Reaction of Tungsten with Melts in the Cu - Co System

We have studied the solubility at 1200°C of tungsten in Cu - Co melts and the growth kinetics of a W₆Co₇ layer at the tungsten — melt interface. We have established the composition of the melt in the three-phase equilibrium tungsten — W₆Co₇ — melt: 0.0195 Co, 4.8 · 10⁻⁵ W, the rest is Cu (in atomic fractions). In the studied composition range for the melt, the solubility of tungsten is described well by the expression: lgX_W = −7.117 + 25.7 · X_Co ^1/2 − 41.06 · X_Co, where X_W and X_Co are the atomic fractions of the corresponding elements in the melt. We have determined the correlation between the growth rate for a layer of tungsten-containing phase at the tungsten — melt interface and the thermodynamic characteristics of the melt. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(443), pp. 81–86, May–June, 2005.     

Structural investigations of detonation-deposited tungsten carbide-cobalt coatings

Conclusions The phase composition of detonation-deposited VK type coatings differs from that of the starting mixtures. A deposited layer contains about 40–50% of the tungsten carbide WC present in the starting mixture, metallic tungsten and cobalt, the intermetallic compounds Co₇W₆ and Co₃W, traces of W₂C, and a small amount of complex carbides. The amount and particle size of the tungsten carbide in a coating are determined by the particle size distribution of the powder being deposited and detonation process parameters. Our investigations have shown that the optimum powder particle size range for the process of detonation deposition of hard-metal mixtures may be taken to be 5–40 m. During detonation deposition under the conditions investigated the usual hard-metal structure and composition are not obtained. Most of the starting tungsten carbide decomposes with the formation of tungsten and cobalt intermetallic compounds. Attempts should now be made to improve deposition conditions so as to increase the phase and structural homogeneity of layers being deposited. In coatings deposited by an optimum method the ductile Co binder will be present in an amount close to that in a sintered VK type hard metal. The fracture of coatings and basis metal on the antivibration shelves of compressor blades has a fatigue character. Crack initiation takes place in the surface and inner layers of detonation-deposited coatings as a result of their considerable brittleness, which is due to the presence of metastable intermetallic phases as well as of defects in the form of pores and blowholes.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 24–29, October, 1982.     

The eta carbides in the Fe−W−C and Co−W−C systems

The η carbides in Fe?W?C are Fe₆W₆C with an edge length of the cubic unit cella=10.956 to 10.958Å and Fe₃W₃C witha=11.102 to 11.146Å. In Co?W?C the η carbides solidity from the melt as Co₆W₆C and ～Co₂W₄C. Co₆W₆C retains a narrow homogeneity range down to 1000°C with an observed range of lattice constants between 10.894 and 10.902Å. The compositional range of Co₂W₄C spreads out between Co₃W₃C and Co₂W₄C at 1400°C and narrows back down to a small range around Co₂W₄C at 1000°C. Its lattice constants vary between 11.066 and 11.251Å. In Fe-W-C a new hexagonal phase is found at FeW₃C. It has the structure reported by Schönberg for Co₃W₉C₄ and its lattice parameters area=c=7.806 to 7.810Å.     

Characterization of the W2C phase formed during the high velocity oxygen fuel spraying of a WC + 12 pct Co powder

A variety of experimental techniques have been used to study a WC-12 pct Co powder and the coatings produced by high velocity oxygen fuel (HVOF) spraying of the powder onto a steel substrate. Many of the structural characteristics of the powder were also found in the coating. However, when the metallic matrix of the powder was melted during thermal spraying, the carbides were partially dissolved and a very heterogeneous liquid phase was produced in which the W/C ratio varied from about 1 to 4. These variations have been linked with oxidation of the liquid phase during spraying. The factors influencing the formation of W₂C in the coating have been identified as (1) an in situ transformation of WC into W₂C maintaining the original WC faceted morphology and (2) the precipitation of W₂C from the W-rich liquid phase matrix as the coating cools. A cobalt containing carbide of the M₆C-M₁₂C type has also precipitated from the liquid phase when the W/C and W/Co ratios were high. deceased.     

Growth Features of Diamond Films on the Tungsten Carbide Surface with a Copper Sublayer

The surface preparation is a prerequisite for ensuring the required properties of a diamond film formed by gas-phase deposition. The influence of the temperature and concentration of the CuSO4 etchant on the structural and phase compositions of the surface of hard-alloy materials is considered in this work, and the structural and phase compositions of a continuous polycrystalline diamond film at its growth stages is also considered. The adhesion of the diamond films to the surface of hard-alloy materials is determined qualitatively. It is established that the treatment of the hard-alloy surface in the CuSO4 solution at t = 23°C leads to the inhomogeneous removal of a cobalt binder with chipping WC grains and the formation of the porous structure in the surface layer of the WC–6% Co alloy. The treatment by the CuSO4 etchant at t = –2°C provides the homogeneous etching of the Co binder over the boundaries of WC grains and the formation of a chemically homogeneous surface. The orientation growth and adhesion of the diamond film depend on the elemental composition of the WC–Co alloy surface after treatment by the CuSO4 solution. If the treatment was performed at tsln = 23°C, then the removal of copper from the defect surface WC layer is complicated during the synthesis, which provides the multidirectional growth of diamond crystals in the film along two directions, 〈111〉 and 〈110〉, which causes critical biaxial compressive stresses (2.5 GPa) and leads to low film adhesion to the hard alloy surface. If the treatment is performed at tsln = –2°C, then the orientational growth of diamond crystals in the film occurs in one preferential crystallographic direction 〈111〉, which lowers the biaxial compressive stresses (1.7 GPa) and increases the adhesive bond of the film to the hard alloy surface. The structure imperfection calculated from the ratio of the lines of integrated intensities I1333/I1580 using the Raman spectroscopy method decreases with an increase in concentration at negative temperatures and increases at positive temperatures of the CuSO4 solution during surface preparation.     

Structure of the surface layer of a hard-metal cold-upsetting tool

Conclusions In the structure of the WC and Co phases making up VK20K hard metal the presence was discovered, in the as-sintered condition, of dislocations and twins, which may be attributed to the generation, during the postsintering cooling of the alloy, of large thermal stresses as a result of the difference in the coefficients of thermal expansion of its tungsten carbide grains and metallic cobalt interlayers. Operation increased the number of dislocations and twins and brought about fragmentation of twins in the Co and WC phases, which is evidence that their wear was preceded by appreciable plastic deformation. In the structure of VK20K alloy in the as-sintered condition and in its surface layer after wear the carbide W₂C was found on the surface of the carbide WC between the WC grains and Co-phase interlayers, and it is therefore reasonable to assume the existence under such conditions of an extremely thin transition layer of virtually pure tungsten.Translated from Poroshkovaya Metallurgiya, No. 12(216), pp. 83–88, December, 1980.     

WC-Co by Direct Reduction Carburisation Reactions

Abstract

Thermogravimetry, XRD and SEM have been used to study the direct reduction/carburisation of non-stoichiometric CoW0₄ with hydrogen/methane mixtures over the temperature range 900°C to 1100°C. The evolution of phases during reduction/carburisation has been related to differences in starting materials, temperature and gas phase composition. The thermochemistry of the reduction/carburisation process is detailed and actual results of processing are related to thermodynamic predictions. It is shown that the first stage of reaction is reduction of the tungstate to give the intermetallic compounds Co₃W and Co₇W₆ as well as tungsten metal. During this stage no carburisation occurs. The subsequent carburisation stage of the process yields a sequence of phases passing through carbides of the general formulae M₁₂C and M₆C before giving a mixture of WC and Co. In W-rich mixtures a transitory existence of W₂C is shown to exist.     

Interaction of W<Subscript>2</Subscript>B<Subscript>5</Subscript> with Me<Superscript>IV,V</Superscript>C carbides

It is established that the W₂B₅-Me^IV,VC join in the W-B-C-Me^IV,V quaternary systems is quasibinary, while polythermal joins are described by eutectic phase diagrams. It is shown that the eutectic composition and temperature correlate with the melting points of carbides and the Me ^d C content varies from 50 mol % in the W₂B₅-VC system to 4–6 mol % in the W2B5-TaC system. The existence of Me ^d C-Me ^d B₂-W₂B₅ ternary eutectic systems promising for the development of new ceramics is proved.     

Contribution of electrospark alloying to the oxidation resistance of hard tungsten alloys

The paper examines the contribution of electrospark alloying to the oxidation resistance of hard tungsten alloys. It is established that the oxidation of carbides results from their electronic structure. When WC and hard tungsten alloys are heated to 1000°C, a brittle scale consisting of WO₃ and CoWO₄ rapidly forms. The oxidation resistance reduces as follows: TiC → Co → W → HTA (if TiC is more than 10%) → WC-Co → WC. The oxidation rate of hard tungsten alloys may be a criterion of their serviceability. It is shown that the oxidation resistance of hard tungsten alloys becomes much higher after their electrospark alloying with aluminum, titanium, and chromium and with wear-resistant composite TsLAB-2 ceramics based on the ZrB₂-ZrSi₂-LaB₆ system with Ni-Cr-Al (30 mole%) binder. __________ Translated from Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 145–150, 2008.     

Tensile properties and microstructure of haynes 25 alloy after aging at elevated temperatures for extended times

The tensile properties of Haynes 25 alloy have been measured after various aging treatments, time, and temperature: as received; and aged at 600 °C for three months; 800 °C for 6 and 12 months; and 1000 °C for 3 and 6 months. Contour plots in temperature-ln (time) space were constructed based on the literature and our own data, detailing changes in yield strength, ultimate tensile strength, and tensile elongation. Scanning electron microscopy and transmission electron microscopy observations of the Haynes 25 alloy microstructure provided an explanation of why the properties changed with aging. Intense lattice distortions after aging at 600 °C, the presence of an α-Co₃W, a L1₂-ordered, fcc phase, a=0.357 nm, after aging at 800 °C, and the nucleation and growth of W₃Co₃C carbides from aging at 800 °C and 1000 °C produced the changes in tensile properties. We did not observe either the Co₂W Laves phase or Co₇W₆ γ phase in any of the material conditions we examined, using TEM of thin foils: as received and aged at 600 °C, 800 °C, and 1000 °C. Other researchers believe these phases cause a loss of ductility in the Haynes 25 alloy with prolonged high-temperature exposure.     

Influence of the matrix composition,structure, and properties on the service life of a diamond drilling tool

The microstructure and mechanical properties of four matrices of a complex chemical composition—VK15–Cu, VK15–Cu–Sn, VK20–Cu, and VK20–Cu–Sn—which are applied when fabricating diamond drilling tools, are investigated. The samples of matrices were formed by the infiltration of formations of the hard-alloy charge with copper at t = 1100°C and with bronze at t = 1000°C. The slices of diamond-free matrices were fabricated; their metallographic and electron probe microanalysis were performed; and their hardness, microhardness, density, and porosity were determined. Hardness of matrices was established by the HRC scale and microhardness was established using a PMT-3 device under a load of 2 N. It is revealed that (i) hard and strong diamond-tool matrices with open porosity no larger that 3% are formed by the infiltration of formations of the hard-alloy charge of VK15 and VK20 grades by the melts based on copper and bronze, and infiltration passes more completely when using bronze; (ii) matrices formed by the infiltration of formations of the hard-alloy charge of the VK15 grade with bronze are hardest. The application of matrices based on the hard-alloy charge of the VK20 grade when fabricating the drilling diamond tool is unreasonable because of an abrupt decrease in its abrasive ability when drilling hard materials.     

Surface Hardening of Hard Tungsten-Carbide Alloys: A Review

Russian and non-Russian research on the surface hardening of hard tungsten-carbide alloys to improve the wear resistance is reviewed. There is great scope for improving the wear resistance and durability of hard-alloy components by surface strengthening on the basis of various coatings, including coatings with 100-nm structural components. On hard tungsten-carbide alloys, the most common coatings consist of titanium carbide TiC and nitride TiN, characterized by high lattice binding energy and high melting point. If such coatings are applied to hard-alloy tools, the frictional coefficient is reduced by a factor of 1.5–2.0 when cutting steel. The use of a TiN + ZrN ion-plasma coating reduces the frictional coefficient by a factor of 5.9. At present, multilayer coatings are widely employed. The most widespread are TiN + TiC and Al₂O₃ + TiC coatings. Their wear is proportional to the coating thickness. These multilayer coatings still leave room for improvement in the wear resistance of hard alloys. In Russia, the potential of hard alloys with a strength gradient from a ductile and high-strength core to a wear-resistant surface is being explored. At the Research Institute of Refractory Metals and Hard Alloys, a method has been developed for producing alloys with variable cobalt content over the thickness of the cutting insert. That permits change in alloy composition from VK20 to VK2 over the sample thickness. Correspondingly, the wear resistance of the insert’s working section is equivalent to that of VK2 alloy, while the base is able to withstand considerable flexural stress. Recently, cutting tools with a diamond coating on hard alloys have been adopted in practice. To increase the durability of hard-alloy VK inserts, strengthening based on concentrated energy fluxes may be employed. Examples include treatment of hard-alloy surfaces by γ quanta, ion beams, and laser beams, electroexplosive alloying, and electrospark strengthening.     

Kinetics of dissolution of cobalt oxides in acidic media

The kinetics of dissolution of cobalt oxides Co₂O₃ and Co₃O₄ in aqueous solutions of acids (H₂SO₄, EDTA) is experimentally studied. Dissolution rate W increases with the temperature or the EDTA concentration. The reaction orders of dissolution for hydrogen ions in sulfuric acid and EDTA (dlogW/dpH = 0.5 ± 0.1) and for anions (dlogW/dlog[An ⁻] = 0.5 ± 0.1) are determined. A specific feature of the dissolution kinetics in EDTA is a maximum in the dissolution rate of the cobalt oxides at pH −1. The activation energy of the process E _a is 70 kJ/mol in H₂SO₄ and 60 kJ/mol in EDTA. The modeling of the process shows that the CoOH⁺ ion is a surface particle controlling the dissolution rate in mineral acids and the CoHY⁻ ion, in the complexone.     

Multiple twins of τ2-Al13Co4 showing fivefold symmetry

The presence of a monoclinic Τ²-A1₁₃Co₄ phase, whosea andc are about Τ² = 2.618 (Τ = times greater than the respective parameters of the monoclinic Al₁₃Co₄, and its (100) and (001) twins has been reported earlier. Like its prototype structure, the monoclinic Al₁₃Fe₄/Al₁₃Co₄, this Τ²-A1₁₃Co₄ also shows a tendency to form multiple twins displaying five-fold symmetry. A transmission electron microscopic study of the twin mechanisms has been carried out. In addition to the well-known (001) and (100) glide twins, (20-1) glide twins have also been found. Alternate (100)-(20-l) or (100)-(001)-(100) twinning will yield twins display-ing fivefold symmetry. The distribution of Co atoms in the pentagonal layer in Τ²-A1₁₃Co₄ and the tessellation of pentagons in the (100) and (20-1) glide twins have also been studied by high-resolution electron microscopy.     

The Transformation of Carbides during Austenization and Its Effect on the Wear Resistance of High Speed Steel Rolls

The transformation of carbides with austenization time of a high speed steel (HSS) roll material, manufactured by a centrifugal casting method, has been studied. The correlation between wear resistance and the type, morphology, volume fraction, and distribution of the carbides has also been investigated. Microstructural observations, X-ray diffraction (XRD) analysis, hardness measurements, and energy dispersive spectroscopy (EDS) have been used to characterize the carbides. The type and volume fraction of carbides were found to change with austenizing time. During austenization, the transformation of the M₃C carbides can be postulated as M₃C + γ-Fe → M₂C, with much finer nodular and rodlike MC carbides also forming through a solid-state transformation. The M₂C carbide decomposes as M₂C + γ-Fe → MC + M₇C₃ + M₆C. The decomposed carbide substantially maintains a platelike shape until the end of decomposition. The most important finding of this study is that austenization results in changes in the type, morphology, volume fraction, and distribution of carbides and that it can be controlled to produced a homogeneous distribution of hard carbides, resulting in an improvement in the wear resistance of HSS rolls. This finding may be of great use for the industrial production of HSS rolls.     

Reaction of Tungsten with Liquid Co ― Sn Alloys

The solubility of tungsten in Co Sn melts and the growth kinetics of a W₆Co₇ phase layer at the tungsten melt interface were studied at 1200°C. The liquid alloys composition in the three-phase equilibrium W W₆Co₇ melt was established as (at. fraction) 0.51 Co, 0.49 Sn, 2.3·10^–3 W. The solubility of tungsten in the investigated range of melt compositions is well represented by the equation lgx _W = –0.964-3.420x _Sn, where x _W and x _Sn are atomic fractions of the elements in the melt. The calculated thermodynamic properties can be used for the analysis of other systems which include cobalt and tungsten.     

Catalytic effect of cobalt on the carburization kinetics of tungsten

The kinetics of the gas-phase carburization of tungsten by methane were studied and found to be controlled by two surface reactions and a later diffusion limiting reaction. The two surface reactions were found to produce the compounds W₂C and W₂C-WC respectively. The diffusion limiting reaction produced only WC. Extensive carburization caused cracking of the tungsten material. Cobalt was found to form a thin film on each tungsten surface which was easily converted to eta-phase (Co₃W₃C) when carburization began. A model was proposed and kinetic equations were developed which predict the rate of reaction for both cylinders and powders. The reaction rate curves for the uncatalyzed reaction using the same equations was also developed. CHARLES F. DAVIDSON, formerly with Fansteel Research Center, Salt Lake City, Utah     

Crystallographic characteristics of the Al-Co decagonal quasicrystal and its monoclinic approximant τ2-AI13Co4

The binary Al-Co decagonal quasicrystal (DQC) is stable between 800 °C and 900 °C and transforms to a primitive monoclinic phase (a = 3.984 nm,b = 0.8148 nm,c = 3.223 nm, and β = 107.97 deg) above 950 °C or below 750 °C. The composition of this crystalline approximant is similar to theC-centered monoclinic Al₁₃Co₄ but witha andc parameters τ² (τ = (1 + √5)/2 = 1.61803. . .) times larger. This τ²-A1₁₃Co₄ occurs frequently as (001) and (100) twins. The transformation of the Al-Co DQC to (100) twins of τ²-A1₁₃Co₄ has been followed by selected-area electron diffraction. With reference to the main zonal electron diffraction patterns and the X-ray diffractogram of the Al-Co DQC, the X-ray powder diffractogram of τ²-A1₁₃Co₄ with a giant unit cell can be successfully indexed.     

Growth of a layer of W6Co7 phase in systems with participation of phases containing cobalt

Growth kinetics for a layer of W₆Co₇ at the interfaces tungsten-cobalt and tungsten-melts based on copper and tin containing cobalt at 1200 °C are studied. It is established that rate constant for growth of this layer in the system with participation of tin (k_Sn) is much greater than in the system with copper (k_Cu). Dependences are determined for these constants on cobalt activity in the melt (a_Co): k_Cu(m²/sec) = = 1.72 · 10^?15(a_Co ? 0.221) + 2.99 · 10^?14 (a_Co ? 0.221)²; k_Sn(m²/sec) = 1.8 · 10^?14 (a_Co ? 0.221) + + 9.077 · 10^?12 (a_Co ? 0.221)³.