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Co composite powder prepared by high energy ball milling
Scripta Materialia 49 (2003) 1123–1128 www.actamat-journals.com
Nanostructured WC/Co composite powder prepared by high energy ball milling F.L. Zhang
a,b,*
, C.Y. Wang b, M. Zhu
a
a
b
Department of Mechanical Engineering, South China University of Technology, Guangzhou 510640, PR China Department of Mechanical and Electronic Engineering, Guangdong University of Technology, Dongfeng East Road 729, Guangzhou 510090, PR China Received 1 October 2002; received in revised form 26 July 2003; accepted 11 August 2003
Abstract The microstructure of WC–10%Co nanocomposite prepared by high energy ball milling was investigated by X-ray diffraction and transmission electron microscopy. The WC phase was refined to a grain size of 11 nm after milling. The dislocation density in WC was high and caused severe lattice distortion. Dislocations with Burgers vector 1/3[1 2 1 0] were identified in some areas. Ó 2003 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. Keywords: Microstructure; Ball milling; WC–Co; Nanostructure; Dislocation
1. Introduction WC–Co hardmetals are widely used in metal cutting tools because of their high hardness, good wear-resistance, good fracture resistance and high temperature strength. Mechanical properties of hardmetals are strongly dependent on the microstructure of the WC–Co hardmetal, and additionally affected by the microstructure of WC powders before sintering. An important feature is that the toughness and the hardness increase simultaneously with the refining of WC. Therefore, development of nanostructured WC–Co hard* Corresponding author. Address: Department of Mechanical and Electronic Engineering, Guangdong University of Technology, Dongfeng East Road 729, Guangzhou 510090, PR China. Tel.: +86-02037627005; fax: +86-02037627005. E-mail address: [email protected] (F.L. Zhang).
metal has been extensively studied. The methods of synthesizing the nanostructured WC or WC–Co composite include spraying conversion process [1], co-precipitation [2], displacement reaction process [3], mechanochemical synthesis [4,5], high energy ball milling and so on. High energy ball milling is a simple and efficient way of manufacturing the fine powder with nanostructure except for the problem of contamination [6]. However, when milling WC– Co powder, the milling media can be made of WC–Co hardmetal to eliminate the contamination. Therefore it is a simple method to synthesize nanostructured WC–Co powder by high energy ball milling. After a certain period of high energy milling, the grain size of WC can be reduce to nanoscale together with a considerable internal strain [7]. Generally, the grain size reduction in ball milling is mainly due to the comminution, dislocationsÕ annihilation and recombining to small
1359-6462/$ - see front matter Ó 2003 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. doi:10.1016/j.scriptamat.2003.08.009
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angle grain boundaries separating individual grains [8]. As it has been proved, dislocations with the Burgers vectors of [0 0 0 1], 1/3[1 2 1 3] and 1/ 3[1 2 1 0], appeared in WC phase when the WC–Co hardmetal is deformed by compression [9]. The 1/ 3[1 2 1 3] and its dissociated fault 1/6[1 2 1 3] are also observed when WC is deformed by indentation [10]. But high energy ball milling is a different process, in which the repeated deformation, fracture and cold welding caused by continuous impact take place. So the different deformation may cause the difference of microstructure and defect structure. Yang et al. [11] found that high energy ball milling can partially change hexagonal WC into a orthorhombic phase by introduced stacking fault 1/6[ 12 1 3] on the plane of {1 0 1 0}. While, as for the detail of the defect structure and microstructure of WC phase in high energy ball-milled WC–Co composite, few results were reported, especially the direct observation by high resolution electron microscopy (HREM). Thus the purposes of this paper is to characterize the microstructure and the defect structure of WC in high energy ball milled WC–10%Co composite by means of X-ray diffraction, SEM, TEM and HREM.
covered with carbon after being shocked by ultrasonic for 20 min in dispersant (ethanol). TEM and HREM observation were carried out in JEM100CX and PHILIPS-CM300 respectively. SEM observations were carried out on PHILIPS XL30FEG.
2. Experimentals
b ¼ bd þ be ¼ Kk=ðd cos hÞ þ 4e tan h
WC (99.8%) with mean Fisher sub-sieve size (Fsss) particle size of 5.6 lm and Co (99%) with mean Fsss particle size of 1.0 lm were used as starting materials. Powder mixture of WC– 10%wt.Co mixed with ethanol were sealed in WC– Co hardmetal vial with WC–Co hardmetal balls (8 and 12 mm in diameter and 50–50 in weight percentage, respectively) in a glove box containing inert atmosphere of Ar. The ball to powder weight ratio is controlled to be 15:1. The milling process was carried on a high energy ball mill (Fritsch Pulverisette-5) with the selected rotation velocity of 250 rpm. After 10 h of milling, the powder was taken from the vial and dried at room temperature in Ar. The X-ray diffraction analysis was carried out in the PHILIPS X-Pert diffractometer with the Cu Ka radiation (k ¼ 0:15406 nm). The sample of TEM and HREM were dripped onto Cu grid
where b is the full width at half-maximum (FWHM) of the diffraction peak after instrument
3. Results and discussion X-ray diffraction pattern of WC–10%Co powder before and after high energy ball milling are given in Fig. 1. The starting WC–10%Co powder contains small amounts of W2 C phase, and the Co phase is composed of fcc(a) and hcp(e) phases. After 10 h of ball milling, diffraction peaks of a-Co disappear. This is because that fcc-Co transforms into hcp-Co due to mechanically induced transformation in the ball milling. This kind of allotropic transition in Co was reported in previous work and was considered depending on ball milling intensity [12,13]. It can also be seen from Fig. 1 that the diffraction peak of WC is broaden apparently after 10 h of milling, which is due to refining of grain size and increasing of internal strain resulted from ball milling. The grain size and the internal stress are calculated by Stokes and WilsonÕs formula, ð1Þ
Fig. 1. X-ray diffraction patterns of WC–10%Co powder before (a) and after 10 h milling (b).
F.L. Zhang et al. / Scripta Materialia 49 (2003) 1123–1128
correction; bd and be are FWHM caused by small grain size and internal stress, respectively; K is constant as 0.9; k is wavelength of the X-ray radiation; d and e are grain size and internal stress, respectively; and h is the Bragg angle. b and bs follow Cauchy form with the relationship: B0 ¼ b þ bs , where B0 and bs are FWHM of broadened Bragg peaks and the standard sampleÕs Bragg peaks, respectively. The starting WC with coarse grain size is set as standard sample. Bragg peak (0 0 0 1), (1 0 1 0), (1 0 1 1) is selected to calculate the average grain size of WC in high energy ball milled WC–10%Co mixture by using the Eq. (1). It is found that after 10 h milling, the grain size is reduced to 11 nm with internal stress reach to 1.44%. This is similar to the result obtained by Gillies [7]. It is also found that the peaks of W2 C and e-Co weakened and broadened after milling. But their accurate grain size and internal strain are difficult to estimate because of the small ratio of their weak peak intensity to background. Therefore, they were not further investigated in this work. The morphologies of SEM for WC–10%Co before and after milling are showed in Fig. 2(a) and (b). It can be seen that the particle size has been reduced after milling, with the small particles agglomerating into large particles. The large particles are about 2–5 lm. The discernable small particles are about 100–500 nm with the shape of sphere. The details of microstructure for milled WC– 10%Co powder can be revealed from TEM observation. Fig. 3(a) and (b) shows the bright field image and electron diffraction pattern of one particle of about 400 nm in diameter. The diffraction halos given in Fig. 3(b) are nearly continuous rings, which proved that the grains with different orientation are quite small. Diffraction halos of e-Co and W2 C are also found in Fig. 3(b), which match the result obtained from the X-ray diffraction profile of milled WC–10%Co in Fig. 1(b). The dark field image in Fig. 3(c) is taken by encircling a small part of {1 0 1 0} diffraction halo of WC phase. From the bright field image and the dark field image shown in Fig. 3(a) and (c), it can be seen that the particle is composed of WC phase with the size of about 10 nm, which corresponds to the result obtained from the X-ray diffraction
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Fig. 2. SEM images of WC–10%Co powder mixture before (a) and after 10 h milling (b).
analysis. This means that WC phase has been refined to nanocrystalline in WC–10%Co mixture by milling. The refining of grains during milling process resulted from the movement, annihilation and recombination of large number of dislocations. Therefore, a large amount of high angle grain boundaries are formed to separate nanosize grains. Seen in Fig. 4, the HREM images are given for one WC particle arbitrarily chosen from the asmilled WC–10%Co powder mixture. At the brim of the particle, the microstructure can be observed clearly. It can be seen that the grains separated by large angle boundaries or amorphous zones are from several nanometers to 20 nm in size, which is in agreement with the results of X-ray diffraction analysis and dark field image observation mentioned above. To show the microstructure of the
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Fig. 3. TEM images for one particle of WC–10%Co composite after 10 h milling: bright field image (a), electron diffraction pattern (b) and dark field image (c).
particle in details, different regions labeled with A1 , A2 , B and C and marked by white rectangles are enlarged and put together in Fig. 4(A1 , A2 , B and C). The particle is smooth and with round shape. But at the edge of the particle labeled with A1 and A2 , a number of straight planes were observed. After the examination in the enlarged area, it is found that the straight plane is the {1 0 1 0} of WC, and the length of the straight plane is about 2–3 nm in region (A1 ) and 5–6 nm in region (A2 ).
These straight plane may come from the fracture through {1 0 1 0} plane in WC in milling. Because the prismatic plane of {1 0 1 0} is one of the main slip plane of WC [14], and the interaction of the dislocations will generate the crack, which spread across {1 0 1 0} plane then fracture on it [15]. In the early milling stage, the fracture on planes {1 0 1 0} may cover a large area. Because of the high frequent deformation, however, the initial fracture planes may break and deform along with milling,
F.L. Zhang et al. / Scripta Materialia 49 (2003) 1123–1128
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Fig. 4. HREM image of one particle of WC and its enlarged images of the regions labeled as A1 , A2 , B and C.
and some of the planes can also change orientation. Therefore the remaining straight {1 0 1 0} planes are short in length and oriented in different directions.
In regions (B) and (C), some zones marked with white arrows are strongly distorted, which means the existence of dislocation in high density. The planes of {0 0 0 1}, {1 0 1 0} and {0 1 1 1}
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are identified and marked by pairs parallel segments and opposite arrows. In region (C), four dislocations with same orientation marked with inverse ‘‘T’’ are observed. According to image (C), the electron beam is parallel to 1/6[1 2 1 3]. By drawing the Burgers circuit of one dislocation as shown in (C), the direction and the displacement of the Burgers ! vector b can be determined. It is found that the dislocations have the Burgers vector of 1/3[1 2 1 0], ! which correspond to the vector of a in WC unit cell [10]. In the report of Greenwood [9], the dislocations involved in the WC deformed by compression include [0 0 0 1], 1/3[1 2 1 0] and 1/3[1 2 1 3] in different slip planes. When the WC is deformed by indentation [10], Burger vectors [0 0 0 1], 1/3[1 2 1 0] and 1/ 3[1 2 1 3] are observed at distances greater than 2 lm from indentation. But the deformed areas in indentation are filled with stacking fault R ¼ 1=6½1 2 1 3 which result from the dislocation of 1/3[1 2 1 3]. In our work, although only the dislocations with Burgers 1/3[1 2 1 0] are identified in the region of Fig. 4(C), we believe that other types of dislocations also are present. Similar to result of Ref. [10], dislocations with Burgers 1/3[1 2 1 0] appear in the area with less deformation. Nevertheless, in the highly deformed place, 1/3[1 2 1 3] maybe exists in a large quantity. But due to the high density of dislocations, severe lattice distortion and the shape of the small particles, it is difficult to identify the Burgers vectors of those dislocations in this study. 4. Conclusion 1. High energy ball milling can efficiently refine the microstructure of tungsten carbide in a WC/Co composite. After 10 h milling, the grain size of WC is reduced to 11 nm, a severe internal strain is introduced, and fcc-cobalt has transformed into hcp-cobalt by a mechanical induced allotropic transformation.
2. At the edge of one WC particle, a number of straight planes are observed. The straight planes are shown to be the {1 0 1 0} plane of WC, which is the main slip and fracture plane. 3. Dislocations with Burgers vector 1/3[1 2 1 0] are identified in one WC particle arbitrarily selected from the milled WC–10%Co composite. The dislocation density is high in some regions, which causes the internal strain.
Acknowledgements This work is supported by NSFC with grant no. 559925102 and key projects of Guangdong province with the grant no. A1070105.
References [1] McCandish LE, Keer BH, Bhatia S. International Patent WO91/07244, 1991. [2] Zhang ZY, Wahlberg S, Wang MS, et al. Nanostruct Mater 1999;12:163. [3] Gao L, Kear BH. Nanostruct Mater 1997;9:205. [4] Tan GL, Wu XJ. Powder Metall 1998;41:300. [5] Sherif El-Eskandarany M, Mahday Amir A, Ahmed HA, et al. J Alloy Comp 2000;312:315–25. [6] Suryanrayana C. Prog Mater Sci 2001;46:1. [7] Gillies DC, Lewis D. Powder Metall 1968;11:400. [8] Fecht HJ. Nanostruct Mater 1995;6:33. [9] Greenwood RM, Loretto MH, Smallman RE. Acta Metall 1982;30:1193. [10] Hibbs MK, Sinclair R. Acta Metall 1981;29:1645. [11] Yang ZM, Mao CH, Du J. Trans Nonferrous Met Soc China 2001;11:529. [12] Huang JY, Wu YK, Ye HQ. Acta Mater 1996;44:1201. [13] Sui HX, Zhu M, Li GB, Yang DZ. J Appl Phys 1992;71:2945. [14] Hakan WH, Ulf J, Borje J, et al. Science 2001;293:2434. [15] Jayaram V, Kronenberg A, Kirby SH. Scripta Metall 1986;20:701–5.
Nanostructured WC/Co composite powder prepared by high energy ball milling F.L. Zhang
a,b,*
, C.Y. Wang b, M. Zhu
a
a
b
Department of Mechanical Engineering, South China University of Technology, Guangzhou 510640, PR China Department of Mechanical and Electronic Engineering, Guangdong University of Technology, Dongfeng East Road 729, Guangzhou 510090, PR China Received 1 October 2002; received in revised form 26 July 2003; accepted 11 August 2003
Abstract The microstructure of WC–10%Co nanocomposite prepared by high energy ball milling was investigated by X-ray diffraction and transmission electron microscopy. The WC phase was refined to a grain size of 11 nm after milling. The dislocation density in WC was high and caused severe lattice distortion. Dislocations with Burgers vector 1/3[1 2 1 0] were identified in some areas. Ó 2003 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. Keywords: Microstructure; Ball milling; WC–Co; Nanostructure; Dislocation
1. Introduction WC–Co hardmetals are widely used in metal cutting tools because of their high hardness, good wear-resistance, good fracture resistance and high temperature strength. Mechanical properties of hardmetals are strongly dependent on the microstructure of the WC–Co hardmetal, and additionally affected by the microstructure of WC powders before sintering. An important feature is that the toughness and the hardness increase simultaneously with the refining of WC. Therefore, development of nanostructured WC–Co hard* Corresponding author. Address: Department of Mechanical and Electronic Engineering, Guangdong University of Technology, Dongfeng East Road 729, Guangzhou 510090, PR China. Tel.: +86-02037627005; fax: +86-02037627005. E-mail address: [email protected] (F.L. Zhang).
metal has been extensively studied. The methods of synthesizing the nanostructured WC or WC–Co composite include spraying conversion process [1], co-precipitation [2], displacement reaction process [3], mechanochemical synthesis [4,5], high energy ball milling and so on. High energy ball milling is a simple and efficient way of manufacturing the fine powder with nanostructure except for the problem of contamination [6]. However, when milling WC– Co powder, the milling media can be made of WC–Co hardmetal to eliminate the contamination. Therefore it is a simple method to synthesize nanostructured WC–Co powder by high energy ball milling. After a certain period of high energy milling, the grain size of WC can be reduce to nanoscale together with a considerable internal strain [7]. Generally, the grain size reduction in ball milling is mainly due to the comminution, dislocationsÕ annihilation and recombining to small
1359-6462/$ - see front matter Ó 2003 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. doi:10.1016/j.scriptamat.2003.08.009
1124
F.L. Zhang et al. / Scripta Materialia 49 (2003) 1123–1128
angle grain boundaries separating individual grains [8]. As it has been proved, dislocations with the Burgers vectors of [0 0 0 1], 1/3[1 2 1 3] and 1/ 3[1 2 1 0], appeared in WC phase when the WC–Co hardmetal is deformed by compression [9]. The 1/ 3[1 2 1 3] and its dissociated fault 1/6[1 2 1 3] are also observed when WC is deformed by indentation [10]. But high energy ball milling is a different process, in which the repeated deformation, fracture and cold welding caused by continuous impact take place. So the different deformation may cause the difference of microstructure and defect structure. Yang et al. [11] found that high energy ball milling can partially change hexagonal WC into a orthorhombic phase by introduced stacking fault 1/6[ 12 1 3] on the plane of {1 0 1 0}. While, as for the detail of the defect structure and microstructure of WC phase in high energy ball-milled WC–Co composite, few results were reported, especially the direct observation by high resolution electron microscopy (HREM). Thus the purposes of this paper is to characterize the microstructure and the defect structure of WC in high energy ball milled WC–10%Co composite by means of X-ray diffraction, SEM, TEM and HREM.
covered with carbon after being shocked by ultrasonic for 20 min in dispersant (ethanol). TEM and HREM observation were carried out in JEM100CX and PHILIPS-CM300 respectively. SEM observations were carried out on PHILIPS XL30FEG.
2. Experimentals
b ¼ bd þ be ¼ Kk=ðd cos hÞ þ 4e tan h
WC (99.8%) with mean Fisher sub-sieve size (Fsss) particle size of 5.6 lm and Co (99%) with mean Fsss particle size of 1.0 lm were used as starting materials. Powder mixture of WC– 10%wt.Co mixed with ethanol were sealed in WC– Co hardmetal vial with WC–Co hardmetal balls (8 and 12 mm in diameter and 50–50 in weight percentage, respectively) in a glove box containing inert atmosphere of Ar. The ball to powder weight ratio is controlled to be 15:1. The milling process was carried on a high energy ball mill (Fritsch Pulverisette-5) with the selected rotation velocity of 250 rpm. After 10 h of milling, the powder was taken from the vial and dried at room temperature in Ar. The X-ray diffraction analysis was carried out in the PHILIPS X-Pert diffractometer with the Cu Ka radiation (k ¼ 0:15406 nm). The sample of TEM and HREM were dripped onto Cu grid
where b is the full width at half-maximum (FWHM) of the diffraction peak after instrument
3. Results and discussion X-ray diffraction pattern of WC–10%Co powder before and after high energy ball milling are given in Fig. 1. The starting WC–10%Co powder contains small amounts of W2 C phase, and the Co phase is composed of fcc(a) and hcp(e) phases. After 10 h of ball milling, diffraction peaks of a-Co disappear. This is because that fcc-Co transforms into hcp-Co due to mechanically induced transformation in the ball milling. This kind of allotropic transition in Co was reported in previous work and was considered depending on ball milling intensity [12,13]. It can also be seen from Fig. 1 that the diffraction peak of WC is broaden apparently after 10 h of milling, which is due to refining of grain size and increasing of internal strain resulted from ball milling. The grain size and the internal stress are calculated by Stokes and WilsonÕs formula, ð1Þ
Fig. 1. X-ray diffraction patterns of WC–10%Co powder before (a) and after 10 h milling (b).
F.L. Zhang et al. / Scripta Materialia 49 (2003) 1123–1128
correction; bd and be are FWHM caused by small grain size and internal stress, respectively; K is constant as 0.9; k is wavelength of the X-ray radiation; d and e are grain size and internal stress, respectively; and h is the Bragg angle. b and bs follow Cauchy form with the relationship: B0 ¼ b þ bs , where B0 and bs are FWHM of broadened Bragg peaks and the standard sampleÕs Bragg peaks, respectively. The starting WC with coarse grain size is set as standard sample. Bragg peak (0 0 0 1), (1 0 1 0), (1 0 1 1) is selected to calculate the average grain size of WC in high energy ball milled WC–10%Co mixture by using the Eq. (1). It is found that after 10 h milling, the grain size is reduced to 11 nm with internal stress reach to 1.44%. This is similar to the result obtained by Gillies [7]. It is also found that the peaks of W2 C and e-Co weakened and broadened after milling. But their accurate grain size and internal strain are difficult to estimate because of the small ratio of their weak peak intensity to background. Therefore, they were not further investigated in this work. The morphologies of SEM for WC–10%Co before and after milling are showed in Fig. 2(a) and (b). It can be seen that the particle size has been reduced after milling, with the small particles agglomerating into large particles. The large particles are about 2–5 lm. The discernable small particles are about 100–500 nm with the shape of sphere. The details of microstructure for milled WC– 10%Co powder can be revealed from TEM observation. Fig. 3(a) and (b) shows the bright field image and electron diffraction pattern of one particle of about 400 nm in diameter. The diffraction halos given in Fig. 3(b) are nearly continuous rings, which proved that the grains with different orientation are quite small. Diffraction halos of e-Co and W2 C are also found in Fig. 3(b), which match the result obtained from the X-ray diffraction profile of milled WC–10%Co in Fig. 1(b). The dark field image in Fig. 3(c) is taken by encircling a small part of {1 0 1 0} diffraction halo of WC phase. From the bright field image and the dark field image shown in Fig. 3(a) and (c), it can be seen that the particle is composed of WC phase with the size of about 10 nm, which corresponds to the result obtained from the X-ray diffraction
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Fig. 2. SEM images of WC–10%Co powder mixture before (a) and after 10 h milling (b).
analysis. This means that WC phase has been refined to nanocrystalline in WC–10%Co mixture by milling. The refining of grains during milling process resulted from the movement, annihilation and recombination of large number of dislocations. Therefore, a large amount of high angle grain boundaries are formed to separate nanosize grains. Seen in Fig. 4, the HREM images are given for one WC particle arbitrarily chosen from the asmilled WC–10%Co powder mixture. At the brim of the particle, the microstructure can be observed clearly. It can be seen that the grains separated by large angle boundaries or amorphous zones are from several nanometers to 20 nm in size, which is in agreement with the results of X-ray diffraction analysis and dark field image observation mentioned above. To show the microstructure of the
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Fig. 3. TEM images for one particle of WC–10%Co composite after 10 h milling: bright field image (a), electron diffraction pattern (b) and dark field image (c).
particle in details, different regions labeled with A1 , A2 , B and C and marked by white rectangles are enlarged and put together in Fig. 4(A1 , A2 , B and C). The particle is smooth and with round shape. But at the edge of the particle labeled with A1 and A2 , a number of straight planes were observed. After the examination in the enlarged area, it is found that the straight plane is the {1 0 1 0} of WC, and the length of the straight plane is about 2–3 nm in region (A1 ) and 5–6 nm in region (A2 ).
These straight plane may come from the fracture through {1 0 1 0} plane in WC in milling. Because the prismatic plane of {1 0 1 0} is one of the main slip plane of WC [14], and the interaction of the dislocations will generate the crack, which spread across {1 0 1 0} plane then fracture on it [15]. In the early milling stage, the fracture on planes {1 0 1 0} may cover a large area. Because of the high frequent deformation, however, the initial fracture planes may break and deform along with milling,
F.L. Zhang et al. / Scripta Materialia 49 (2003) 1123–1128
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Fig. 4. HREM image of one particle of WC and its enlarged images of the regions labeled as A1 , A2 , B and C.
and some of the planes can also change orientation. Therefore the remaining straight {1 0 1 0} planes are short in length and oriented in different directions.
In regions (B) and (C), some zones marked with white arrows are strongly distorted, which means the existence of dislocation in high density. The planes of {0 0 0 1}, {1 0 1 0} and {0 1 1 1}
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are identified and marked by pairs parallel segments and opposite arrows. In region (C), four dislocations with same orientation marked with inverse ‘‘T’’ are observed. According to image (C), the electron beam is parallel to 1/6[1 2 1 3]. By drawing the Burgers circuit of one dislocation as shown in (C), the direction and the displacement of the Burgers ! vector b can be determined. It is found that the dislocations have the Burgers vector of 1/3[1 2 1 0], ! which correspond to the vector of a in WC unit cell [10]. In the report of Greenwood [9], the dislocations involved in the WC deformed by compression include [0 0 0 1], 1/3[1 2 1 0] and 1/3[1 2 1 3] in different slip planes. When the WC is deformed by indentation [10], Burger vectors [0 0 0 1], 1/3[1 2 1 0] and 1/ 3[1 2 1 3] are observed at distances greater than 2 lm from indentation. But the deformed areas in indentation are filled with stacking fault R ¼ 1=6½1 2 1 3 which result from the dislocation of 1/3[1 2 1 3]. In our work, although only the dislocations with Burgers 1/3[1 2 1 0] are identified in the region of Fig. 4(C), we believe that other types of dislocations also are present. Similar to result of Ref. [10], dislocations with Burgers 1/3[1 2 1 0] appear in the area with less deformation. Nevertheless, in the highly deformed place, 1/3[1 2 1 3] maybe exists in a large quantity. But due to the high density of dislocations, severe lattice distortion and the shape of the small particles, it is difficult to identify the Burgers vectors of those dislocations in this study. 4. Conclusion 1. High energy ball milling can efficiently refine the microstructure of tungsten carbide in a WC/Co composite. After 10 h milling, the grain size of WC is reduced to 11 nm, a severe internal strain is introduced, and fcc-cobalt has transformed into hcp-cobalt by a mechanical induced allotropic transformation.
2. At the edge of one WC particle, a number of straight planes are observed. The straight planes are shown to be the {1 0 1 0} plane of WC, which is the main slip and fracture plane. 3. Dislocations with Burgers vector 1/3[1 2 1 0] are identified in one WC particle arbitrarily selected from the milled WC–10%Co composite. The dislocation density is high in some regions, which causes the internal strain.
Acknowledgements This work is supported by NSFC with grant no. 559925102 and key projects of Guangdong province with the grant no. A1070105.
References [1] McCandish LE, Keer BH, Bhatia S. International Patent WO91/07244, 1991. [2] Zhang ZY, Wahlberg S, Wang MS, et al. Nanostruct Mater 1999;12:163. [3] Gao L, Kear BH. Nanostruct Mater 1997;9:205. [4] Tan GL, Wu XJ. Powder Metall 1998;41:300. [5] Sherif El-Eskandarany M, Mahday Amir A, Ahmed HA, et al. J Alloy Comp 2000;312:315–25. [6] Suryanrayana C. Prog Mater Sci 2001;46:1. [7] Gillies DC, Lewis D. Powder Metall 1968;11:400. [8] Fecht HJ. Nanostruct Mater 1995;6:33. [9] Greenwood RM, Loretto MH, Smallman RE. Acta Metall 1982;30:1193. [10] Hibbs MK, Sinclair R. Acta Metall 1981;29:1645. [11] Yang ZM, Mao CH, Du J. Trans Nonferrous Met Soc China 2001;11:529. [12] Huang JY, Wu YK, Ye HQ. Acta Mater 1996;44:1201. [13] Sui HX, Zhu M, Li GB, Yang DZ. J Appl Phys 1992;71:2945. [14] Hakan WH, Ulf J, Borje J, et al. Science 2001;293:2434. [15] Jayaram V, Kronenberg A, Kirby SH. Scripta Metall 1986;20:701–5.