Role of Co Content on Densification and Microstructure of WC–Co Cemented Carbides Prepared by Selective Laser Melting

WC–Co cemented carbides, well-known as the conventional tooling materials, have not been successfully produced by one step additive manufacturing processes such as selective laser melting(SLM) yet. The microstructure evolution as well as WC grain growth behavior has rarely been investigated in detail during SLM process. In this study, the WC–Co cemented carbides with different Co contents(12–32 wt%) were prepared by optimized SLM processes for comparative investigation of densification behavior, microstructure characterization and mechanical property. The increase in Co content in feedstock carbide granules can improve the densification behavior during SLM process. The SLM processed WC-12 Co shows larger average WC grain size and higher percentage of coarser WC grains as compared with both WC-20 Co and WC-32 Co. The microstructure characterization, combined with finite element simulation, shows the WC grain growth mechanisms include agglomeration and dissolution-deposition of WC during SLM process and agglomeration of WC is an important mechanism especially for WC–Co cemented carbides with Co content as low as 12 wt%. The comparison between horizontal(perpendicular to the SLM laser beam) and vertical(parallel to the SLM laser beam) cross sections of carbides shows that SLM process introduces a certain degree of microstructure and mechanical behavior anisotropy for WC-12 Co, WC-20 Co, and WC-32 Co.     

Non-equilibrium solidification of undercooled Co–Si melts

The effect of melt undercooling on solidification modes of peritectic Co–Si alloys has been studied by electromagnetic levitation. For Co_82.5Si_17.5, Co₈₁Si₁₉ and Co₇₅Si₂₅ alloys there are transitions towards direct crystallization of peritectic ε-Co and a metastable Co₃Si phase, respectively. For Co₇₃Si₂₇ a morphology change of the equilibrium α-Co₂Si phase was observed.     

Viscosity of WC–Co compacts during sintering

This paper describes experiments performed on WC–Co compacts in order to measure the viscosities of a Newtonian constitutive law commonly used to simulate sintering. An intermittent loading method is used during two series of experiments. The first series are dedicated to determining the axial viscosity and takes place in a loading-dilatometer. The second one takes advantage of a video-extensometry device and provides results about the viscous Poisson's ratio. The axial viscosity is obtained as a function of relative density and temperature. Viscosity shows strong exponential increase with increasing density during isothermal conditions but decreases from 10 to 1 GPa·s between 1100°C and 1325°C during a conventional sintering cycle. Viscous Poisson's ratio shows low values at low densities and increases to 0.5 at full density.     

Influence of O and Co on the early stages of sintering of WC–Co: a surface study by AES and STM

The influence of oxygen on the sintering behavior of WC–Co has been investigated by Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). Deposition of Co on the WC(0001) surface and subsequent annealing at 650°C results in a 2×2 reconstructed pre-cursor layer on top of which Co grows in weakly bound islands which can be moved on the surface by the STM tip. Annealing at 850°C removes excess Co and leaves only the 2×2 surface. Oxygen exposure of the 2×2 surface results in a “clustered” cobalt oxide overlayer which on annealing at 750°C breaks up and restores the 2×2 structure as the metallic Co wets the surface.     

Pulsed-laser fabrication of gas-filled hollow Co–Pt nanospheres

We report on nitrogen-filled hollow Co–Pt nanospheres produced via pulsed-laser ablation in ambient nitrogen gas. The resulting nanospheres are characterized by a single-crystalline face-centred cubic Co_55±3Pt_45±3 shell and a void filled with molecular nitrogen, typically occupying the sphere’s central region. The average diameter of the spheres and the voids is 35 ± 8 and 16 ± 2 nm, respectively. The calculated number density of nitrogen atoms, measured within these voids, is 1.58 ± 0.4 nm^?3. The resulting pressure in the voids near ambient temperature (300 K) and at the boiling temperature for the Co–Pt alloy (～3000 K) is estimated to be 1.9 ± 0.3 and 34.3 ± 9 MPa, respectively. The gas-filled Co–Pt hollow spheres are formed in only one step involving two physical processes. First, after each laser pulse, the vaporized, supersaturated Co–Pt ablated species are condensated in the plume under high pressure and temperature, resulting in nitrogen gas trapping. Between two laser pulses, the pressure and temperature in the plume drop rapidly, the nitrogen-rich liquid nanospheres become thermodynamically unstable and the nitrogen gas bubble starts to expand until the solidification of the nanospheres. The fast solidification of the solid shell prevents further outward diffusion of nitrogen and thus an amount of nitrogen gas is preserved in the void. These nanospheres have the potential in biomedical, magnetic and catalytic applications.     

Quantitative characterization of the microstructure and properties of nanocrystalline WC–Co bulk

Microstructure of the nanocrystalline WC–Co cermet bulk was quantitatively described by transmission electron microscopy based precession electron diffraction technology. It is discovered that the fraction of the Σ2 grain boundaries increases with the decrease of WC grain size. The effect of microstructure on mechanical properties depends on Co distribution, Σ2 boundaries fraction and WC grain contiguity.

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Electrodeposition of Ni–Co alloy films onto titanium substrate

Binary bright Ni–Co alloy films were electrodeposited on titanium in the chloride–sulfate electrolytes.The influences of Co2?concentration, current density, and temperature on the Ni–Co alloy films electrodeposition were investigated. The films were analyzed by scanning electron microscope(SEM), energy dispersive spectroscopy(EDS), and X-ray diffraction(XRD). Cathodic polarization for Ni–Co codeposition was performed on Ti working electrodes. With the increase of Co2?concentration, the Ni content in the films decreases and the current efficiency increases slightly. The Ni content increases with the increase of temperature, while it decreases with the increase of current density to a minimum and then increases. The cathodic reduction peak potential is measured to be-1.34 V. Anomalous deposition is found to occur in the Ni–Co codeposition. The SEM of Ni–Co alloy films shows that hydroxide particles are not present on the surface and fine grain, compact, smooth, and bright Ni–Co alloy films are obtained. The XRD result indicates that the deposited Ni–Co alloy film is Ni-solid solution with a facecentered cubic in structure.     

Strain-magnetization property of Ni-Mn-Ga(Co,Cu) microwires

Magnetization associated with reversible phase transformation or rearrangement of martensite variants of two kinds of shape memory alloys under the coupling of tensile stress were investigated.One is the austenitic Ni₄₆Mn₂₈Ga₂₀Co₃Cu₃ micro wire with the [001] preferred orientation,which exhibits enhanced cyclic stability and large fully recoverable strain(> 8%) due to the stress-induced reversible martensitic transformation at room temper...     

Evaluation of High Temperature Oxidation Behaviour of Nanostructured Cr/Co–Al Coatings

The high temperature oxidation behavior of sputtered Cr/Co–Al coatings fabricated by DC/RF magnetron sputtering on a superalloy substrate has been studied in the present work. The microstructural features and phase formation of the as-deposited coatings were characterized by FE-SEM, AFM, and XRD, respectively. Weight-change measurements were made to calculate the cyclic oxidation kinetics of the coated superalloy exposed to air at 900 °C. It was observed that the corrosion rate of sputtered Cr/Co–Al coated superalloy is lower than that of the uncoated superalloy owing to the formation of continuous, dense, adherent and protective oxide scales over the surface of the coatings. The protective oxide scales in the corroded coatings were basically the thin layer of Cr₂O₃, CoO, Al₂O₃ and CoCr₂O₄, which provide protection to the base superalloy at high temperature.     

Optimization of Cr-Content for High-Temperature Oxidation Behavior of Co–Re–Si-Base Alloys

The oxidation behavior of Co-17Re-xCr-2Si alloys containing 23, 25, 27 and 30 at.% chromium at 1,000 and 1,100 °C were investigated. Alloy Co–17Re–23Cr–2Si showed a poor oxidation resistance during exposure to laboratory air forming a two-layer external scale and a very thin discontinuous Cr₂O₃ layer at the oxide/substrate interface. The outer layer of the oxide scale consisted of CoO, whereas the inner layer was a porous mixture of CoCr₂O₄ spinel particles in a CoO matrix. The oxide scale was found to be non-protective in nature as the vaporization of Re-oxide took place during oxidation. An increase of chromium content from 23 at.% to 25 at.% improved significantly the alloy oxidation resistance; a compact protective Cr₂O₃-scale formed and prevented the rhenium oxide evaporation. The oxidation behavior of alloys containing 27 at.% and 30 at.% chromium were quite similar to that of Co–17Re–25Cr–2Si. The oxidation mechanism for Co–17Re–25Cr–2Si alloy was established and the subsurface microstructural changes were investigated by means of EBSD characterization.     

Electrodeposition of high performance multilayer coatings of Zn–Co using triangular current pulses

Abstract

Compositionally modulated alloy (CMA) coatings of Zn–Co were electrodeposited on to mild steel from an acid chloride bath containing thiamine hydrochloride, as an additive. Electroplating was carried out galvanostatically from a single bath containing Zn²⁺ and Co²⁺ ions. Gradual change in composition in each layer was effected by triangular current pulses, cycling between two cathode current densities. Compositionally modulated alloy coatings were developed under different conditions of cyclic cathode current density and number of layers, and their corrosion resistances were evaluated by potentiodynamic polarisation and electrochemical impedance spectroscopy. The formation of multilayer and corrosion mechanism was analysed using scanning electron microscopy. The corrosion resistances of CMA and monolithic alloy coatings were compared with that of the base metal. Compositionally modulated alloy coating at optimal configuration, represented as (Zn–Co)_{2·0/4·0/300}, was found to exhibit ～80 times better corrosion resistance compared with monolithic (Zn–Co)_3·0 alloy, deposited for the same length of time from the same bath. Improved corrosion resistance was attributed to the formation of n-type semiconductor film at the interface, supported by Mott–Schottky plots. Decrease in corrosion resistance at high degree of layering was found, and is due to lower relaxation time for redistribution of solutes in the diffusion double layer, during plating.     

Electrodeposition of Ni–Co alloys from a deep eutectic solvent

Pure Ni and three Ni–Co alloys films, i.e. Ni–4 wt.%Co, Ni–18 wt.%Co, and Ni–40 wt.%Co, are electrodeposited at room temperature from the choline chloride/ethylene glycol deep eutectic solvent dissolved by nickel or/and cobalt chlorides. Electrodeposition mechanism, microstructure, and corrosion properties of the films are investigated. Surface morphology and chemical composite of the films are significantly dependent on the Ni²⁺ and Co²⁺ concentrations in the electrolytes. Interestingly, it is found that the amount of cobalt in the Ni–Co alloy films is significantly lower than that present in the electrolytes, which indicates an absence of anomalous codeposition process for the non-aqueous electrolytes. However, anomalous codeposition of Ni–Co deposits is frequently observed for the aqueous electrolytes. The Ni–Co alloy films possess face-centered cubic structures and refined grains revealed by X-ray diffractometer and scanning electron microscope. Potentiodynamic polarization measurements show that the Ni film exhibits the noblest corrosion potential and the lowest corrosion current compared with the Ni–Co alloys films. Moreover, the more Co content the Ni–Co films have, the more negative corrosion potential and the higher corrosion current the films exhibit.     

Influences of sub-micrometer Ta and Co dopants on microstructure and properties of tungsten heavy alloys

Tungsten heavy alloys are aggregates of particles of tungsten bonded with Ni/Fe or Ni/Cu via liquidphase sintering. The sub-micrometer Ta Co powder was added to this aggregate to strengthen the bonding phase. It is found that the main fracture pattern of the alloys is cleavage of tungsten grains and ductile rupture of bond phase,leading to improved tensile strength and elongation. Dopant Ta can act as grain size inhibitor in tungsten heavy alloys.     

Influence of Cs+ ions on codeposition of SiC particles with Ni–Co alloy

Abstract

Ni–Co/SiC composite coatings were produced by electrodeposition from a Watt's-type bath. The effect of current density and SiC concentration on the weight percentage of embedded particles was determined. Enhanced SiC incorporation was observed in the presence of small amount of cesium ions in the plating bath. It was attributed to increased adsorption of Co²⁺ and Ni²⁺ on the particles induced by Cs⁺ ions. Preferential adsorption of Cs⁺ was also observed. Validation of the Guglielmi model was confirmed for the codeposition process in the Ni–Co/SiC system. The incorporation of SiC within the alloy matrix resulted in the improvement of the microhardness of the deposits. Morphology and particle distribution in the deposits was studied by optical and electron (SEM, TEM) microscopy.     

First Principles Study of Structural Stability and Magnetic Property of Non-equilibrium Co–Mo Alloys

First principles calculations are carried out to investigate the structural stability of several non-equilibrium intermetallic phases in the cobalt(Co)–Mo system using spin polarized projected augmented-wave potentials. It is revealed that the Co3Mo, CoMo, and CoMo3 alloys are energetically favored to be in D019, B11, and A15 structures, respectively,and that the magnetic moments of Co atoms would decrease rapidly with an increasing percentage of Mo content and would most probably disappear when the content of Mo is no less than 50 at%. Generally, the calculated results in the present work match well with the available experimental observations.     

Effect of citrate additive on the electrodeposition and corrosion behaviour of Zn–Co alloy

The electrodeposition of Zn–15Co alloy on a stainless steel substrate from a citrate bath has been investigated as a function of pH and citrate concentration in the electroplating bath. UV–vis results and speciation diagrams of zinc and cobalt in aqueous citrate solution were used to select deposition baths with different dominant Zn–Co citrate complexes. Voltammetry and potentiostatic methods were used to study the codeposition mechanism. The corrosion behaviour of electrodeposits of Zn–15Co alloys was studied by means of electrochemical experiments in a solution of 3.5% NaCl using potentiodynamic polarisation and open circuit potential. The results showed that the corrosion resistance of the deposits was highly influenced by the presence of citrate in the electroplating bath.