- Email: [email protected]
Solid-solution hardening of WC by rhenium
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Solid-solution hardening of WC by rhenium Chong Zhaoa, Hao Lua, Haibin Wanga, Fawei Tanga, Hongbo Nieb, Chao Houa, Xuemei Liua, ⁎ Xiaoyan Songa, , Zuoren Niea a College of Materials Science and Engineering, Key Laboratory of Advanced Functional Materials, Education Ministry of China, Beijing University of Technology, Beijing 100124, China b Xiamen Tungsten Co., Ltd., Xiamen 361009, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Tungsten carbide Solid-solution hardening Mechanical alloying Rhenium
A novel binderless tungsten carbide bulk alloyed with Re was prepared for the first time. The formation of solid solution of Re in WC was demonstrated by detailed microstructural and chemical analyses. The Re alloyed WC has very high indentation modulus and nano-hardness of 771 GPa and 36.0 GPa, respectively, which increased by 38% and 52% respectively as compared with those of the conventional tungsten carbide. The mechanisms for the hardening of the (W,Re) C solid solution were analyzed by the first-principles calculations. It indicated that alloying Re can increase the charge density and occupy more bonding states of WC, leading to strong chemical bonds hence high hardness. The present results show the feasibility of solid-solution hardening for WC to obtain even higher hardness. It thus provides a new approach to improve the mechanical properties of cement carbides by tailoring the composition and features of the hard matrix.
1. Introduction Tungsten carbide based cemented carbide has been widely used in a broad range of industrial applications, especially for the wear-resistant machining tools, due to the high wear resistance and good combination of high hardness and satisfactory toughness [1–3]. It is well known that the mechanical properties of cemented carbides are highly dependent on their microstructures, such as the grain size of WC, the volume fraction of the metallic binder, and the contiguity of WC grains [4–6]. Substantial efforts have been made to prepare cemented carbide with fine or nanoscale grain sizes due to the potential to dramatically improve the mechanical properties [7,8]. Because of the industrial significance, extensive studies are being conducted to investigate different process techniques and additions [9–12]. However, little attention has been paid to a different way to tailor the mechanical properties of cemented carbide by changing the mechanical properties of WC crystals themselves. If the hardness of WC itself could be increased, possible improvements of the mechanical properties and wear resistance of cemented carbide are expected. It would be also useful for developing binderless WC with fine microstructure and high hardness for some special applications in harsh working conditions, such as abrasively loaded water jet nozzles. Theoretically, this would be done by partially substituting the W atoms in WC by other metallic atoms. Such a strategy, which is well
⁎
known as solution hardening, has long been applied in many metallic and ceramic systems [13–15]. For WC, this requires that there is a solubility of another element in the WC lattice. It was found that Ti, V, Cr, Nb, Ta, etc. can be dissolved in WC, and the solubilities were measured by atom probe analysis (APT) and relatively low (10−3∼10-5) [16–19]. A series of tungsten carbides with substituting elements, such as (W,Ta)C and (W,Cr)C, have been prepared by carburization processes combined with high temperature liquid phase sintering with the addition of cobalt [20,21]. Unfortunately, the hardness of the co-doped tungsten carbides is significantly lower than the undoped WC. Until now, only the dissolution of aluminum has been claimed to be able to enhance the hardness and bending strength of WC [22–24]. According to the DFT calculations, tungsten aluminum ternary carbides synthesized from the three elements or from WC and Al are more thermodynamically favorable than from binary carbides WC and Al4C3 [25]. However, all the calculated tungsten aluminum ternary carbides are nonequilibrium phases and less stable than WC. Thus, the possibility to prepare (W,Al)C solid solution is questionable and unclear. Herrmann et al. argued that it is unlikely to form a (W,Al)C solid solution [26]. Only Al/Al2O3 as inclusions in WC grains were found in their experiments. Re is another promising solute element to harden WC. Recently, first-principles calculations study conducted by Yu et al. has found that alloying Re into WC crystal may increase its hardness [27]. Similar to
Corresponding author. E-mail address: [email protected] (X. Song).
https://doi.org/10.1016/j.jeurceramsoc.2019.09.050 Received 20 July 2019; Received in revised form 27 September 2019; Accepted 29 September 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Chong Zhao, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.09.050
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
sample was cooled down to the room temperature naturally. The control sample, pure WC, was prepared by the same procedure. The assintered samples were mechanically ground and then polished using diamond slurry (1–6 μm) for the subsequent microstructure observations.
W, Re has a variety of special potential applications depending on its unique properties, such as high melting temperature and high density. For instance, Re is widely used in the preparation for superalloys. W and Re alloys have been applied in aerospace and nuclear industries due to their high melting temperature, good corrosion resistance and heat resistance [28,29]. Re is one of the candidates alloying elements that would improve the mechanical properties of cemented carbides because of the potential to improve strength, especially at high temperatures. It is claimed that Re is dissolved in the cobalt based metallic binder and acts as a strong grain growth inhibitor during the liquid phase sintering [30]. However, it is unclear whether Re can be dissolved into WC grains to enhance the hardness of WC itself to improve the mechanical properties of cemented carbides. The difficulty to prepare ternary tungsten carbide solid solutions is mainly due to the high stability of WC and the nature of low solubilities of the substituting elements in WC crystal [17]. It is hard to prepare ternary tungsten carbide solid solutions from conventional experimental process, such as liquid phase sintering which may result in serious segregation of the solute elements. It is suggested that the nonequilibrium ternary tungsten carbide solid solution phases could be prepared by mechanical alloying (MA) achieved by high energy ball milling [31] and following spark plasma sintering (SPS) process. By repeated fracture and cold welding of the powder particles during the MA process, it is possible to make alloys from normally immiscible components [32]. It is also believed that the defects formed during the high energy ball milling are related to the formation of non-equilibrium structure solid solution [31]. Thanks to the advantages of the SPS process with a rapid heating rate, a short holding time and a fast cooling rate, the supersaturated solid solution in a nonequilibrium state and fine microstructure as that of the powder can be inherited and kept in the sintered bulk [33]. To the best of our knowledge, there are seldom experimental studies on solid-solution hardening for WC due to the aforementioned issues of low solubility and no reports on the effects of Re on the mechanical properties of WC. In this study, we prepared a new type of WC alloyed with Re, possessing increased high hardness, by combining mechanical alloying and spark plasma sintering. The microstructure of the prepared (W,Re)C was analyzed and the mechanism of the high hardness was studied by first-principles calculations.
2.2. Sample characterization The phase constitutions of the milled powders as well as the sintered bulk materials were examined by X-ray diffraction (XRD, Rigaku Ultimate IV diffractometer) using Cu Kα radiation in the range of 2θ = 20-90° with a scanning rate of 0.04°/s. The WC lattice parameters of the powders were evaluated using the Jade 6.0 software. The density of the sintered alloys was determined by Archimede method with an analytical balance. The morphology of the sample was observed by field emission scanning electron microscopy (SEM, Nova NanoSEM 200), and the distribution of the elements was determined by X-ray energydispersive spectrometer (EDS). The microstructure of the sintered sample was investigated via high resolution transmission electron microscopy (HRTEM) operated at 200 kV (JEM-2100 F) and high-angle annular dark-field (HAADF) imaging in an aberration-corrected scanning transmission electron microscope (STEM, FEI Titan, G2) operated at 300 kV. TEM samples of about 3 × 6 μm were lifted out from the polished surface via FIB by a Ga ion beam. DigitalMicrograph software provided by Gatan Inc. was used to analyze the selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) images. An X-ray photoelectron spectroscope (XPS, Kratos XSAM 800) was used to characterize the surface chemical state of the sintered bulk to analyze the chemical bonding of Re. The nanoindentation tests were performed using Berkovich diamond indenter on Nano Indenter G200 (Agilent Technologies). The continuous stiffness measurement (CSM) mode was applied, which could provide the indentation depth varying with indentation load. The indenter has a tip radius less than 20 nm and an included angle of 120°. Before measurement, the machine was calibrated carefully on a fused silica sample. The obtained data were recorded and analyzed using the NanoSuite software. 3. Results and discussion
2. Experimental
3.1. Phase constitutions and morphology of the powders
2.1. Materials preparation
The X-ray diffraction (XRD) patterns of the milled (W,Re)C powders as a function of milling time are shown in Fig. 1a and b. It can be seen that the raw powder of WC and Re shows distinct and sharp diffraction peaks at the initial stages. As milling time increased, intensities of Re peaks decreased continuously, and disappeared completely till the sample was milled for 30 h. It may imply that Re was dissolved into WC and (W,Re)C solid solution was obtained. In addition, it can be also seen that a remarkable peak broadening of WC was induced by the high energy ball milling. The analysis achieved by the Rietveld method of peak fitting indicates that the broadening of the peak can be ascribed to the reduction of crystallite size down to about 7 nm. The trends of average crystallite size of WC in the milled powders, as calculated by the Rietveld method, are illustrated in Fig. 1c as a function of milling time. It can be noted that ball-milling treatment is effective to reduce the average crystallite size to nanoscale in a relatively short time. The crystallite size decreased rapidly at the initial stage of milling and the average crystallite size of WC phase reached a minimal value of about 10 nm after milling for 20 h. The morphology of the fabricated (W,Re)C powders is shown in Fig. 1d and e. As shown, the particle size decreased as the milling time increased. Many platelets structures can be observed in the powders after milled by 36 h, indicating the formation of WC plates already during the high energy ball milling. According to the first-principles study [33], the equilibrium structures of WC should be truncated
Commercially available WC (mean particle size 23 μm, > 99.8% purity, Shanghai Zaibang Chemical Co. Ltd., China) and Re (mean particle size: 74 μm, > 99.99% purity, Advanced Technology & Materials Co. Ltd., China) were used as raw materials. The stearic acid was used as the milling media and carbon supplement (1 wt.%) in order to avoid the carbon deficiency during sintering. The molar ratio of WC to Re was 26:1. Ball milling of the mixed powders was performed in the planetary ball miller (Nanjing), equipped with special jars of 500 ml and three types of milling balls with 1–5 mm in diameter. Both the lining of the milling jars and the milling balls were made of cemented carbide (i.e. WC-8Co) to prevent contamination. In order to prevent oxidation, the mixed powders were weighed and put into the jar in an argon-filled glove box. The ball-to-powder weight ratio was fixed at 30:1 and the as-received mixtures were milled for 36 h (with half an hour cooling every 1.5 h) at a high rotation speed. In order to improve the density of the sintered bulk, the as-synthesized powders were firstly cold compacted under 500 MPa. The green samples had a diameter of 20 mm. Then the compacted powder was put into a graphite die and sintered in the spark plasma sintering (SPS, 3.20-MK-V, Sumitomo Coal Mining Co. Ltd., Japan) system. The compacted powder was heated to 1500 °C with a heating rate of 100 °C/min under an external pressure of 50 MPa, and was held at this temperature for 10 min. The sintered bulk 2
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
Fig. 1. Characterization of the milled (W,Re)C powders: (a) XRD patterns of the milled powders after various milling time; (b) Magnified view of the Re peaks; (c) Calculated average crystallite size as a function of milling time; (d, e) Morphologies of the milled powders after milling for 6 h and 36 h, respectively; (f) Particle size distribution of the 36 h-milled powder.
−10 > structure and the zone axes of the four grains are along the < 12 − − − 4 23 1 0− 1 > , < 2− , < 1− > and < 2 4 23 > , respectively. The interplanar 1 1) planes are measured to be 2.761 Å and distances of (0001) and (10− 1.838 Å, respectively, which are slightly smaller than the standard values of 2.831 Å and 1.883 Å for pure WC [34]. Similarly, the measured 1 0 ) planes are 2.474 Å and 2.445 Å, which interplanar distance of (10− are also smaller than the standard value of 2.512 Å. The shrinkage of the lattice is thought to be attributed to the dissolving of Re into the WC grains, since the size of Re atom is smaller than that of W atom (Rw = 1.30 Å vs RRe = 1.28 Å) [35]. Fig. 4 illustrates the High-Angle Annular Dark Field (HAADF) TEM image as well as the elements mappings of Re doped WC. The analyzed area is the boxed top right-hand corner of the sample shown in Fig. 3. As demonstrated, the sample only contains WC grains and no particles are precipitated. Re distributes relatively homogenously in the grains. Fig. 5 shows the HAADF atomic image of the sintered bulk. The measured interplanar distances of (0001) plane is 2.743 Å, which is smaller than the standard values of 2.831 Å for pure WC. The interplanar distance analysis based on the HAADF image is consistent to that based on the HRTEM image and SAED pattern. Fig. 5c illustrates the image analysis for the HAADF atomic image. In the HAADF atomic image, the heavy elements are brighter than the light ones. The brightness of Re and W atoms are similar to each other since Re and W are neighbors on the periodic table. Thus, the brightness of the atoms is analyzed by the image processing software, Image J, to distinguish Re and W. As illustrated, the Re atoms distribute relatively homogeneously. This further confirms the formation of (W,Re)C solid solution. In order to get detailed chemical state information of Re element in the sintered sample, XPS analysis were conducted and the spectra are shown in Fig. 6. The peaks for W and C can be observed in the survey spectrum from Fig. 6a, which is reasonable. However, the Re peak cannot be observed clearly due to the low amount of Re and the positions of Re peak and W peak are close to each other. Fig. 6b displays the Re 4f high resolution spectrum. As demonstrated, there are four strong peaks in the high resolution spectrum. The two peaks of elemental Re locate at 40.26 eV and 42.66 eV, correspond to the Re0 binding energy
polygon rather than platelet. Hence, the formed structure by ball milling is a non-equilibrium structure. The distribution of the (W,Re)C particle sizes is relatively homogeneous, which is evaluated by the linear intercept method and shown in Fig. 1f. The mean particle size is about 120 nm. It is thought that the powder and crystals in nanoscales are more reactive, which would promote the dissolving of Re into WC grains. 3.2. Microstructures of the sintered bulk Fig. 2a shows the surface morphology of the sintered bulk. The sample was nearly fully consolidated by the sintering process and no obvious pores or defects could be observed. The relative density was measured to be 98.3%, which is comparable to that of binderless WC reported in the literature [16,18]. Due to the short holding time and relatively low temperature in the SPS process, a small amount of micropores inevitably remained trapped within the grains/at the grain junctions. The EDS mapping was carried out for the red frame region of the sintered bulk surface as shown in Fig. 2(a). Only W, Re and C peak were detected as expected. And the corresponding composition is tabulated in the inset of Fig. 2(b). The content of Re was estimated to be about 2 at%, which was close to the designed ratio. Fig. 2c shows the Electron Backscattered Diffraction (EBSD) image of the sintered bulk. As illustrated, all grains have the WC hcp structure and no obvious texture can be observed. The largest grains were about 3 μm and the mean grain size was estimated to be around 0.55 μm. Only WC grains without metallic binder can be observed in the SEM/BSE and EBSD micrograph, which indicates the dissolution of Re into WC grains and the formation of (W,Re)C solid solution. The XRD pattern of the sintered bulk shown in Fig. 2d also confirms the formation of (W,Re)C solid solution. Only the peaks for WC are shown up and the peaks of Re or any other phases cannot be observed. High resolution TEM and SAED analyses were conducted to further confirm the formation of WC solid solution with the dissolving of Re. Four randomly selected grains with different sizes and shapes were analyzed. As shown in Fig. 3, all the grains have typical hcp type 3
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
Fig. 2. Characterization of the sintered (W,Re)C bulk: (a) SEM image of the microstructure; (b) EDS analysis of the composition in the marked region in (a); (c) EBSD image of the grain structure; (d) XRD pattern of the bulk sample.
carbon which has a large electronegativity, leading to Re partially loses electrons to form Re-C chemical bonds. Due to the absence of ReC precipitates in the SEM and TEM images, it is believed that Re is dissolved into WC grains and substitutes the W atoms. The XPS analysis
of Re4f7/2 and Re4f5/2, respectively [36]. This indicates that some of the Re atoms are not dissolved into WC and still in the pure metal state. The other two peaks at 40.91 eV and 43.31 eV, which have higher binding energy than the standard Re0 peaks. This implies that Re reacts with
Fig. 3. Microstructures of the Re doped WC bulk sample, with the high resolution TEM analyses and the SAED patterns of the marked grains. 4
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
Fig. 4. HAADF-STEM image of the sintered sample and the corresponding mapping of elements.
different crystal planes were averaged. As illustrated in Fig.7, there are relatively large variations in the nano-hardness and indentation modulus values, which can be attributed to the fact that crystal plane dependence of the mechanical properties. The averaged nano-hardness and indentation modulus of (W,Re)C are around 36.0 GPa and 771 GPa, respectively. Both of them are higher than those of WC, around 21.8 GPa and 616 GPa. The indentation modulus and nano-hardness of WC are enhanced by 38% and 52% when WC is alloyed with Re. It was found that the Young’s modulus of WC with Mo addition decreased with increasing the Mo content [19]. The reason was explained as the heterogeneous distribution of Mo in WC and the formation of Mo2C with a relatively low theoretical modulus value of 400 GPa. Similarly, the degradation in performance of WC solid solution were also found with the addition of Cr [21]. The Young’s modulus and hardness of WC with Ta addition are 531 ± 92 GPa and 28.5 ± 3.0 GPa tested by nanoindentation [21], which are much lower than the results shown in the present study. Compared to the reported nanoindentation results of (W,Ta)C, (W,Mo)C, (W,Cr)C crystallite in the literature, the addition of Re element has a significant effect to improve
also suggests the formation of (W,Re)C solid solution. Based on the above analysis (phase constitutions, microstructures, elements distributions, binding state), it is believed that Re would dissolve into WC to form (W,Re)C solid solution by the process combined high energy ball milling and SPS. 3.3. Mechanical properties of the sintered sample Nanoindentation tests were performed to measure the nano-hardness and indentation modulus of the sintered samples. The reason of using nano-indentation technique is to avoid the influence of grain size and grain boundaries. In the present study, a maximum depth of indentation was set to 120 nm for statistically reliable results. The Poisson’s ratios of the investigated samples were chosen to be ν = 0.24 [37]. It is known that the mechanical properties of WC grains have a strong crystal anisotropy, which has been shown in our previous study [38,39]. Particularly, the hardness of (0001) crystal plane is about 1 0 ) crystal plane. In order to exclude this potential twice of that of (10− effect of anisotropy, 100 indentation tests were performed on a 10 × 10 array with a distance of 3 μm between the indents. All tested values of
Fig. 5. Atomic images of the (W,Re)C bulk sample observed along < 1− 2 10 > direction: (a) HAADF image corresponding to the region marked “E” in Fig. 3; (b) The interplanar distance of (0001) plane; (c) Image analysis and identification of Re atoms. 5
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
Fig. 6. XPS spectra of the sintered bulk sample: (a) Survey spectrum; (b) High resolution spectrum of Re 4f. Fig. 7. Mechanical properties of the sintered bulk samples: (a) Nano-hardness and indentation modulus of (W,Re)C; (b) Comparison of the indentation modulus and nano-hardness between WC and (W,Re)C; (c) Comparison of the present results with those reported in the literature. Solid symbols represent the experimental data, hollow symbols represent the calculated data.
(W,Re)C are higher than those of WC, which is consistent to experimental measurements. The parameter of (G/B)2G is always used to predict the hardness of materials because this parameter reflects both elasticity and plasticity [45]. As demonstrated, the hardness of (W,Re)C would be higher than WC. Fig. 8b and c illustrate the charge distribu− tions on the (0001) plane and (11 2 0) plane of (W,Re)C. It can be seen that alloying Re may increase the charge density in WC and strengthen − the chemical bonds, especially for metal-carbon bonds on the (11 2 0) plane. This may explain the reason why alloying Re may harden WC. Fig. 8d illustrates the density of states (DOS) of WC with and without Re. As can be seen, alloying Re only changes the feature of DOS of WC slightly since the alloying content is low. A deep valley close to the Fermi level can be seen in the DOS curves. The electronic states below the valley are bonding states, and those above the valley are antibonding states. Fermi level is situated near the minimum of the DOS, and the valley separates the bonding states and the antibonding states. Alloying Re may shift the DOS curve towards low energy and the Fermi level becomes closer to the minimum of the DOS. Thus, more bonding states can be occupied and it may strengthen the chemical bonds by alloying Re into WC. Therefore, doping Re into WC may increase the elastic modulus and hardness of WC.
the mechanical properties of WC. Fig. 7(c) summaries the data for the indentation modulus and nano-hardness of WC and WC with the dissolving elements in the literature, together with the measured data in the present work. As illustrated, the sintered (W,Re)C in the present work has the highest indentation modulus and nano-hardness, which is much harder than the other WC based compounds reported in the literature. It is supposed that the novel solid solution (W,Re)C with high indentation modulus and nano-hardness is promising for applications in harsh abrasive working environments. The present study may provide a new approach to improve the mechanical properties of WC and cemented carbide by the strategy of solution hardening for WC. 3.4. Mechanisms for solid-solution hardening of (W,Re)C First-principles calculations were conducted to understand the mechanism of the high hardness of (W,Re)C. The calculations were carried out using the density functional theory (DFT) implemented in the Vienna Ab initio Simulation Package (VASP) [40–42] with the projector-augmented wave (PAW) potential [43]. The generalized gradient approximation (GGA) with the exchange-correlation functional of Perdew, Burke and Ernzerhof (PBE) [44] was employed. A 3 × 3 × 3 supercell for WC on its primitive cell was constructed. One W atom in the supercell was substituted by Re atom as shown in Fig. 8a. As demonstrated in Table 1, the calculated elastic moduli of 6
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
Fig. 8. Analyses on the hardening effect of the (W,Re)C solid solution: (a) The calculation model of (W,Re)C; (b, c) Distributions of the charge density on the (0001) and (11− 2 0) planes, respectively; (d) Densities of states of WC and (W,Re)C. The vertical dashed line denotes the Fermi energy (EF = 0 eV). [4] H. Engqvist, B. Uhrenius, Determination of the average grain size of cemented carbides, Int. J. Refract. Metals Hard Mater. 21 (2003) 31–35. [5] A.V. Shatov, S.S. Ponomarev, S.A. Firstov, R. Warren, The contiguity of carbide crystals of different shapes in cemented carbides, Int. J. Refract. Metals Hard Mater. 24 (2006) 61–74. [6] W.B. Zhan, H.B. Wang, S.H. Liang, X.M. Liu, X.Y. Song, Acceleration effect of cobalt on carburization of tungsten at low temperature, J. Alloys Compd. 732 (2018) 429–435. [7] S.X. Zhao, X.Y. Song, J.X. Zhang, X.M. Liu, Effects of scale combination and contact condition of raw powders on SPS sintered near-nanocrystalline WC–Co alloy, Mater. Sci. Eng. A 473 (2008) 323–329. [8] X.W. Liu, X.Y. Song, H.B. Wang, X.M. Liu, X.L. Wang, G.S. Guo, Preparation and mechanisms of cemented carbides with ultrahigh fracture strength, J. Appl. Cryst. 48 (2015) 1254–1263. [9] W.B. Liu, X.Y. Song, J.X. Zhang, F.X. Yin, G.Z. Zhang, A novel route to prepare ultrafine-grained WC–Co cemented carbides, J. Alloys Compd. 458 (2008) 366–371. [10] X.M. Liu, X.Y. Song, J.X. Zhang, S.X. Zhao, Temperature distribution and neck formation of WC–Co combined particles during spark plasma sintering, Mater. Sci. Eng. A 488 (2008) 1–7. [11] S. Patel, M. Kuttolamadom, Powder roll-compaction process for controlling grain orientation texture and size in spark plasma sintered carbides, Mater. Lett. 211 (2018) 153–156. [12] S. Farag, I. Konyashin, B. Ries, The influence of grain growth inhibitors on the microstructure and properties of submicron, ultrafine and nano-structured hardmetals–A review, Int. J. Refract. Metals Hard Mater. 77 (2018) 12–30. [13] M. Gutierrez, D. Araujo, P. Jurczak, J. Wu, H. Liu, Solid solution strengthening in GaSb/GaAs: a mode to reduce the TD density through Be-doping, Appl. Phys. Lett. 110 (2017) 92–103. [14] H.L. Shang, B.Y. Ma, K.C. Shi, R.B. Li, G.Y. Li, The strengthening effect of boron interstitial supersaturated solid solution on aluminum films, Mater. Lett. 192 (2017) 104–106. [15] L. Silvestroni, S. Failla, V. Vinokurov, I. Neshpor, O. Grigoriev, Core-shell structure: an effective feature for strengthening ZrB2 ceramics, Scripta Mater. 160 (2019) 1–4. [16] H.C. Kim, H.K. Park, I.K. Jeong, I.Y. Ko, I.J. Shon, Sintering of binderless WC–Mo2C hard materials by rapid sintering process, Ceram. Int. 34 (2008) 1419–1423. [17] J. Weidow, S. Johansson, H.O. Andren, G. Wahnstrom, Transition metal solubilities in WC in cemented carbide materials, J. Am. Ceram. Soc. 94 (2011) 605–610. [18] L. Zhang, S. Chen, C. Shan, F.J. Huang, X. Cheng, Y. Ma, Hot pressing densification of WC-MoxC binderless carbide, Trans. Nonferrous Met. Soc. China 22 (2012) 2027–2031. [19] C. Liu, M. Komatsu, A. Nino, S. Sugiyama, H. Taimatsu, Preparation of WC-MoC ceramics and their mechanical properties, J. Jpn. Soc. Powder Powder Metall. 59 (2012) 479–483. [20] L.S. Wan, L. Yang, X.P. Zeng, X.R. Lai, L.F. Zhao, Synthesis of Cr-doped APT in the evaporation and crystallization process and its effect on properties of WC-Co cemented carbide alloy, Int. J. Refract. Met. Hard Mater. 64 (2017) 248–254. [21] J. Weidow, A. Blomqvist, J. Salomonsson, S. Norgren, Cemented carbides based on WC pre-alloyed with Cr or Ta, Int. J. Refract. Met. Hard Mater. 49 (2015) 36–41. [22] Z.H. Qiao, X.F. Ma, W. Zhao, H.G. Tang, S.G. Cai, B. Zhao, A novel (W–Al)–C–Co composite cemented carbide prepared by mechanical alloying and hot-pressing
Table 1 The calculated elastic modulus of WC and (W,Re)C.
WC (W,Re)C
B (GPa)
G (GPa)
E (GPa)
(G/B)2G (GPa)
378 390
283 292
680 701
159 164
4. Conclusion In this work, a novel solid solution bulk of rhenium doped tungsten carbide has been preapred combining mechanical alloying and spark plasma sintering. The prepared (W,Re)C bulk material had a high relative density of 98.3%. It has high nano-hardness and indentation modulus of about 36.0 GPa and 771 GPa, respectively, which are 52% and 38% higher than those of WC. The mechanism of the hardening effect was revealed by the first-principles calculations. It indicated that Re alloying can increase the charge density and tailor the position of Fermi level of WC, leading to stronger chemical bonding and thus higher hardness. With its high nano-hardness and indentation modulus, the (W,Re)C solid solution is promising for applications in harsh abrasive working environments. The present study also provides a new approach to improve the mechanical properties of cemented carbides by tailoring the composition and features of the hard WC matrix. Acknowledgments This work was supported by the National Key Program of Research and Development (2018YFB0703902), the National Natural Science Foundation of China (51631002, 51425101, 51621003), and the Beijing Natural Science Foundation (2194068). References [1] B. Roebuck, E.A. Almond, Deformation and fracture processes and the physical metallurgy of WC-Co hardmetals, Int. Met. Rev. 33 (1988) 90–110. [2] G.S. Upadhyaya, Materials science of cemented carbides - an overview, Mater. Design 22 (2001) 483–489. [3] Z.Z. Fang, X. Wang, T. Ryu, K.S. Hwang, H.Y. Sohn, Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide—a review, Int. J. Refract. Metals Hard Mater. 27 (2009) 288–299.
7
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
[34] ICSD, (2012), 43380; 66751. [35] K. Barbalace. Periodic Table of Elements. EnvironmentalChemistry.com. 1995–2019, https://EnvironmentalChemistry.com/yogi/periodic/. [36] National Institute of Standard and Technology, NIST on line Databases, X-ray Photoelectron Spectroscopy Database, (2019) https://srdata.nist.gov/xps/ ElmComposition.aspx. [37] T. Csanadi, M. Blanda, N.Q. Chinh, P. Hvizdos, J. Dusza, Orientation-dependent hardness and nanoindentation-induced deformation mechanisms of WC crystals, Acta Mater. 83 (2015) 397–407. [38] X.L. Wang, H.B. Wang, R. Moscatelli, X.M. Liu, X.Y. Song, Cemented carbides with highly oriented WC grains and formation mechanisms, Mater. Sci. Eng. A 659 (2016) 76–83. [39] X.W. Liu, X.Y. Song, H.B. Wang, X.M. Liu, X.L. Wang, G.S. Guo, Preparation and mechanisms of cemented carbides with ultrahigh fracture strength, J. Appl. Cryst. 48 (2015) 1254–1263. [40] T. Sahraoui, A. Kellou, H.I. Faraoun, N. Fenineche, H. Aourag, C. Coddet, Ab initio calculations and experimental studies of site substitution of ternary elements in WC, Mater. Sci. Eng. B 107 (2004) 1–7. [41] G. Kresse, J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comp. Mater. Sci. 6 (1996) 15–50. [42] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 16 (1996) 11169–11186. [43] G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmentedwave method, Phys. Rev. B 59 (1999) 1758–1775. [44] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. [45] X.Q. Chen, H.Y. Niu, D.Z. Li, Y.Y. Li, Modeling hardness of polycrystalline materials and bulk metallic glasses, Intermetallics 19 (2011) 1275–1281.
sintering, Int. J. Refract. Met. Hard Mater. 26 (2008) 251–255. [23] Z.H. Qiao, X.F. Ma, W. Zhao, H.G. Tang, Microstructure, thermal stability and mechanical properties of the novel (W1-xAlx)C–Co (x = 0.2, 0.33, 0.4, 0.5) cemented carbide, Int. J. Refract. Met. Hard Mater. 27 (2009) 48–51. [24] M. Zakeri, M.R. Rahimipour, S.K. Sadrnezhad, R. Yazdanni, Preparation of alumina–tungsten carbide nanocomposite by mechano-chemical reduction of WO3 with aluminum and graphite, J. Alloys Compd. 491 (2010) 203–208. [25] G.D. Suetin, I.R. Shein, A.L. Ivanovskii, Electronic structure of tungsten aluminum carbides W2AlC and WAlC2, Russ. J. Inorg. Chem. 54 (2009) 1433–1439. [26] M. Herrmann, J. Raethel, L.M. Berge, On the possibility of the incorporation of Al into the WC lattice, Int. J. Refract. Met. Hard Mater. 41 (2013) 495–500. [27] R. Yu, Qi Zhang, Qian Zhan, Softest elastic mode governs materials hardness, Chin. Sci. Bull. 59 (2014) 1747–1754. [28] Y.C. Chen, Y.H. Li, N. Gao, H.B. Zhou, W.Y. Hu, G.H. Lu, F. Gao, H.Q. Deng, New interatomic potentials of W, Re and W-Re alloy for radiation defects, J. Nucl. Phys. Mater. Sci. Radiat. Appl. 502 (2018) 141–153. [29] A. Xu, D. Armstrong, C. Beck, M. Moody, G. Smith, P. Bagot, S. Roberts, Ion-irradiation induced clustering in W-Re-Ta, W-Re and W-Ta alloys: an atom probe tomography and nanoindentation study, Acta Mater. 124 (2017) 71–78. [30] I. Konyashina, S. Faraga, B. Riesa, B. Roebuckc, WC-Co-Re cemented carbides: structure, properties and potential applications, Int. J. Refract. Met. Hard Mater. 78 (2019) 247–253. [31] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001) 1–184. [32] X.M. Liu, X.Y. Song, S.X. Zhao, J.X. Zhang, Spark plasma sintering densification mechanism for cemented carbides with different WC particle sizes, J. Am. Ceram. Soc. 93 (2010) 3153–3158. [33] Y. Zhong, H. Zhu, L.L. Shaw, R. Ramprasad, The equilibrium morphology of WC particles – a combined ab initio and experimental study, Acta Mater. 59 (2011) 3748–3757.
8
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Solid-solution hardening of WC by rhenium Chong Zhaoa, Hao Lua, Haibin Wanga, Fawei Tanga, Hongbo Nieb, Chao Houa, Xuemei Liua, ⁎ Xiaoyan Songa, , Zuoren Niea a College of Materials Science and Engineering, Key Laboratory of Advanced Functional Materials, Education Ministry of China, Beijing University of Technology, Beijing 100124, China b Xiamen Tungsten Co., Ltd., Xiamen 361009, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Tungsten carbide Solid-solution hardening Mechanical alloying Rhenium
A novel binderless tungsten carbide bulk alloyed with Re was prepared for the first time. The formation of solid solution of Re in WC was demonstrated by detailed microstructural and chemical analyses. The Re alloyed WC has very high indentation modulus and nano-hardness of 771 GPa and 36.0 GPa, respectively, which increased by 38% and 52% respectively as compared with those of the conventional tungsten carbide. The mechanisms for the hardening of the (W,Re) C solid solution were analyzed by the first-principles calculations. It indicated that alloying Re can increase the charge density and occupy more bonding states of WC, leading to strong chemical bonds hence high hardness. The present results show the feasibility of solid-solution hardening for WC to obtain even higher hardness. It thus provides a new approach to improve the mechanical properties of cement carbides by tailoring the composition and features of the hard matrix.
1. Introduction Tungsten carbide based cemented carbide has been widely used in a broad range of industrial applications, especially for the wear-resistant machining tools, due to the high wear resistance and good combination of high hardness and satisfactory toughness [1–3]. It is well known that the mechanical properties of cemented carbides are highly dependent on their microstructures, such as the grain size of WC, the volume fraction of the metallic binder, and the contiguity of WC grains [4–6]. Substantial efforts have been made to prepare cemented carbide with fine or nanoscale grain sizes due to the potential to dramatically improve the mechanical properties [7,8]. Because of the industrial significance, extensive studies are being conducted to investigate different process techniques and additions [9–12]. However, little attention has been paid to a different way to tailor the mechanical properties of cemented carbide by changing the mechanical properties of WC crystals themselves. If the hardness of WC itself could be increased, possible improvements of the mechanical properties and wear resistance of cemented carbide are expected. It would be also useful for developing binderless WC with fine microstructure and high hardness for some special applications in harsh working conditions, such as abrasively loaded water jet nozzles. Theoretically, this would be done by partially substituting the W atoms in WC by other metallic atoms. Such a strategy, which is well
⁎
known as solution hardening, has long been applied in many metallic and ceramic systems [13–15]. For WC, this requires that there is a solubility of another element in the WC lattice. It was found that Ti, V, Cr, Nb, Ta, etc. can be dissolved in WC, and the solubilities were measured by atom probe analysis (APT) and relatively low (10−3∼10-5) [16–19]. A series of tungsten carbides with substituting elements, such as (W,Ta)C and (W,Cr)C, have been prepared by carburization processes combined with high temperature liquid phase sintering with the addition of cobalt [20,21]. Unfortunately, the hardness of the co-doped tungsten carbides is significantly lower than the undoped WC. Until now, only the dissolution of aluminum has been claimed to be able to enhance the hardness and bending strength of WC [22–24]. According to the DFT calculations, tungsten aluminum ternary carbides synthesized from the three elements or from WC and Al are more thermodynamically favorable than from binary carbides WC and Al4C3 [25]. However, all the calculated tungsten aluminum ternary carbides are nonequilibrium phases and less stable than WC. Thus, the possibility to prepare (W,Al)C solid solution is questionable and unclear. Herrmann et al. argued that it is unlikely to form a (W,Al)C solid solution [26]. Only Al/Al2O3 as inclusions in WC grains were found in their experiments. Re is another promising solute element to harden WC. Recently, first-principles calculations study conducted by Yu et al. has found that alloying Re into WC crystal may increase its hardness [27]. Similar to
Corresponding author. E-mail address: [email protected] (X. Song).
https://doi.org/10.1016/j.jeurceramsoc.2019.09.050 Received 20 July 2019; Received in revised form 27 September 2019; Accepted 29 September 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Chong Zhao, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.09.050
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
sample was cooled down to the room temperature naturally. The control sample, pure WC, was prepared by the same procedure. The assintered samples were mechanically ground and then polished using diamond slurry (1–6 μm) for the subsequent microstructure observations.
W, Re has a variety of special potential applications depending on its unique properties, such as high melting temperature and high density. For instance, Re is widely used in the preparation for superalloys. W and Re alloys have been applied in aerospace and nuclear industries due to their high melting temperature, good corrosion resistance and heat resistance [28,29]. Re is one of the candidates alloying elements that would improve the mechanical properties of cemented carbides because of the potential to improve strength, especially at high temperatures. It is claimed that Re is dissolved in the cobalt based metallic binder and acts as a strong grain growth inhibitor during the liquid phase sintering [30]. However, it is unclear whether Re can be dissolved into WC grains to enhance the hardness of WC itself to improve the mechanical properties of cemented carbides. The difficulty to prepare ternary tungsten carbide solid solutions is mainly due to the high stability of WC and the nature of low solubilities of the substituting elements in WC crystal [17]. It is hard to prepare ternary tungsten carbide solid solutions from conventional experimental process, such as liquid phase sintering which may result in serious segregation of the solute elements. It is suggested that the nonequilibrium ternary tungsten carbide solid solution phases could be prepared by mechanical alloying (MA) achieved by high energy ball milling [31] and following spark plasma sintering (SPS) process. By repeated fracture and cold welding of the powder particles during the MA process, it is possible to make alloys from normally immiscible components [32]. It is also believed that the defects formed during the high energy ball milling are related to the formation of non-equilibrium structure solid solution [31]. Thanks to the advantages of the SPS process with a rapid heating rate, a short holding time and a fast cooling rate, the supersaturated solid solution in a nonequilibrium state and fine microstructure as that of the powder can be inherited and kept in the sintered bulk [33]. To the best of our knowledge, there are seldom experimental studies on solid-solution hardening for WC due to the aforementioned issues of low solubility and no reports on the effects of Re on the mechanical properties of WC. In this study, we prepared a new type of WC alloyed with Re, possessing increased high hardness, by combining mechanical alloying and spark plasma sintering. The microstructure of the prepared (W,Re)C was analyzed and the mechanism of the high hardness was studied by first-principles calculations.
2.2. Sample characterization The phase constitutions of the milled powders as well as the sintered bulk materials were examined by X-ray diffraction (XRD, Rigaku Ultimate IV diffractometer) using Cu Kα radiation in the range of 2θ = 20-90° with a scanning rate of 0.04°/s. The WC lattice parameters of the powders were evaluated using the Jade 6.0 software. The density of the sintered alloys was determined by Archimede method with an analytical balance. The morphology of the sample was observed by field emission scanning electron microscopy (SEM, Nova NanoSEM 200), and the distribution of the elements was determined by X-ray energydispersive spectrometer (EDS). The microstructure of the sintered sample was investigated via high resolution transmission electron microscopy (HRTEM) operated at 200 kV (JEM-2100 F) and high-angle annular dark-field (HAADF) imaging in an aberration-corrected scanning transmission electron microscope (STEM, FEI Titan, G2) operated at 300 kV. TEM samples of about 3 × 6 μm were lifted out from the polished surface via FIB by a Ga ion beam. DigitalMicrograph software provided by Gatan Inc. was used to analyze the selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) images. An X-ray photoelectron spectroscope (XPS, Kratos XSAM 800) was used to characterize the surface chemical state of the sintered bulk to analyze the chemical bonding of Re. The nanoindentation tests were performed using Berkovich diamond indenter on Nano Indenter G200 (Agilent Technologies). The continuous stiffness measurement (CSM) mode was applied, which could provide the indentation depth varying with indentation load. The indenter has a tip radius less than 20 nm and an included angle of 120°. Before measurement, the machine was calibrated carefully on a fused silica sample. The obtained data were recorded and analyzed using the NanoSuite software. 3. Results and discussion
2. Experimental
3.1. Phase constitutions and morphology of the powders
2.1. Materials preparation
The X-ray diffraction (XRD) patterns of the milled (W,Re)C powders as a function of milling time are shown in Fig. 1a and b. It can be seen that the raw powder of WC and Re shows distinct and sharp diffraction peaks at the initial stages. As milling time increased, intensities of Re peaks decreased continuously, and disappeared completely till the sample was milled for 30 h. It may imply that Re was dissolved into WC and (W,Re)C solid solution was obtained. In addition, it can be also seen that a remarkable peak broadening of WC was induced by the high energy ball milling. The analysis achieved by the Rietveld method of peak fitting indicates that the broadening of the peak can be ascribed to the reduction of crystallite size down to about 7 nm. The trends of average crystallite size of WC in the milled powders, as calculated by the Rietveld method, are illustrated in Fig. 1c as a function of milling time. It can be noted that ball-milling treatment is effective to reduce the average crystallite size to nanoscale in a relatively short time. The crystallite size decreased rapidly at the initial stage of milling and the average crystallite size of WC phase reached a minimal value of about 10 nm after milling for 20 h. The morphology of the fabricated (W,Re)C powders is shown in Fig. 1d and e. As shown, the particle size decreased as the milling time increased. Many platelets structures can be observed in the powders after milled by 36 h, indicating the formation of WC plates already during the high energy ball milling. According to the first-principles study [33], the equilibrium structures of WC should be truncated
Commercially available WC (mean particle size 23 μm, > 99.8% purity, Shanghai Zaibang Chemical Co. Ltd., China) and Re (mean particle size: 74 μm, > 99.99% purity, Advanced Technology & Materials Co. Ltd., China) were used as raw materials. The stearic acid was used as the milling media and carbon supplement (1 wt.%) in order to avoid the carbon deficiency during sintering. The molar ratio of WC to Re was 26:1. Ball milling of the mixed powders was performed in the planetary ball miller (Nanjing), equipped with special jars of 500 ml and three types of milling balls with 1–5 mm in diameter. Both the lining of the milling jars and the milling balls were made of cemented carbide (i.e. WC-8Co) to prevent contamination. In order to prevent oxidation, the mixed powders were weighed and put into the jar in an argon-filled glove box. The ball-to-powder weight ratio was fixed at 30:1 and the as-received mixtures were milled for 36 h (with half an hour cooling every 1.5 h) at a high rotation speed. In order to improve the density of the sintered bulk, the as-synthesized powders were firstly cold compacted under 500 MPa. The green samples had a diameter of 20 mm. Then the compacted powder was put into a graphite die and sintered in the spark plasma sintering (SPS, 3.20-MK-V, Sumitomo Coal Mining Co. Ltd., Japan) system. The compacted powder was heated to 1500 °C with a heating rate of 100 °C/min under an external pressure of 50 MPa, and was held at this temperature for 10 min. The sintered bulk 2
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
Fig. 1. Characterization of the milled (W,Re)C powders: (a) XRD patterns of the milled powders after various milling time; (b) Magnified view of the Re peaks; (c) Calculated average crystallite size as a function of milling time; (d, e) Morphologies of the milled powders after milling for 6 h and 36 h, respectively; (f) Particle size distribution of the 36 h-milled powder.
−10 > structure and the zone axes of the four grains are along the < 12 − − − 4 23 1 0− 1 > , < 2− , < 1− > and < 2 4 23 > , respectively. The interplanar 1 1) planes are measured to be 2.761 Å and distances of (0001) and (10− 1.838 Å, respectively, which are slightly smaller than the standard values of 2.831 Å and 1.883 Å for pure WC [34]. Similarly, the measured 1 0 ) planes are 2.474 Å and 2.445 Å, which interplanar distance of (10− are also smaller than the standard value of 2.512 Å. The shrinkage of the lattice is thought to be attributed to the dissolving of Re into the WC grains, since the size of Re atom is smaller than that of W atom (Rw = 1.30 Å vs RRe = 1.28 Å) [35]. Fig. 4 illustrates the High-Angle Annular Dark Field (HAADF) TEM image as well as the elements mappings of Re doped WC. The analyzed area is the boxed top right-hand corner of the sample shown in Fig. 3. As demonstrated, the sample only contains WC grains and no particles are precipitated. Re distributes relatively homogenously in the grains. Fig. 5 shows the HAADF atomic image of the sintered bulk. The measured interplanar distances of (0001) plane is 2.743 Å, which is smaller than the standard values of 2.831 Å for pure WC. The interplanar distance analysis based on the HAADF image is consistent to that based on the HRTEM image and SAED pattern. Fig. 5c illustrates the image analysis for the HAADF atomic image. In the HAADF atomic image, the heavy elements are brighter than the light ones. The brightness of Re and W atoms are similar to each other since Re and W are neighbors on the periodic table. Thus, the brightness of the atoms is analyzed by the image processing software, Image J, to distinguish Re and W. As illustrated, the Re atoms distribute relatively homogeneously. This further confirms the formation of (W,Re)C solid solution. In order to get detailed chemical state information of Re element in the sintered sample, XPS analysis were conducted and the spectra are shown in Fig. 6. The peaks for W and C can be observed in the survey spectrum from Fig. 6a, which is reasonable. However, the Re peak cannot be observed clearly due to the low amount of Re and the positions of Re peak and W peak are close to each other. Fig. 6b displays the Re 4f high resolution spectrum. As demonstrated, there are four strong peaks in the high resolution spectrum. The two peaks of elemental Re locate at 40.26 eV and 42.66 eV, correspond to the Re0 binding energy
polygon rather than platelet. Hence, the formed structure by ball milling is a non-equilibrium structure. The distribution of the (W,Re)C particle sizes is relatively homogeneous, which is evaluated by the linear intercept method and shown in Fig. 1f. The mean particle size is about 120 nm. It is thought that the powder and crystals in nanoscales are more reactive, which would promote the dissolving of Re into WC grains. 3.2. Microstructures of the sintered bulk Fig. 2a shows the surface morphology of the sintered bulk. The sample was nearly fully consolidated by the sintering process and no obvious pores or defects could be observed. The relative density was measured to be 98.3%, which is comparable to that of binderless WC reported in the literature [16,18]. Due to the short holding time and relatively low temperature in the SPS process, a small amount of micropores inevitably remained trapped within the grains/at the grain junctions. The EDS mapping was carried out for the red frame region of the sintered bulk surface as shown in Fig. 2(a). Only W, Re and C peak were detected as expected. And the corresponding composition is tabulated in the inset of Fig. 2(b). The content of Re was estimated to be about 2 at%, which was close to the designed ratio. Fig. 2c shows the Electron Backscattered Diffraction (EBSD) image of the sintered bulk. As illustrated, all grains have the WC hcp structure and no obvious texture can be observed. The largest grains were about 3 μm and the mean grain size was estimated to be around 0.55 μm. Only WC grains without metallic binder can be observed in the SEM/BSE and EBSD micrograph, which indicates the dissolution of Re into WC grains and the formation of (W,Re)C solid solution. The XRD pattern of the sintered bulk shown in Fig. 2d also confirms the formation of (W,Re)C solid solution. Only the peaks for WC are shown up and the peaks of Re or any other phases cannot be observed. High resolution TEM and SAED analyses were conducted to further confirm the formation of WC solid solution with the dissolving of Re. Four randomly selected grains with different sizes and shapes were analyzed. As shown in Fig. 3, all the grains have typical hcp type 3
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
Fig. 2. Characterization of the sintered (W,Re)C bulk: (a) SEM image of the microstructure; (b) EDS analysis of the composition in the marked region in (a); (c) EBSD image of the grain structure; (d) XRD pattern of the bulk sample.
carbon which has a large electronegativity, leading to Re partially loses electrons to form Re-C chemical bonds. Due to the absence of ReC precipitates in the SEM and TEM images, it is believed that Re is dissolved into WC grains and substitutes the W atoms. The XPS analysis
of Re4f7/2 and Re4f5/2, respectively [36]. This indicates that some of the Re atoms are not dissolved into WC and still in the pure metal state. The other two peaks at 40.91 eV and 43.31 eV, which have higher binding energy than the standard Re0 peaks. This implies that Re reacts with
Fig. 3. Microstructures of the Re doped WC bulk sample, with the high resolution TEM analyses and the SAED patterns of the marked grains. 4
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
Fig. 4. HAADF-STEM image of the sintered sample and the corresponding mapping of elements.
different crystal planes were averaged. As illustrated in Fig.7, there are relatively large variations in the nano-hardness and indentation modulus values, which can be attributed to the fact that crystal plane dependence of the mechanical properties. The averaged nano-hardness and indentation modulus of (W,Re)C are around 36.0 GPa and 771 GPa, respectively. Both of them are higher than those of WC, around 21.8 GPa and 616 GPa. The indentation modulus and nano-hardness of WC are enhanced by 38% and 52% when WC is alloyed with Re. It was found that the Young’s modulus of WC with Mo addition decreased with increasing the Mo content [19]. The reason was explained as the heterogeneous distribution of Mo in WC and the formation of Mo2C with a relatively low theoretical modulus value of 400 GPa. Similarly, the degradation in performance of WC solid solution were also found with the addition of Cr [21]. The Young’s modulus and hardness of WC with Ta addition are 531 ± 92 GPa and 28.5 ± 3.0 GPa tested by nanoindentation [21], which are much lower than the results shown in the present study. Compared to the reported nanoindentation results of (W,Ta)C, (W,Mo)C, (W,Cr)C crystallite in the literature, the addition of Re element has a significant effect to improve
also suggests the formation of (W,Re)C solid solution. Based on the above analysis (phase constitutions, microstructures, elements distributions, binding state), it is believed that Re would dissolve into WC to form (W,Re)C solid solution by the process combined high energy ball milling and SPS. 3.3. Mechanical properties of the sintered sample Nanoindentation tests were performed to measure the nano-hardness and indentation modulus of the sintered samples. The reason of using nano-indentation technique is to avoid the influence of grain size and grain boundaries. In the present study, a maximum depth of indentation was set to 120 nm for statistically reliable results. The Poisson’s ratios of the investigated samples were chosen to be ν = 0.24 [37]. It is known that the mechanical properties of WC grains have a strong crystal anisotropy, which has been shown in our previous study [38,39]. Particularly, the hardness of (0001) crystal plane is about 1 0 ) crystal plane. In order to exclude this potential twice of that of (10− effect of anisotropy, 100 indentation tests were performed on a 10 × 10 array with a distance of 3 μm between the indents. All tested values of
Fig. 5. Atomic images of the (W,Re)C bulk sample observed along < 1− 2 10 > direction: (a) HAADF image corresponding to the region marked “E” in Fig. 3; (b) The interplanar distance of (0001) plane; (c) Image analysis and identification of Re atoms. 5
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
Fig. 6. XPS spectra of the sintered bulk sample: (a) Survey spectrum; (b) High resolution spectrum of Re 4f. Fig. 7. Mechanical properties of the sintered bulk samples: (a) Nano-hardness and indentation modulus of (W,Re)C; (b) Comparison of the indentation modulus and nano-hardness between WC and (W,Re)C; (c) Comparison of the present results with those reported in the literature. Solid symbols represent the experimental data, hollow symbols represent the calculated data.
(W,Re)C are higher than those of WC, which is consistent to experimental measurements. The parameter of (G/B)2G is always used to predict the hardness of materials because this parameter reflects both elasticity and plasticity [45]. As demonstrated, the hardness of (W,Re)C would be higher than WC. Fig. 8b and c illustrate the charge distribu− tions on the (0001) plane and (11 2 0) plane of (W,Re)C. It can be seen that alloying Re may increase the charge density in WC and strengthen − the chemical bonds, especially for metal-carbon bonds on the (11 2 0) plane. This may explain the reason why alloying Re may harden WC. Fig. 8d illustrates the density of states (DOS) of WC with and without Re. As can be seen, alloying Re only changes the feature of DOS of WC slightly since the alloying content is low. A deep valley close to the Fermi level can be seen in the DOS curves. The electronic states below the valley are bonding states, and those above the valley are antibonding states. Fermi level is situated near the minimum of the DOS, and the valley separates the bonding states and the antibonding states. Alloying Re may shift the DOS curve towards low energy and the Fermi level becomes closer to the minimum of the DOS. Thus, more bonding states can be occupied and it may strengthen the chemical bonds by alloying Re into WC. Therefore, doping Re into WC may increase the elastic modulus and hardness of WC.
the mechanical properties of WC. Fig. 7(c) summaries the data for the indentation modulus and nano-hardness of WC and WC with the dissolving elements in the literature, together with the measured data in the present work. As illustrated, the sintered (W,Re)C in the present work has the highest indentation modulus and nano-hardness, which is much harder than the other WC based compounds reported in the literature. It is supposed that the novel solid solution (W,Re)C with high indentation modulus and nano-hardness is promising for applications in harsh abrasive working environments. The present study may provide a new approach to improve the mechanical properties of WC and cemented carbide by the strategy of solution hardening for WC. 3.4. Mechanisms for solid-solution hardening of (W,Re)C First-principles calculations were conducted to understand the mechanism of the high hardness of (W,Re)C. The calculations were carried out using the density functional theory (DFT) implemented in the Vienna Ab initio Simulation Package (VASP) [40–42] with the projector-augmented wave (PAW) potential [43]. The generalized gradient approximation (GGA) with the exchange-correlation functional of Perdew, Burke and Ernzerhof (PBE) [44] was employed. A 3 × 3 × 3 supercell for WC on its primitive cell was constructed. One W atom in the supercell was substituted by Re atom as shown in Fig. 8a. As demonstrated in Table 1, the calculated elastic moduli of 6
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
Fig. 8. Analyses on the hardening effect of the (W,Re)C solid solution: (a) The calculation model of (W,Re)C; (b, c) Distributions of the charge density on the (0001) and (11− 2 0) planes, respectively; (d) Densities of states of WC and (W,Re)C. The vertical dashed line denotes the Fermi energy (EF = 0 eV). [4] H. Engqvist, B. Uhrenius, Determination of the average grain size of cemented carbides, Int. J. Refract. Metals Hard Mater. 21 (2003) 31–35. [5] A.V. Shatov, S.S. Ponomarev, S.A. Firstov, R. Warren, The contiguity of carbide crystals of different shapes in cemented carbides, Int. J. Refract. Metals Hard Mater. 24 (2006) 61–74. [6] W.B. Zhan, H.B. Wang, S.H. Liang, X.M. Liu, X.Y. Song, Acceleration effect of cobalt on carburization of tungsten at low temperature, J. Alloys Compd. 732 (2018) 429–435. [7] S.X. Zhao, X.Y. Song, J.X. Zhang, X.M. Liu, Effects of scale combination and contact condition of raw powders on SPS sintered near-nanocrystalline WC–Co alloy, Mater. Sci. Eng. A 473 (2008) 323–329. [8] X.W. Liu, X.Y. Song, H.B. Wang, X.M. Liu, X.L. Wang, G.S. Guo, Preparation and mechanisms of cemented carbides with ultrahigh fracture strength, J. Appl. Cryst. 48 (2015) 1254–1263. [9] W.B. Liu, X.Y. Song, J.X. Zhang, F.X. Yin, G.Z. Zhang, A novel route to prepare ultrafine-grained WC–Co cemented carbides, J. Alloys Compd. 458 (2008) 366–371. [10] X.M. Liu, X.Y. Song, J.X. Zhang, S.X. Zhao, Temperature distribution and neck formation of WC–Co combined particles during spark plasma sintering, Mater. Sci. Eng. A 488 (2008) 1–7. [11] S. Patel, M. Kuttolamadom, Powder roll-compaction process for controlling grain orientation texture and size in spark plasma sintered carbides, Mater. Lett. 211 (2018) 153–156. [12] S. Farag, I. Konyashin, B. Ries, The influence of grain growth inhibitors on the microstructure and properties of submicron, ultrafine and nano-structured hardmetals–A review, Int. J. Refract. Metals Hard Mater. 77 (2018) 12–30. [13] M. Gutierrez, D. Araujo, P. Jurczak, J. Wu, H. Liu, Solid solution strengthening in GaSb/GaAs: a mode to reduce the TD density through Be-doping, Appl. Phys. Lett. 110 (2017) 92–103. [14] H.L. Shang, B.Y. Ma, K.C. Shi, R.B. Li, G.Y. Li, The strengthening effect of boron interstitial supersaturated solid solution on aluminum films, Mater. Lett. 192 (2017) 104–106. [15] L. Silvestroni, S. Failla, V. Vinokurov, I. Neshpor, O. Grigoriev, Core-shell structure: an effective feature for strengthening ZrB2 ceramics, Scripta Mater. 160 (2019) 1–4. [16] H.C. Kim, H.K. Park, I.K. Jeong, I.Y. Ko, I.J. Shon, Sintering of binderless WC–Mo2C hard materials by rapid sintering process, Ceram. Int. 34 (2008) 1419–1423. [17] J. Weidow, S. Johansson, H.O. Andren, G. Wahnstrom, Transition metal solubilities in WC in cemented carbide materials, J. Am. Ceram. Soc. 94 (2011) 605–610. [18] L. Zhang, S. Chen, C. Shan, F.J. Huang, X. Cheng, Y. Ma, Hot pressing densification of WC-MoxC binderless carbide, Trans. Nonferrous Met. Soc. China 22 (2012) 2027–2031. [19] C. Liu, M. Komatsu, A. Nino, S. Sugiyama, H. Taimatsu, Preparation of WC-MoC ceramics and their mechanical properties, J. Jpn. Soc. Powder Powder Metall. 59 (2012) 479–483. [20] L.S. Wan, L. Yang, X.P. Zeng, X.R. Lai, L.F. Zhao, Synthesis of Cr-doped APT in the evaporation and crystallization process and its effect on properties of WC-Co cemented carbide alloy, Int. J. Refract. Met. Hard Mater. 64 (2017) 248–254. [21] J. Weidow, A. Blomqvist, J. Salomonsson, S. Norgren, Cemented carbides based on WC pre-alloyed with Cr or Ta, Int. J. Refract. Met. Hard Mater. 49 (2015) 36–41. [22] Z.H. Qiao, X.F. Ma, W. Zhao, H.G. Tang, S.G. Cai, B. Zhao, A novel (W–Al)–C–Co composite cemented carbide prepared by mechanical alloying and hot-pressing
Table 1 The calculated elastic modulus of WC and (W,Re)C.
WC (W,Re)C
B (GPa)
G (GPa)
E (GPa)
(G/B)2G (GPa)
378 390
283 292
680 701
159 164
4. Conclusion In this work, a novel solid solution bulk of rhenium doped tungsten carbide has been preapred combining mechanical alloying and spark plasma sintering. The prepared (W,Re)C bulk material had a high relative density of 98.3%. It has high nano-hardness and indentation modulus of about 36.0 GPa and 771 GPa, respectively, which are 52% and 38% higher than those of WC. The mechanism of the hardening effect was revealed by the first-principles calculations. It indicated that Re alloying can increase the charge density and tailor the position of Fermi level of WC, leading to stronger chemical bonding and thus higher hardness. With its high nano-hardness and indentation modulus, the (W,Re)C solid solution is promising for applications in harsh abrasive working environments. The present study also provides a new approach to improve the mechanical properties of cemented carbides by tailoring the composition and features of the hard WC matrix. Acknowledgments This work was supported by the National Key Program of Research and Development (2018YFB0703902), the National Natural Science Foundation of China (51631002, 51425101, 51621003), and the Beijing Natural Science Foundation (2194068). References [1] B. Roebuck, E.A. Almond, Deformation and fracture processes and the physical metallurgy of WC-Co hardmetals, Int. Met. Rev. 33 (1988) 90–110. [2] G.S. Upadhyaya, Materials science of cemented carbides - an overview, Mater. Design 22 (2001) 483–489. [3] Z.Z. Fang, X. Wang, T. Ryu, K.S. Hwang, H.Y. Sohn, Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide—a review, Int. J. Refract. Metals Hard Mater. 27 (2009) 288–299.
7
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
C. Zhao, et al.
[34] ICSD, (2012), 43380; 66751. [35] K. Barbalace. Periodic Table of Elements. EnvironmentalChemistry.com. 1995–2019, https://EnvironmentalChemistry.com/yogi/periodic/. [36] National Institute of Standard and Technology, NIST on line Databases, X-ray Photoelectron Spectroscopy Database, (2019) https://srdata.nist.gov/xps/ ElmComposition.aspx. [37] T. Csanadi, M. Blanda, N.Q. Chinh, P. Hvizdos, J. Dusza, Orientation-dependent hardness and nanoindentation-induced deformation mechanisms of WC crystals, Acta Mater. 83 (2015) 397–407. [38] X.L. Wang, H.B. Wang, R. Moscatelli, X.M. Liu, X.Y. Song, Cemented carbides with highly oriented WC grains and formation mechanisms, Mater. Sci. Eng. A 659 (2016) 76–83. [39] X.W. Liu, X.Y. Song, H.B. Wang, X.M. Liu, X.L. Wang, G.S. Guo, Preparation and mechanisms of cemented carbides with ultrahigh fracture strength, J. Appl. Cryst. 48 (2015) 1254–1263. [40] T. Sahraoui, A. Kellou, H.I. Faraoun, N. Fenineche, H. Aourag, C. Coddet, Ab initio calculations and experimental studies of site substitution of ternary elements in WC, Mater. Sci. Eng. B 107 (2004) 1–7. [41] G. Kresse, J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comp. Mater. Sci. 6 (1996) 15–50. [42] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 16 (1996) 11169–11186. [43] G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmentedwave method, Phys. Rev. B 59 (1999) 1758–1775. [44] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. [45] X.Q. Chen, H.Y. Niu, D.Z. Li, Y.Y. Li, Modeling hardness of polycrystalline materials and bulk metallic glasses, Intermetallics 19 (2011) 1275–1281.
sintering, Int. J. Refract. Met. Hard Mater. 26 (2008) 251–255. [23] Z.H. Qiao, X.F. Ma, W. Zhao, H.G. Tang, Microstructure, thermal stability and mechanical properties of the novel (W1-xAlx)C–Co (x = 0.2, 0.33, 0.4, 0.5) cemented carbide, Int. J. Refract. Met. Hard Mater. 27 (2009) 48–51. [24] M. Zakeri, M.R. Rahimipour, S.K. Sadrnezhad, R. Yazdanni, Preparation of alumina–tungsten carbide nanocomposite by mechano-chemical reduction of WO3 with aluminum and graphite, J. Alloys Compd. 491 (2010) 203–208. [25] G.D. Suetin, I.R. Shein, A.L. Ivanovskii, Electronic structure of tungsten aluminum carbides W2AlC and WAlC2, Russ. J. Inorg. Chem. 54 (2009) 1433–1439. [26] M. Herrmann, J. Raethel, L.M. Berge, On the possibility of the incorporation of Al into the WC lattice, Int. J. Refract. Met. Hard Mater. 41 (2013) 495–500. [27] R. Yu, Qi Zhang, Qian Zhan, Softest elastic mode governs materials hardness, Chin. Sci. Bull. 59 (2014) 1747–1754. [28] Y.C. Chen, Y.H. Li, N. Gao, H.B. Zhou, W.Y. Hu, G.H. Lu, F. Gao, H.Q. Deng, New interatomic potentials of W, Re and W-Re alloy for radiation defects, J. Nucl. Phys. Mater. Sci. Radiat. Appl. 502 (2018) 141–153. [29] A. Xu, D. Armstrong, C. Beck, M. Moody, G. Smith, P. Bagot, S. Roberts, Ion-irradiation induced clustering in W-Re-Ta, W-Re and W-Ta alloys: an atom probe tomography and nanoindentation study, Acta Mater. 124 (2017) 71–78. [30] I. Konyashina, S. Faraga, B. Riesa, B. Roebuckc, WC-Co-Re cemented carbides: structure, properties and potential applications, Int. J. Refract. Met. Hard Mater. 78 (2019) 247–253. [31] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001) 1–184. [32] X.M. Liu, X.Y. Song, S.X. Zhao, J.X. Zhang, Spark plasma sintering densification mechanism for cemented carbides with different WC particle sizes, J. Am. Ceram. Soc. 93 (2010) 3153–3158. [33] Y. Zhong, H. Zhu, L.L. Shaw, R. Ramprasad, The equilibrium morphology of WC particles – a combined ab initio and experimental study, Acta Mater. 59 (2011) 3748–3757.
8