Mechanical properties and rapid sintering of nanostructured WC and WC–TiAl3 hard materials by the pulsed current activated heating

Tungsten carbides are primarily used as cutting tools and abrasive materials in the form of composites with a binder metal, such as Co or Ni. However, these binder phases have inferior chemical characteristics compared to the carbide phase and the high cost of Ni or Co. Therefore, low corrosion resistance of the WC–Ni and WC–Co cermets has generated interest in recent years for alternative binder phases. In this study, TiAl₃ was used as a novel binder and consolidated by the pulsed current activated sintering (PCAS) method. Highly dense WC–TiAl₃ with a relative density of up to 99% was obtained within 2 min by PCAS under a pressure of 80 MPa. The method was found to enable not only the rapid densification but also the inhibition of grain growth preserving the nano-scale microstructure. The average grain sizes of the sintered WC and WC–TiAl₃ were lower than 100 nm. The addition of TiAl₃ to WC enhanced the toughness without great decrease of hardness due to crack deflection and decrease of grain size.     

Effect of the Cr content on the sintering behaviour of TiCN–WC–Ni–Cr3C2 powder mixtures

In TiCN–W–Cr–Ni cermets produced by liquid phase sintering melting occurs at lower temperatures as their Cr content increases. For low Cr additions (up to 4 wt.%) eutectic temperatures are close to those found in the TiC–WC–Ni system. For 8 wt.% Cr and above, temperatures are similar to those found in the Cr–Ni–C system. The precipitation of M₇C₃ carbides is observed to start at 8 wt.% Cr in samples sintered at 1425 °C for 1 h. This sets a limit for the Cr solubility in the binder phase of these cermets around 18 wt.%. The dissolution of WC and Cr₃C₂ particles starts at temperatures as low as 1150 °C, but that the homogenization of the binder phase is only achieved after melting. The carbonitride phase exhibits the typical precipitation of inner and outer rims onto Ti(C,N) cores. However, a fine precipitation of Ni-rich particles is found inside Ti(C,N) cores, likely related to coalescence phenomena.     

Effect of binder-phase modification and Cr3C2 addition on properties of WC-IOC0 cemented carbide

For production of fine-grained and corrosion-resistant tungsten carbide (WC) based cemented carbides, addition of chromium carbide (Cr₃3C₂) in small amounts is standard practice. No systematic study, however, has been made of the effects of large additions (maximum 6 wt % ) of Cr₃C₂ as a substitute for tungsten carbide. This study focuses on the effect of hard-phase substitution by C₃C₂ in WC-1OCo cemented carbide. An attempt is also made to modify the binder metal cobalt by partial or complete substitution of nickel. Specimens were prepared using the standard liquid-phase sintering process and were tested for sintered porosity, mechanical properties, corrosion resistance, and microstructural parameters. Results confirm the findings of earlier workers regarding grain refinement and improvement of mechanical properties upon the addition of small amounts (<2 wt%) of Cr₃C₂. Modification of the binder phase improves indentation fracture toughness and corrosion resistance. Addition of Cr₃C₂ independent of the binder type improves corrosion resistance.     

Ni3Al添加量对WC-Co硬质合金中WC晶粒形状的影响

采用粉末冶金制备技术,以粗WC粉末、Co粉和WC＋Ni3Al预合金粉末为原料制备出WC-40vol％（Co—Ni,Al）硬质合金。利用扫描电镜和透射电镜研究了不同NbAl含量对WC-40vol％（Co—Ni3Al）硬质合金中WC晶粒形状的影响规律。结果表明：W在Co粘结相中的固溶度接近25．4wt％,而W在Ni,Al粘结相中的固溶度接近9．5wt％,随着NbAl含量的增加,粘结相对W的固溶度减小,合金中的WC晶粒圆钝和细小;WC晶粒表面上出现明显的台阶。相应的,延长烧结时间,WC—Co—Ni3Al硬质合金具有与WC—Co硬质合金相同的WC生长行为,WC-40vol％（Co—Ni3Al）硬质合金中的WC晶粒表面上的台阶处出现明显的刻面。     

Diffusion parameters of grain-growth inhibitors in WC based hardmetals with Co,Fe/Ni and Fe/Co/Ni binder alloys

The diffusion behaviour of the grain-growth inhibitors (GGI) Cr and V during early sintering stages from 950 to 1150 °C was investigated by means of diffusion couples of the type WC-GGI-binder/WC-binder. Besides Co, also alternative Fe/Ni and Fe/Co/Ni binder alloys were investigated. It was found that the diffusion in green bodies differs significantly from sintered hardmetals. Diffusivities in the binder phase were determined from diffusion couples prepared from model alloys and were found to be almost equal for Co and alternative binder alloys. The diffusion parameters determined from green bodies allowed to estimate the GGI distribution in a hardmetal during heat up. This was subsequently used to estimate an appropriate grain size of VC and Cr₃C₂ in hardmetals, which is required to ensure a sufficient GGI distribution during sintering before WC grain-growth initiates.     

An investigation into the effects of HIP after sintering of WC-ZrC-Co-Cr3C2 cemented carbides

The sintering behaviour of cemented carbides based on WC-ZrC-Co-Cr₃C₂ powder mixtures have been analyzed by dilatometric and calorimetric methods for different cobalt contents and WC/ZrC ratios. As expected, powder oxide reduction in these compositions is mainly of carbothermic nature. However, depending on the milling conditions, some highly stable Zr-rich oxides are retained in the binder phase after sintering. Hot isostatic pressing (HIP) cycles have been successfully applied for closing residual porosity after vacuum sintering. For a fixed amount of binder phase and a WC/ZrC ratio, the hardness of these materials depends on the amount of residual porosity and WC grain growth control. The best combination of hardness and toughness is found for alloys with 8 wt%Co and WC/ZrC wt. ratios of 6.46. HIP treatments induce the formation of a compact and well adhered layer mainly comprised of Zr oxides and WC grains. The cobalt binder phase migrates from this layer towards the sample bulk likely due to the loss of wettability on these Zr rich oxides. Hot hardness is higher for the alloy with higher WC/ZrC ratio suggesting that this property depends on both the volume fraction of (Zr_xW_1-x)C and WC phases and their degree of contiguity.     

Effect of chromium and carbon contents on the sintering of WC-Fe-Ni-Co-Cr multicomponent alloys

WC-Fe-Ni-Co-Cr cemented carbides have been obtained by liquid phase sintering from WC-Fe-Ni-Co-Cr₃C₂ powder mixtures. Taking the 40wt%Fe-40wt%Ni-20wt.%Co alloy as a reference, new binder phases has been prepared by introducing controlled amounts of Cr and C, via Cr₃C₂ and C black powders respectively. As described for WC-Co-Cr materials, Cr additions are observed to reduce the eutectic temperatures of the WC-Fe-Ni-Co system. First liquids detected on heating exhibit wide temperature melting ranges, which become narrower and are displaced to higher temperatures on repeated heating and cooling cycles. Apart from the decarburization associated to the carbothermal reduction of powder oxides, this phenomenon could be also associated to the homogeneization of the chemical composition of these multicomponent binder phases, which is faster as C content decreases. Correlation between experimental melting and solidification temperature ranges and those predicted by Thermocalc® is better as Cr content increases. Experimental C windows, defined in this work by the absence of free C or η phases, are located at C contents higher than those estimated by Thermocalc®. Although the 40wt.%Fe-40wt.%Ni-20wt.%Co alloy is austenitc, BCC phases are partially stabilized at low C and high Cr contents. Although these compositions are free from η phases or free C, a precipitation of Cr-rich carbides is found at the WC-metal interface. These precipitates are not observed in the alloy with 0.75 wt% Cr (i.e. 5 wt% of the nominal metal content) and 5.39 wt%C. This C content is 0.17 wt% higher than that predicted for precipitation of M₇C₃.     

VC- and Cr3C2-doped WC–NbC–Co hardmetals

This study compares the microstructure and mechanical properties of plain and 0.9 or 3.6 wt% VC- or Cr₃C₂-doped WC–12 wt% Co hardmetals with 40 wt% NbC, prepared by pulsed electric current sintering (PECS) in the solid state for 4 min at 1240 °C and conventional pressureless liquid phase sintering (CS) for 1 h at 1420 °C. The addition of VC or Cr₃C₂ was found to inhibit grain growth of the residual WC grains, whereas the size of the solid solution (Nb,W,V/Cr)C grains was hardly influenced. The type of grain growth inhibitor and densification temperature however, strongly influenced the composition of the NbC solid solution formed, which was thermodynamically and experimentally assessed.     

Microstructure and mechanical properties of 90WC-8Ni-2Mo2C cemented carbide developed by conventional powder metallurgy

In this paper, the microstructure and mechanical properties of a WC-Ni based cemented carbide with the addition of 2 wt% Mo₂C, processed by conventional powder metallurgy, was investigated. With the addition of only Mo₂C in the WC-Ni alloy system, the wettability between the WC and Ni binder phase was improved, which was confirmed by the increased density, hardness, fracture toughness and flexure strength of the cemented carbide obtained, which is superior than those observed in WC-10Ni cemented carbides and similar to those observed in WC-Co and WC-Ni-TiC-Mo₂C cemented carbides. Microstructural examinations of the developed cemented carbide 90WC-8Ni-2Mo₂C indicated that there was no excessive grain growth of the WC particles during sintering, confirming that Mo₂C is a grain growth inhibitor as effective as other carbides such as VC, TiC, Cr₂O₃, showing that the addition of only Mo₂C is able to improve the overall mechanical properties of the WC-Ni alloy system without sacrificing the toughness.     

Pulsed electric current sintered Fe3Al bonded WC composites

WC based composites with 5, 10 and 20 vol.% Fe₃Al binder were consolidated via pulsed electric current sintering (PECS) in the solid state for 4 min at 1200 °C under a pressure of 90 MPa. Microstructural analysis revealed a homogeneous Fe₃Al binder distribution, ultrafine WC grains and dispersed Al₂O₃ particle clusters. The WC-5 vol.% Fe₃Al composite combines an excellent Vickers hardness of 25.6 GPa with very high Young’s modulus of 693 GPa, a fracture toughness of 7.6 MPa m^1/2 and flexural strength of 1000 MPa. With increasing Fe₃Al binder content, the hardness and stiffness decreased linearly to 19.9 and 539 GPa, respectively with increasing binder content up to 20 vol.%, while the fracture toughness and flexural strength were hardly influenced by the binder content. Compared to WC–Co cemented carbides processed under exactly the same conditions, the WC–Fe₃Al composites exhibit a substantially higher hardness and Young’s modulus.     

Microstructure and composition of the grain/binder interface in WC–Ni3Al composites

The microstructure and composition of WC/Ni₃Al interface were studied. An orientation relationship of [100]_WC//[110]_Ni3Al and (001)_WC//(001)_Ni3Al with a good coherence, besides many random orientation relationships between WC and Ni₃Al, has been repetitively found by selected area electron diffraction and high resolution TEM observations. The XRD pattern of WC–Ni₃Al composites indicated that the major binder phase was Ni₃Al and showed possibility of coherence between WC and Ni₃Al as common interplanar spacings existed. Electron probe microanalysis results revealed that the atomic ratio of Al:Ni is close to 1:3 in binder phase and WC/Ni₃Al interface in the WC–Ni₃Al composites has a sharper compositional gradient and a smaller width of transition region than the WC/Co interface in WC–Co composites.     

Effect of partial substitution of Cr for Ni on densification behavior, microstructure evolution and mechanical properties of Ti(C,N)-Ni-based cermets

Four cermets of composition TiC-10TiN-16Mo-6.5WC-0.8C-0.6Cr₃C₂-(32 − x)Ni-xCr (x = 0, 3.2, 6.4 and 9.6 wt%) were prepared, to investigate the effect of the partial substitution of Cr for Ni on densification behavior, microstructure evolution and mechanical properties of Ti(C,N)-Ni-based cermets. The partial substitution of Cr for Ni decreased full densification temperature, and the higher the content of Cr additive was, the lower full densification temperature was. The partial substitution of Cr for Ni had no significant effect of the formation of Mo₂C and Ti(C,N) and the dissolution of WC, and however, it had a significant effect on the dissolution of Mo₂C. Cr in Ni-based binder phase diffused into undissolved Mo₂C to form (Mo,Cr)₂C above 1000 °C at 6.4-9.6 wt% Cr additive, and a small amount of (Mo,Cr)₂C did not dissolve after sintering at 1410 °C for 1 h at 9.6 wt% Cr additive. In the final microstructure, Cr content in Ni-based binder phase increased with increasing the content of Cr additive, and however, regardless of the content of Cr additive, coarse Ti(C,N) grains generally consisted of black core, white inner rim and grey outer rim, and fine Ti(C,N) grains generally consisted of white core and grey rim. The partial substitution of Cr for Ni increased hardness and decreased transverse rupture strength (TRS). Ni-based binder phase became hard with increasing the content of Cr additive, therefore resulting in the increase of hardness and the decrease of TRS. TRS was fairly low at 9.6 wt% Cr additive, which was mainly attributed hardening of Ni-based binder phase and undissolved (Mo,Cr)₂C.     

Effects of Cr3C2 addition on the corrosion behavior of Ti(C,N)-based cermets

The Ti(C, N)-based cermets with different Cr₃C₂ addition were prepared and the effects of Cr₃C₂ addition on the microstructure and properties of cermets were discussed. The corrosion behavior of the cermets with different Cr₃C₂ addition was investigated emphatically in 2 mol/L HNO₃ solution. The results indicate that there is no obvious effect of Cr₃C₂ addition on the densification of the cermets, and all cermets are almost fully densified during sintering. The thickness of rim phase is reduced and the core size is increased remarkably in the cermets with 1 wt.% and 3 wt.% Cr₃C₂ addition; the grains are refined significantly in the cermets with the increase of Cr₃C₂ addition to 5 wt.%. The hardness and transverse rapture strength of the cermets are improved with Cr₃C₂ added properly. In HNO₃ solution, the corrosion resistance of cermets is improved remarkably by Cr₃C₂ addition. The corrosion of binder phase is predominant in the cermets in which the Ni binder phase without Cr has lower corrosion resistance than the rim phase; whereas the corrosion resistance of binder phase with high Cr content is better compared to the rim phase, so that the degradation of rim phase is predominant and a reticulate binder phase forms. With the increase of Cr₃C₂ addition, the Mo content in rim increases, and it is bad for the corrosion resistance of rim phase. Additionally, the inner rim phase has lower corrosion resistance than the outer rim phase owing to the higher Mo content.     

Rapid consolidation of nanostuctured WC-FeAl3 by pulsed current activated heating and its mechanical properties

In the case of cemented WC, Ni or Co is added as a binder for the formation of composite structures. However, the high cost of Ni or Co, the low hardness and the low corrosion resistance of the WC-Ni and WC-Co cermets have generated interest in recent years for alternative binder phases. In this study, FeAl₃ was used as a novel binder and consolidated by the pulsed current activated sintering (PCAS) method. Highly dense WC-FeAl₃ with a relative density of up to 97% was obtained within 2 min by PCAS under a pressure of 80 MPa. The average grain sizes of the sintered WC and WC-FeAl₃ were lower than 100 nm. The addition of FeAl₃ to WC enhanced the mechanical properties and increase of relative density.     

Gradient cemented carbide with β-phase free surface layer formed by nitrogen source AlN

Using AlN as nitrogen source, the gradient cemented carbide with β-phase free surface layer was fabricated in situ by one-step vacuum sintering. The β-phase free layer was explored by phase characterization, elemental-distribution analysis, microstructure and fracture observation. The results indicated that it was feasible to obtain β-phase free layer when utilizing AlN as the nitrogen source. The AlN decomposition fell appreciably to lower temperature in the presence of binder Co and Ni under vacuum sintering. The thickness of β-phase free layer could be tailored by controlling AlN contents, and the nitride former Al remained in the β-phase free layer. When applying nitrogen source AlN, the relative Ti content in the subsurface layer was lower than that with the conventional nitrogen source Ti(C,N) or TiN. By contrast, Co enrichment in the β-phase free layer is less significant than Ni as a result of higher solidification temperature. WC phases were much coarser averagely in the β-phase free layer than in the bulk, which was considered to be favorable for resisting against fracture. The β-phase free layer containing Al played a substantially improved role on the transverse rupture strength when AlN addition was 0.6 wt% and 1.2 wt%, while it is detrimental to the transverse rupture strength due to the formation of the intermetallic phase of Al and binder when AlN addition was 1.8 wt%.     

Carbothermic reduction of scheelite (CaWO4) doped with cobalt or nickel

Pure scheelite (CaWO₄) and carbon black mixtures, containing 0 or 2 wt% cobalt or nickel were prepared by 8 or 24 h planetary ball milling (PBM). The mixtures were studied by thermal analysis (TGA-DTA), isothermal annealing at 950, 1000 and 1100 °C, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Independently of the presence of transition metal (Co or Ni), the carbothermic reaction occurs through several steps, where the Ca:O atomic ratio goes from 1:4 to 1:1 according to the sequence: CaWO₄ → Ca₃WO₆ → CaO, with concomitant development of gaseous CO and formation of metal tungsten. Finally, tungsten is carburized to give nanostructured WC.For the first time we here show that Co and Ni have a noteworthy catalytic effect on the carbothermic reduction of CaWO₄. In particular, these transition metals catalyze both the two-step reduction of scheelite and the subsequent carburization of tungsten. In the latter case, formation of intermediate η phases (Me_xW_yC_z, with Me = Co or Ni) occurs.Doping with Co or Ni allows obtaining an almost quantitative (97%) conversion of scheelite into WC after 12 h at a temperature as low as 950 °C. The catalyst allows to reduce PBM duration as well, in that doped mixtures subjected to just 8 h PBM give WC yields larger than 90% after 1 h at 1100 °C, being W₂C the balance.The doping of CaWO₄:C mixtures with few weight percent of Co or Ni allows to produce nanostructured WC by reducing both the milling time and the annealing temperature. These results are particularly appealing from both industrial and sustainability point of view since they allow performing less energy-intensive syntheses of nanostructured WC powders from scheelite.     

Effects of Mo additions on the corrosion behavior of WC–TiC–Ni hardmetals in acidic solutions

The corrosion behavior of WC–TiC–Ni cemented carbides with the additions of molybdenum was investigated in 1 M H₂SO₄ and 1 M HCl solutions utilizing potentiodynamic polarization, long-term immersion tests and surface analytical techniques. The results show that the Mo additions can enhance the corrosion resistance of WC–TiC–Ni hardmetals owing to the formation of new (Ti, W, Mo)C phase based on TiC. Pseudopassivity was observed in the polarization curves of WC–Ni and WC–TiC–Ni hardmetals in HCl. However, the additions of molybdenum can make WC–TiC–Ni hardmetals passivated visibly in curves at 600 mV–1000 mV and enhance the pitting corrosion resistance due to the formation of MoO₃ film. In addition, all nickel based alloys show the typical passivation behavior in H₂SO₄ caused by the formation of NiSO₄, NiWO₄ and continuous metal oxide films.     

Si-modified aluminide coatings deposited on Ti46Al7Nb alloy by slurry method

Ti46Al7Nb alloy has been used as the research substrate material for the deposition of water-based slurries containing Al and Si powders. The diffusion treatment has been carried out at 950 °C for 4 h in Ar atmosphere. The structure of the silicon-modified aluminide coatings 40 μm thick is as follows: (a) an outer zone consisting of TiAl₃ phase and titanium silicides formed on the matrix grain boundaries composed of TiAl₃–type Ti₅Si₃; (b) a middle zone containing the same phase components with the matrix TiAl₃ and the silicides Ti₅Si₃, which formed columnar grains; (c) an inner zone, 2 μm thick, consisting of TiAl₂ phase. Cyclic oxidation tests were conducted in 30 cycles (690 h at high temperature) and showed a remarkably higher oxidation resistance of the Ti46Al7Nb alloy with the protective coating in comparison with the uncoated sample.     

Effect of Mo2C additions on the properties of SPS manufactured WC–TiC–Ni cemented carbides

The effect of spark plasma sintering (SPS) on the microstructure and mechanical properties of WC–Co and WC–Ni cemented carbides was studied, and compared to WC–Co produced by liquid phase sintering (LPS). There were finer WC grains with larger Co pools in the spark plasma sintered WC–Co, resulting in higher hardness and slightly lower fracture toughness than the liquid phase sintered WC–Co. The influence of the addition of 0.5–5 wt.%Mo₂C to WC-based cemented carbide containing 6.25 wt.%TiC and 9.3 wt.%Ni prepared by SPS was also studied. This addition improved the wettability between WC and Ni and lead to the improvements of microstructures, resulting in good combinations of hardness, fracture toughness and modulus of elasticity that were comparable to WC–Co based cemented carbides.     

冷轧热处理钛-铝箔制备微叠层Ti-Al系金属间化合物板材

通过钛箔、铝箔叠加烧结制备出微叠层Ti-Al系金属间化合物合金板材。对不同烧结条件下获得的板材组织和相组成进行分析。结果表明,当烧结温度到达到Al的熔点以上时,高温自蔓延反应（SHS）在Ti箔和Al箔之间发生,生成α-Ti、Ti3Al、TiAl、TiAl2和TiAl3等相;随着烧结时间的延长,α-Ti、TiAl2和TiAl3逐渐消失,最终获得包含Ti3Al和TiAl的叠层结构板材。由于铝的熔化,柯肯达尔效应和反应前、后摩尔体积的变化在烧结过程中产生大量的孔洞,随后的热压处理将孔洞消除并获得致密的合金板材。