On the structure and properties of wear- and corrosion-resistant nickel-chromium- tungsten-carbon-(silicon) alloys

In some applications, for chemical and physical reasons hard nickel-based alloys have to be used instead of cobalt-based alloys but boron must be avoided. The nickel-chromium-tungsten-carbon system with and without silicon was therefore studied in several concentration ranges at 1050°C with respect to structure, phase, hardness and corrosion and wear resistance. Alloys containing 2% carbon, 10% tungsten and more than 10% chromium are composed of a nickel solid solution and an M₇C₃ carbide in both cast and homogenized (1050°C, 180 h) conditions. On increasing the tungsten content up to 20% the M₂C carbide becomes dominant, and this is associated with a remarkable increase in the hardness of the alloys. Additions of 2% silicon do not change the M₇C₃ and M₂C carbides present. In some cases a carbon-stabilized silicide M₅Si(C) was observed. Silicon additions decrease the liquidus temperature range relatively little, but they affect particle shape and size and the grain size distribution. The relation of various chromium, tungsten and silicon contents to corrosion and wear resistance was studied. The corrosion resistance depends on the chromium content of the nickel solid solution but also on carbide formation (tungsten and carbon content). The silicon content of the nickel solid solution is important too.Because their liquidus temperature is close to 1300°C the alloys cannot be used as self-fluxing and fusing powders for flame spraying but they can be sprayed by plasma torches and they can, of course, be welded.     

电沉积钨及钨合金涂层的研究进展

金属钨及含钨涂层具有优良的性能,如高熔点、高硬度、良好的化学稳定性和较低的热膨胀系数,在多个领域被广泛应用,金属钨及含钨涂层有很多制备方法,其中电沉积法具有重要的地位。综述了熔盐电沉积钨涂层及溶液电沉积钨合金涂层的研究进展,并展望了熔盐电沉积钨涂层及溶液电沉积钨合金涂层的发展趋势。     

Evaluation of rhenium carbide as a prospective material for hard coating

The literature reveals that interstitial alloys based on rhenium as a precursor might be extremely hard, becoming suitable to be used as hard coatings. In this work, we have produced rhenium carbide (ReC_x) films by the reactive pulsed laser deposition method. Nanoindentation has been performed to estimate hardness. The maximum hardness value for ReC_x films resulted to be 22.5 GPa. We found no evidence that ReC_x films have hardness, or plasticity, higher than competitive hard coating materials. Our results and the fact that rhenium is expensive and scarce, suggest that preceding reports are overoptimistic on the prospective use of rhenium carbide as hard coatings.     

Prospects of producing hard alloys based on submicron and nanoscale W and WC powders prepared by a chemical metallurgy process and with the use of self-propagating high-temperature synthesis

We present a brief review of research and design work aimed at producing tungsten-containing hard alloys for various applications. We examine the feasibility and prospects of using a chemical metallurgy method and self-propagating high-temperature synthesis (SHS) for the preparation of submicron and nanoscale hard-alloy powders as a key component of highly efficient state-of-the-art materials and products. Particular examples are presented of the use of submicron and nanoscale powders for the preparation of TVS tungsten-based heavy metallic alloys and VK tungsten carbide hard alloys for various applications. Their application fields are discussed and their properties are compared to those of their analogs produced by conventional powder metallurgy methods. Using the SHS of tungsten carbide as an example, we demonstrate a particular path from research to commercialization (from the discovery of SHS processes to commercialscale production) of key modern engineering materials: tungsten-based heavy alloys and tungsten carbidebased hard alloys.     

Influence of crystallite size on the hardness and fatigue life of steel samples coated with electrodeposited nanocrystalline Ni-W alloys

The present work deals with the effect of crystallite size on the hardness and fatigue life of steel samples coated with electrodeposited nanocrystalline Ni-W alloys. The Ni-W alloys were electrodeposited on steel samples at four different current densities (0.05, 0.10. 0.15 and 0.20 A/cm²) and at a temperature of 75 °C. The crystallite size of the deposit reduced (from 40 to 13 nm) with an increase in current density (from 0.05 to 0.20 A/cm²) due to an increase in the tungsten content (from 0.72 to 9.33 at.%). Ni-W alloy containing 9.33 at.% W and having a crystallite size of 13 nm exhibited the maximum hardness of 638 HV. The alloys, with the crystallite size in the range 40-13 nm, followed the direct Hall-Petch relation, i.e. hardness increased with a reduction in the crystallite size. The coated samples exhibited inferior fatigue lives compared to uncoated samples. This may be attributed to the presence of tensile residual stresses and inherent microcracks in the coatings. Among the specimens coated with Ni-W alloys, as the crystallite size decreased, the fatigue life of the specimen increased owing to the increase in hardness values.     

CVD Tungsten and Tungsten-Rhenium Alloys for Structural Applications

The microstructures of CVD tungsten and tungsten-rhenium alloys have been investigated to evaluate their suitability as high temperature materials. We found that the partial pressure in the reaction chamber affected the microstructure and surface roughness of deposited pure tungsten and tungsten-rhenium alloys. Grain sizes smaller than 0.5 im were obtained for both deposited pure tungsten and tungsten-rhenium alloys. Fine-grained tungsten proved to have greater hardness (Hv>1000 at room temperature (R.T.)) bending strength (δ = 700-1230 MPa at R.T.-l 175 K) and thermal coefficient of linear expansion (β = 4.5-4.9 xl0⁶K^-1 at 575-1175 K) characteristics than tungsten with columnar structure.     

Relation between shear banding and penetration characteristics of conventional tungsten alloys

The dynamic compression failure and ballistic penetration characteristics of conventional tungsten alloys similar in strength were investigated. Dynamic compression failure properties were generated with a symmetric Taylor test technique and penetration characteristics were obtained with 44 mm kinetic penetrators against an 300 HB hardness steel target at 1400 m/s. From shear crack length data generated with Taylor specimens impacted at different impact speeds a critical speed characterizing shear band initiation was deduced. The critical equivalent plastic strain at shear band initiation sites, obtained from the numerical simulation of the Taylor test at the critical impact speed, was found to decrease with the increase of the penetration performance. These results reinforce the argument that shear band formation is a failure mechanism associated with the erosion process for conventional tungsten alloys.     

Multimodal grain size distribution and high hardness in fine grained tungsten fabricated by spark plasma sintering

Preparation of fine grained, hard and ductile pure tungsten for future fusion reactor applications was tested using the bottom-up approach via powder consolidation by spark plasma sintering (SPS) at different temperature (1300-1800 °C) and pressure (90-266 MPa) conditions. Pure tungsten powders with an average particle size of about 1 μm were sintered to high density (about 94%) with almost no grain growth at a temperature below 1400 °C and an applied pressure up to 266 MPa. These samples had a multi-modal grain size distribution (resembling the size distribution of the initial powder) and a very high Vickers hardness (up to 530 kg/mm²). Above 1500 °C fast grain growth occurred and resulted in a drop in hardness. XRD on the surface of bulk samples showed a small amount of tungsten oxides; however, XPS and EDS indicated that these oxides were only surface contaminants and suggested a high purity for the bulk samples. The results demonstrate that SPS can lead to ultrafine and nanocrystalline tungsten if used to consolidate pure nano tungsten powders.     

Hardness and structural correlation for electroless Ni alloy deposits

Electroless nickel (EN) plating has received attention as a hard coating for industrial applications due to its high hardness, uniform thickness as well as excellent corrosion and wear resistance. The electroless Ni–P deposit is a supersaturated alloy in as-deposited state, and can be strengthened by precipitation of nickel phosphide crystallites with suitable heat treatments. However, the hardness of Ni–P films degrades with excessive annealing due to grain coarsening. This is the most severe barrier for electroless Ni–P deposition process from replacing chromium plating in industrial sectors. This problem is addressed in the paper by modifying the conventional electroless Ni–P bath to co-deposit tungsten to increase the hardness of the coating. Structural changes in the coating due to incorporation of tungsten are also highlighted. Deposition is done from an alkaline hypophosphite bath. Deposits with varying tungsten content are synthesized. Chemical analysis shows that tungsten incorporation reduces the phosphorus content in the deposit. Phosphorus content varied from 3 to 7 wt.% depending upon the tungsten incorporation in the deposit which in turn varied between 8 and 18 wt.%. Coatings with high tungsten content possess high hardness when compared to binary Ni–P as well as low tungsten ternary alloy deposits.     

Microstructure and mechanical properties of high power diode laser (HPDL) treated cast aluminium alloys

The appliance and development of modern technologies in the areas of surface engineering can be extended by laser surface treatment, especially using high power diode laser (HPDL) for remelting, feeding and/or alloying. The purpose of this work was to determine technological and technical conditions for tungsten carbide (WC) ceramic powder feeding into the surface layer of the laser treated Al–Si–Cu cast aluminium alloys with high power diode laser, as well as to investigate the microstructure and ceramic powder particle distribution in the surface layer. Special attention was devoted to monitoring of the layer morphology of the investigated material and on the particle occurred. Light and scanning electron microscopy as well as X‐Ray diffraction were used to characterize the microstructure of the remelted zone. A wide range of laser powers was choose and implicated by different process speed rates. Also one powder in form of tungsten carbide was used for feeding with the middle particle size of 80 µm. As the main findings there was found that, the obtained surface layer is without cracks and defects as well as has a comparably higher hardness value compared to the non remelted material. The hardness value increases according to the laser power used so that the highest power applied gives the highest hardness value in the remelted layer. Also the distribution of the tungsten carbide particles is good, but there are still possibilities for further modelling. The major purpose of this work is to study the effect of a high power diode laser melting on the cast Al–Si–Cu alloys structure to provide application possibilities for automotive and aviation industry.     

粉末注射成形技术及其在难熔钨合金等材料中的应用

粉末注射成形技术是将现代塑料注射成形技术引入粉末冶金领域而形成的一门近净形成形的新技术,有着巨大的发展前景。本文阐述了粉末注射成形技术及其特点,以及在难熔钨合金和硬质合金及其它材料中的应用。     

Structure, mechanical and tribological properties of diamond-like carbon films on aluminum alloy by arc ion plating

The low hardness and poor tribological performance of aluminum alloys restrict their engineering applications. However, protective hard films deposited on aluminum alloys are believed to be effective for overcoming their poor wear properties. In this paper, diamond-like carbon (DLC) films as hard protective film were deposited on 2024 aluminum alloy by arc ion plating. The dependence of the chemical state and microstructure of the films on substrate bias voltage was analyzed by X-ray photoelectron spectroscopy and Raman spectroscopy. The mechanical and tribological properties of the DLC films deposited on aluminum alloy were investigated by nanoindentation and ball-on-disk tribotester, respectively. The results show that the deposited DLC films were very well-adhered to the aluminum alloy substrate, with no cracks or delamination being observed. A maximum sp³ content of about 37% was obtained at −100 V substrate bias, resulting in a hardness of 30 GPa and elastic modulus of 280 GPa. Thus, the surface hardness and wear resistance of 2024 aluminum alloy can be significantly improved by applying a protective DLC film coating. The DLC-coated aluminum alloy showed a stable and relatively low friction coefficient, as well as narrower and shallower wear tracks in comparison with the uncoated aluminum alloy.     

Microstructure of alumina reinforced with tungsten carbide

Carbides and nitrides reinforced alumina based ceramic composites are generally accepted as a competitive technological alternative to cemented carbide (WC-Co). The aim of this work was to investigate the effect of dispersed tungsten carbide (WC) on the microstructure and mechanical properties of alumina (Al₂O₃). Micron size alumina and tungsten carbide powders were mixed in a ball mill and uniaxially pressed at 1600°C under 20 MPa in an inert atmosphere. The hardness of WC reinforced alumina was 19 GPa and fracture toughness attained up to 7 MPa m^1/2. It was demonstrated by TEM analysis that coarse, micrometersized tungsten carbide grains were located at grain boundaries of the alumina matrix grains. Additionally, sub-micrometer tungsten carbide spheres were found inside the alumina particles. Crack deflection triggered by the tungsten carbide at the grain boundaries of the alumina matrix is supposed to increase fracture toughness whereas the presence of intergranular and intragranular hard tungsten carbide particles are responsible for the increase of the hardness values of the investigated composite materials.     

Effect of cobalt powder morphology on the properties of WC-Co hard alloys

The effect of cobalt powder morphology on the microstructure of WC-Co hard alloys produced by sintering cobalt + tungsten carbide powder mixtures has been studied using X-ray diffraction, laser diffraction, scanning electron microscopy, density measurements, and Vickers microhardness tests. The results indicate that, under identical sintering conditions, the densest and most homogeneous microstructure is formed in hard alloys sintered using cobalt powders consisting of rounded particles. The use of cobalt powders with dendritic morphologies impedes the homogenization of Co + WC powder mixtures and preparation of pore-free WC-Co hard alloys.     

(Mo,W)5Si3–(Mo,W)Si2 Eutectics: Properties and Application in Composite Materials

Experimental data are presented on (Mo,W)₅Si₃ and (Mo,W)Si₂ solid solutions and are analyzed using the known phase diagrams of the binary systems Mo–Si and W–Si. It is shown that, with increasing tungsten content, the melting temperature of the (Mo,W)₅Si₃–(Mo,W)Si₂ eutectic rises. Surprisingly, the alloys with W : Mo atomic ratios close to unity are found to contain, along with the silicide solid solutions, molybdenum disilicide almost free of tungsten. The mean room-temperature hardness of the eutectic alloys varies nonmonotonically with tungsten content and shows maxima at 33 and 75 at. % W. The surface texture is found to have a significant effect on the rate of high-temperature gas corrosion. The possibility of compositional control (variations in the W : Mo and (Mo,W)₅Si₃ : (Mo,W)Si₂ ratios) over the thermal expansion of the alloys is analyzed. Data are presented on the temperature-dependent resistivity of SiC-matrix composites.     

Electrodeposition of cobalt–tungsten alloys from acidic bath containing cationic surfactants

The present work is devoted to the replacement of chromium coatings with a particular coating that has good properties and nonpolluting nature. Cobalt–tungsten alloys having high hardness were prepared by electrodeposition process. Optimum conditions of plating process including temperature, current density, concentration of metal ions and the critical concentration of the surfactants were studied. The surface properties of cationic surfactants were determined through surface tension measurements of the solution/air interface. Data of some surface and thermodynamic properties of the components were calculated. The alloys were subjected to heat treatment under nitrogen atmosphere at 400 °C. The surface morphologies of cobalt–tungsten alloy electrodeposits were studied before and after thermal treatment. The properties of the alloys such as microhardness and corrosion resistance were also examined and compared with chromium deposit.     

Mechanical properties of single crystal tungsten microwhiskers characterized by nanoindentation

The nanoindentation results in this work showed that the one-dimensional single crystal tungsten microwhiskers fabricated by vapor deposition possess unique yielding behavior. The average hardness of the microwhiskers measured on their (1 1 1) surfaces was 8.44 GPa, significantly higher than that of the bulk tungsten ranged from 3.4 to 5.8 GPa. The hardness increase was attributed to the lacking of dislocation avalanche in the 1D single crystal tungsten that was often observed in the nanoindentation of the bulk tungsten. However, the values of elastic modulus of the microwhiskers measured on the (1 1 1) surfaces were considerably scattered, whose average value is much lower than the reported value of 410 GPa for the bulk tungsten.     

Turning and Grinding of Hard Alloys

Hard alloys count among the hard-to-machine materials. Performance characteristics such as high hardness and wear resistance as well as the structural difference between hard phases and metallic matrix impede their machineability. The authors review the effects of the mechanical and thermal energy exerted by turning and grinding processes on the materials surfaces. Since high thermal surface loading may cause crack formation in hard phases as well as hardness increase and softening in the case of iron-based alloys they recommend to always take surface area requirements into account.     

Furnace-melted surface layers from carbide hard alloy powder compounds

No technological difficulties were encountered in the processing of pseudo-hard alloys in the form of powder compounds of conventional nickel-based hard alloys with carbides. The alloy greatly influences the resulting structures of the surface layers. Under some processing conditions tungsten carbide is completely dissolved by the molten alloy matrix. Hard phases based on chromium carbide form on cooling. The induced chromium carbide, Cr₃C₂, retains its structure while absorbing large amounts of iron into its matrix. It can be concluded that not only alloying properties but also to a great extent structural criteria determine the stability of the applied supplementary hard phases.     

Influence of aging treatments and alloying additives on the hardness of Al–11Si–2.5Cu–Mg alloys

This study investigated the effects of cooling rate, heat treatment as well as additions of Mn and Sr on hardness and hardening characteristics in Al–11Si–2.5Cu–Mg alloys. The results of scanning electron microscopy reveal that the age-hardening behaviour is related to the precipitation sequence of alloy. An energy dispersive spectroscopy analysis was used to identify the precipitated phases. The results also show that the hardness of the solution heat-treated samples is higher in air-cooled alloys than in furnace-cooled ones. Furthermore, the hardness observed in solution heat-treated samples is higher than in as-cast samples for air-cooled alloys, with the highest hardness level in the non-modified alloys. The highest hardness levels among the artificially aged samples were observed in the non-modified, air-cooled alloys. These levels occur after aging for longer times at lower temperatures (e.g. 30 h at 155 °C). The alloys studied did not display any softening after 44 h at 155 °C, whereas at 180 °C, softening was noted to occur after 10–15 h. At short aging times of 5–10 h, high hardness values may be obtained by aging at 180 °C. At aging temperatures of 200 °C, 220 °C and 240 °C, softening began after 2 h had elapsed. The cooling rate during solidification does not appear to have any significant effect on the precipitation characteristics and hardness of the Sr-modified alloys at certain aging temperatures. On the other hand, the effects of cooling rate may be clearly observed in the non-modified alloys. Manganese has a minimal effect on the hardness of the aged samples as it diminishes the potential action of age-hardening, while strontium lessens the hardness of the artificially aged samples. The effect of strontium, however, is more pronounced in the air-cooled alloys than in the furnace-cooled alloys. Strontium also has a noticeable effect on the reduction of hardness in aged Mg-containing Al–Si–Cu alloys, in that it affects the precipitates containing Cu and Mg.