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Scripta Materialia 68 (2013) 108–110

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On the formation of WC1�x in nanocrystalline cemented carbides

Yang Gao, Xiaoyan Song,⇑ Xuemei Liu, Chongbin Wei, Haibin Wang andGuangsheng Guo

College of Materials Science and Engineering, Key Lab of Advanced Functional Materials, Education Ministry of China, Beijing

University of Technology, Beijing 100124, People’s Republic of China

Received 23 May 2012; accepted 19 September 2012Available online 25 September 2012

In contrast to the general assumption that the WC1�x phase is stable only at temperatures above 2500 �C, in the present studyWC1�x is discovered to exist stably at room temperature in nanocrystalline cemented carbides. Based on characterization of themicrostructure, crystallographic features and orientation relationships, the formation mechanism for the WC1�x phase in the nano-crystalline structure is proposed, and the conditions of the surface energy and composition are discussed.� 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Cemented carbides; Nanocrystalline; Interface; Orientation relationship

Tungsten carbides are widely used in industry as,for example, cutting tools and wear-resistant materials[1]. It is generally known that WC is the dominant stablephase among the tungsten carbides. However, Kim et al.proposed that tungsten carbides have three stablephases, namely WC, W2C and WC1�x [2]. Based on den-sity of states calculations, it was estimated that the den-sity of states near the Fermi levels of the WC1�x phasewas about twice as large as that of the W2C phase andabout six times larger than that of the WC phase [3].Thus the WC1�x phase may be less stable than theWC and W2C phases that are often present in conven-tional polycrystalline tungsten carbides. Rudy and Hoff-man [4] proposed that the WC1�x phase formed ratherreadily from the melt, but the direct solid-state transfor-mation WCWC1�x was hindered and required that thetemperature should be varied slowly near the transfor-mation temperature. Kurlov and Gusev [5] proposedthat the WC1�x phase could be stable only at tempera-tures higher than 2516 �C (in the range 2516–2785 �C),from which a phase diagram for the W–C system wasdescribed by Okamoto [6].

The condition that the WC1�x phase exists at extre-mely high temperatures makes experimental work onthis phase very complicated and comprehensive investi-gations have not yet been performed. Hence, the litera-

1359-6462/$ - see front matter � 2012 Acta Materialia Inc. Published by Elhttp://dx.doi.org/10.1016/j.scriptamat.2012.09.016

⇑Corresponding author. Tel./fax: +86 10 67392311; e-mail:xysong@bjut.edu.cn

ture contains few systematic studies, particularlyexperimental ones, on the formation and the microstruc-tural characterization of the WC1�x phase. In our recentstudies, it was found that the high-temperature phases inconventional polycrystalline materials could exist stablyat the room temperature in nanocrystalline materialswith the same composition [7,8]. In the present work,we report the discovery of the WC1�x phase formed innanocrystalline tungsten carbides prepared by a novelroute combining in situ synthesis of composite powderand spark plasma sintering (SPS). Based on the stabil-ization of the WC1�x phase in the nanostructure, thecrystallographic characteristics involved in the coexis-tence of WC and WC1�x phases are demonstrated, andfurthermore the formation mechanism of the WC1�x

phase is proposed.Commercial blue tungsten oxide (WO2.9), cobalt

oxide (Co3O4) and carbon black powders were used asthe raw materials. WC–10 wt.% Co cemented carbidebulk was taken as the target material. The raw powderswere milled for 50 h using pure ethanol as the liquidmedium, with a ball-to-powder weight ratio of 10:1and a rotation speed of 480 rpm. The as-milled powdermixture was put in a vacuum furnace and heated at850 �C for 1 h to conduct the in situ reduction and car-bonization reactions [9], from which the nanoscale WC–Co composite powder was synthesized. Then 2.0 wt.%VC was added to the composite powder as a graingrowth inhibitor, and the powder was consolidated bythe SPS method. With reference to our previous study

sevier Ltd. All rights reserved.

Figure 2. TEM analyses on the phase boundary in the preparednanocrystalline WC-Co bulk material: (a) HRTEM analyses of theWC1�x phase at the (10�10)WC/(10�11)Co boundary; (b) composi-tion analysis of the Co phase in the vicinity of the phase boundary.

Y. Gao et al. / Scripta Materialia 68 (2013) 108–110 109

on the SPS preparation of nanocrystalline WC–Co bulkmaterials [10], a heating rate of 100 �C min�1, a sinteringtemperature of 1130 �C, a holding time of 5 min and aconstant pressure of 60 MPa were used as the sinteringparameters. The phase constitution of the compositepowder and the sintered bulk material was determinedby X-ray diffraction (XRD, Rigaku D/max-3c) withCu Ka radiation. The microstructure of the bulk mate-rial was examined by transmission electron microscopy(TEM, JEOL JEM-2010F) and high-resolution TEM(HRTEM).

The detailed TEM analyses of the microstructures ofthe prepared WC–10Co bulk material are shown in Fig-ure 1. It is observed that the sintered bulk has a rela-tively homogeneous grain structure (Fig. 1a), with amean grain size measured as 82 nm (see the grain sizedistribution in the inset in Fig. 1a). Figure 1b showsthe selected-area electron diffraction pattern (SADP)and its indexing for the sample, which indicates thatthe microstructure has the WC phase with hexagonalclose-packed crystal structure (WC-hcp) and Co phaseswith hexagonal (Co-hcp) and face-centered cubic (Co-fcc) structures, respectively. Local nanograin structuresare shown by the HRTEM images in Figure 1c and d,which correspond to the regions marked by the dashedcircle and square in Figure 1a, respectively. As indicatedby the indexing of the crystal planes, the two neighbor-ing WC grains have a distinctly large misorientation. Itis interesting to find that in the vicinity of the WC/WCgrain boundary, there exists a different phase extending�4–5 atomic layers, as shown by the enlarged images inthe insets in Figure 1c and d. From the indexing of thecrystallographic characteristics, information concerningthe crystal planes and the lattice parameter are obtained,i.e. the lattice constant of the cubic structure is 4.286 A.

Figure 1. TEM analyses of the microstructures in the preparednanocrystalline WC–Co bulk material: (a) bright-field image of thenanograin structure and the statistical grain size distribution; (b)SADP and its indexing of the WC–Co phase structure; (c) HRTEManalyses of the region marked by the dashed circle in (a), i.e. the(0001)WC/(111)WC1�x interface; (d) HRTEM analyses of the regionmarked by the dashed square in (a), i.e. the (1�101)WC/(110)WC1�x

interface.

Based on its crystallographic characteristics, the phaseformed at the WC/WC grain boundary is identified asWC1�x.

At some of the WC/Co phase boundaries, the WC1�x

phase was also found. As shown in Figure 2a, WC1�x

exists in the layer region of the (10–10)WC/(10–11)Co

phase boundary, as indexed in the inset. Compositionsat different positions in the Co phase were detected.For two kinds of representative positions, as indicatedin Figure 2b, the C concentrations are different in theCo phase. The energy spectrum analyses (element linescanning) show that in the vicinity of the WC1�x/Cointerface, the C concentration in the Co phase is clearlyhigher than that in the region close to the Co center, asshown by the data in the insets in Figure 2b. In addition,there is a gradient of the W concentration in the Cophase, indicating that a certain amount of C and Watoms dissolve in the Co phase. Since the C atoms havea higher solubility in the Co lattice structure than the Watoms, the atomic ratio in the WC phase close to thephase boundary no longer remains at 1:1. Consequently,a composition of WC1�x is obtained. From the compo-sition measurements in the local microstructure, asshown in Figure 2b, at the WC/Co phase boundary aphase with a composition estimated as WC0.64 can bedetermined.

From the viewpoint of crystallography, the resistanceof phase transformation from the hexagonal WC to thecubic WC1�x is reduced when the new phase forms onthe coherent or semicoherent crystal planes. As indi-cated in the diagrams of the local crystal structuresshown in Figure 3, the atomic arrangement on the(0001)WC plane matches well with that on the(111)WC1�x plane, with the interatomic distance of Watoms on the (00 01)WC plane (2.907 A) very close tothat on the (111)WC1�x plane (2.995 A), i.e. with a smallmisfit of �0.03. Therefore, the coherent relationship caneasily form at the (0001)WC/(11 1)WC1�x interface forthe two phases WC and WC1�x. On the (1�101)WC

plane, the interatomic distances of the W atoms are2.907 and 4.062 A, respectively, which are very closeto those on the (110)WC1�x plane with the values of2.995 and 4.235 A, with corresponding misfits of �0.03and 0.08. However, the position of C atoms on the(1�101)WC plane is obviously different from that onthe (110)WC1�x plane. Thus a semicoherent relationshipmay form at the (1�101)WC/ (110)WC1�x interface be-tween the two phases.

Figure 3. Diagrams of crystal structures of hexagonal WC and cubicWC1�x: (a) WC lattice cell; (b) (0001)WC plane; (c) (1�101)WC plane;(d) WC1�x lattice cell; (e) (111)WC1�x plane; (f) (110)WC1�x plane. Thebigger and smaller spheres represent W and C atoms, respectively.

110 Y. Gao et al. / Scripta Materialia 68 (2013) 108–110

Using the “slab” model [11,12], it is possible to calcu-late the surface energies of the low-index crystal planesin the WC and WC1�x structures. It was reported thatthe surface energy of the (0001)WC plane was about3.5 J m�2 [13], which was lower than the surface energiesof all the other WC planes. Moreover, the surface en-ergy of the (111)WC1�x plane was estimated to be about1.9 J m�2 [14]. Therefore, the interface energy of(0001)WC/(111)WC1�x is lower than that of any of theWC/WC grain boundaries. The surface energies of the(1�101)WC and (11 0)WC1�x planes were calculated tobe about 3.8 and 2.2 J m�2, respectively [13]. Thus theformation energy of the (1�101)WC/(110)WC1�x inter-face is also relatively lower than the WC/WC grainboundary energy. Therefore, the formation of theWC1�x phase at the (000 1)WC/(111)WC1�x and(1�101)WC/(110)WC1�x interfaces is favorable basedon energy considerations. The crystallographic analysiscombined with the energy estimation of the low-indexplanes in the crystal structures of the hexagonal WCand cubic WC1�x phases provide a reasonable explana-tion for the appearance of the WC1�x phase at the bor-der of the WC grains.

A nanocrystalline cemented carbide bulk materialwith a mean grain size of 82 nm was prepared by a newlydeveloped route combining in situ reduction and car-bonization reactions and the SPS technique. It wasfound that the cubic WC1�x phase coexists stably withthe hexagonal WC phase at room temperature at theWC/WC or WC/Co interfaces in the nanostructured ce-mented carbides. This is an unusual phase stability ascompared with the general assumption that the WC1�x

phase can exist stably only at temperatures above

2500 �C. It was demonstrated that the formation ofthe layer-like WC1�x at the (0001)WC/(11 1)WC1�x inter-face results from the low surface energy and the coher-ent relationship between the two phases, which areboth favorable for overcoming the phase transformationbarrier. Moreover, due to the higher solubility of Catoms in the Co phase than that of W atoms, the atomicratio of C and W in the vicinity of the nanoscale WC/Cophase boundaries is likely to be less than 1, leading to acomposition of WC1�x. The present study verified thatthe high-temperature WC1�x phase in conventionalpolycrystalline materials can be stable at room tempera-ture in nanostructures, so long as certain conditions,such as the interface energy, orientation relationshipand local atomic concentration, are fulfilled.

The work was supported by the National Nat-ural Science Foundation of China (51174009), BeijingNatural Science Foundation (2112006), German Re-search Foundation (DFG, SPP 1473-SO 1075/1-1) andthe Chinese National Programs for Fundamental Re-search and Development (2011CB612207).

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