% Co to the binary Ni_xCu_100?x, where x = 40, 50 and 60. Cube texture was induced by conventional cold rolling followed by high-temperature annealing. The structural, microstructural, morphological, electrical, magnetic, mechanical and oxidation properties were evaluated and compared with those exhibited by the binary Ni–Cu alloy, as well as by Ni–W and pure Ni. A low Ni content is detrimental for the development of the cube texture with respect to higher concentrations. Nevertheless, the use of high annealing temperatures enabled an area fraction of cube orientation as high as 95% to be obtained for x = 40, and >97.5% in the case of Ni-richer alloys. Compared with Ni and Ni–W, Ni–Cu–Co alloys oxidize more easily and exhibit higher electrical resistance. In addition, the presence of copper enables the Curie temperature to be reduced to 60 K for x = 40 and to 155 K for x = 50. Furthermore, the introduction of cobalt reduces the oxidation rate at temperatures normally used for the deposition of ceramic buffer layers, thus allowing the successful development of a CeO₂/YSZ/CeO₂ architecture on ternary Ni–Cu–Co alloy. YBCO/buffer multilayer architecture deposited by pulsed laser deposition on a selected alloy tape exhibits a critical current density exceeding 1 MA cm^?2 at 77 K in self-field, indicating that this alloy substrate is suitable for YBCO-coated conductor application.     

The kinetics of incongruent reduction of tungsten carbide via reaction with a hafnium–copper melt

The reduction of WC into W by reaction with a Hf–Cu melt has been found to proceed by the formation of layers of W and HfC. The extent of such incongruent WC reduction has been examined after the immersion of dense, polycrystalline WC plates in a vertical orientation in Hf–Cu melts at 1150–1300 °C for 1–24 h. A W layer of uniform thickness formed adjacent to the WC, whereas an irregular, but generally continuous HfC layer formed adjacent to the W layer. The rate of WC reaction was evaluated by measuring the thickness of the W layer as a function of reaction time, temperature and vertical position along the WC surface. Such kinetic data and microstructural analyses indicated that the incongruent reduction of WC in molten Hf–Cu is likely to be controlled by the diffusion of carbon through the W and/or HfC layers.     

Phase stability for the Cu–Zr system: First-principles,experiments and solution-based modeling

First-principles calculations and experimental methods were employed to investigate the relative stability of intermetallic phases in the Cu–Zr system. Computed enthalpies of formation indicate that Cu₅₁Zr₁₄-β and CuZr₂-C11_b are stable phases, while Cu₅Zr-C15_b, Cu₁₀Zr₇-? and CuZr-B2 are metastable at 0 K. Heat treatment and microanalysis revealed two important findings which clarify the phase equilibria. First, the stability range for the Cu₅Zr-C15_b phase was found to have a lower bound associated with an eutectoid invariant between 802 and 955 K, below which it decomposes to face-centered cubic Cu plus Cu₅₁Zr₁₄-β. Second, the Cu₅Zr₈ phase, previously reported as stable, was not observed in a Cu–56.4 at.% Zr alloy after holding at 955 and 1036 K for >100 h. This phase, therefore, was not considered to be stable. Based on computational and experimental results, Gibbs free energies were modeled, including the Cu₂Zr-σ, Cu₂₄Zr₁₃-μ and metastable CuZr-(B19′ and B33) phases. The associated phase diagrams are presented.     

An investigation on the mechanochemical behavior of the Ca–C–Cu2O–WO3 quaternary system to synthesize the Cu–WC nanocomposite powder

Fundamental aspects of the reaction path in the Ca–C–Cu₂O–WO₃ quaternary system to synthesize a copper matrix nanocomposite with reinforcement particles of tungsten carbide have been studied. The mechanism of reactions was specified through the analysis of the relevant sub-reactions. In the presence of carbon as a reducing agent (without Ca), the carbothermal reaction partially occurred even after 40 h of milling. On the other hand, calcium (without C) reduced both Cu₂O and WO₃ after 15 min of milling in a self-sustaining mode. In the simultaneous presence of Ca and C, the products included Cu and W₂C as well as a significant amount of remaining unreacted W. The Cu–WC nanopowder, with no trace of W₂C, was synthesized by the addition of excess carbon to the initial mixture. SEM observations indicated that the composite powders were agglomerated and the range of the particle size was within 100 nm. Elemental mapping spectra showed a relatively uniform distribution of WC in the Cu matrix.     

Anomalous glass transition behavior in Cu–Zr–Sn alloy system

We discuss the effect of Sn addition on anomalous glass transition behavior in Cu–Zr bulk-forming metallic glasses. We found that an unusual endothermic reaction in Cu₅₅Zr₄₀Sn₅ ribbon can originate from the growth reaction of quenched-in nuclei from 0.4 nm to 3.7 nm in the supercooled liquid region. This anomalous devitrification may prove useful for the synthesis of a novel composite with uniform atomic/nanometer scale heterogeneity modulated by controlling cooling rate as well as by tailoring alloy composition.     

Near-nano and coarse-grain WC powders obtained by the self-propagating high-temperature synthesis and cemented carbides on their basis. Part I: Structure,composition and properties of WC powders

Near-nano WC powders with mean grain sizes of about 200 nm were prepared by the SHS method including the reduction of WO₃ by Mg in the presence of carbon and regulating additives. The chemical leaching and refinement of the SHS reaction products allowed one to obtain stoichiometric WC containing only traces of oxygen and magnesium. The thermal reduction of WO₃ and V₂O₅ by magnesium in the presence of carbon resulted in obtaining two carbide phases of WC and complex carbide (W,V)C with the fcc crystal lattice having a grain size of less than 300 nm. It was established that the tungsten oxide reduction by magnesium in the presence of carbon cannot be used to synthesize coarse-grain WC powders. Coarse-grained WC powders were obtained using the W + C mixture heated to high temperatures by a simultaneous exothermic reaction of interaction between magnesium perchlorate Mg(ClO₄) and magnesium. The coarse-grain WC powder synthesized in such a way is nearly stoichiometric and consists of sintered round-shaped agglomerates with the average grain size of up to 16 μm and containing only traces of magnesium and oxygen. The agglomerates comprise WC single-crystals of roughly 1 μm to 8 μm in size.     

Effect of elements with positive enthalpy of mixing on mechanical properties of bulk metallic glasses

Samples of (Cu₄₆Zr₄₆Al₈)_100?xZ_x metallic glass forming alloys with diameters 2–6 mm were prepared by injection casting. The effect of minor amounts of elements Z = Gd, Co and Re with positive enthalpy of mixing within the Gd–Zr, Cu–Co and Cu–Re terminal systems was compared. The addition of Gd up to x = 2 slightly enhances the glass forming ability, Co reduces the critical diameter of bulk metallic glass formation, whereas even for small fractions of Re bulk samples were crystalline, but only amorphous splats can be prepared. Both Gd and Co diminish the crystallization temperature T_x with respect to the Cu₄₆Zr₄₆Al₈ master alloy, but in Re-bearing splats T_x is increased. Alloying with optimum amounts of Gd and Co up to x = 2 leads to plastic deformability of rods, 2 and 3 mm in diameter, in comparison with the brittle Cu₄₆Zr₄₆Al₈ bulk metallic glass.     

Diffusion and solubility of Cr in WC

Although no detailed study on the Cr solubility in WC exists the compilation on the various C–Cr–W phase diagrams [1] suggests this behaviour. In order to prove this and to estimate the diffusivity of Cr in WC we prepared diffusion couples of the type Cr₃C₂–WC by joining and annealing polished fully dense counterparts of the two carbides (temperature range 1550–1750 °C). After thermal treatment the diffusion couples were cut, polished and investigated by metallography. For the measurement of the diffusion profiles the couples were subjected to WDS-EPMA (Cameca SX 100 microprobe). W, Cr, and C concentration profiles were obtained from line scans performed perpendicular to the interface. The analysis of diffusion couples of WC contacted to other carbides used for doping of hardmetals (VC, TaC, NbC, and TiC) did not yield perceptible solubility of the respective metals in WC with respect to the detection limit of EPMA.From the Cr diffusion profiles a diffusion coefficient of Cr in WC of approximately D = 1.70–2.20 × 10^?11cm²/s and an activation energy of E_A = 0.75 eV was estimated. In addition the composition of the ternary phase (W,Cr)₂C in equilibrium with WC and Cr₃C₂ could be measured. For example, in couples annealed at 1750 °C the composition reaches from (W_0.5Cr_0.5)₂C (in equilibrium with WC) to (W_0.2Cr_0.8)₂C (in equilibrium with Cr₃C₂).With the results obtained from the analysis of diffusion couples, the Cr uptake of WC powder as a function of grain size, time and temperature was calculated. Cr saturation in idealised spherical particles of 1 μm occurs only within a few minutes.     

Dense sub-micron-sized ZrC-W composite produced by reactive melt infiltration at 1200 °C

ZrC-W composite was produced by reactive infiltration of Zr₂Cu into WC preform at 1200 °C. The WC reacted completely with Zr in the alloy to form 61.8 vol% ZrC and 38.2 vol% W. The infiltrated composite reached a relative density of 98.3% and average grains sizes of about 0.5 μm. The flexural strength and the fracture toughness of the ZrC-W composite was improved by the addition of W.