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Nanostructured Ni-bonded hard materials (W0.6Al0.4)C0.5–Ni prepared by mechanical alloying and hot-pressing
Materials Science and Engineering A 532 (2012) 146–150
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Nanostructured Ni-bonded hard materials (W0.6 Al0.4 )C0.5 –Ni prepared by mechanical alloying and hot-pressing Jianwei Liu a,b , Xianfeng Ma a,∗ , Huaguo Tang a , Wei Zhao a a b
State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China Graduate School of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 9 June 2011 Received in revised form 16 October 2011 Accepted 20 October 2011 Available online 28 October 2011 Keywords: Nanostructured materials Composites Mechanical characterization Mechanical alloying
a b s t r a c t In this paper, a novel nanostructured Ni bonded hard alloys (W0.6 Al0.4 )C0.5 –Ni with different nickel contents were prepared by mechanical alloying and hot-pressing sintering. Nanocrystalline (W0.6 Al0.4 )C0.5 powders with “rounded” particle shape were prepared by mechanical alloying. The novel materials were easy to process nanoscale sintering and remain the rounded particles in the bulk materials. The relative density of the bulk samples can reach over 98% under the hot-pressing sintering. The sintered samples have superior mechanical properties. The effect of Ni content and on the mechanical properties of bulk sintered (W0.6 Al0.4 )C0.5 –Ni were investigated. The mechanical properties of nanostructured and conventional (W0.6 Al0.4 )C0.5 –Ni cemented carbides compared with (W0.6 Al0.4 )C0.5 –Co were also investigated. The particles size of the starting material (W0.6 Al0.4 )C0.5 appears to play an important role in determining the mechanical properties of the obtained bulk alloys. As the (W0.6 Al0.4 )C0.5 grains refined, the use of Ni instead of Co does not suffer from any reduction in mechanical properties. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Due to its exceptional hardness and superior bending strength, cemented carbide (W1−x Alx )Cy (x = 0.1–0.86, y = 0.1–0.5) is becoming the most important hard material for tool-cutting and mining industry. (W1−x Alx )Cy is a solid solution of Al replacing W in WC lattice, and it is found to crystallize in the hexagonal space group P-6m2 (187), and has the WC-type structure [1,2]. In our previous work, we used cobalt as the binder phase and prepared a new hard alloys (W0.5 Al0.5 )C0.5 –Co by hot-pressing sintering [3]. It has superior mechanical properties than WC–Co system. As nickel is also a good binder metal for sintering WC [4], so it can use nickel as the binder phase to sinter (W0.6 Al0.4 )C0.5 instead of cobalt. In the past decade, nanocrystalline tungsten carbide powders have been produced by a variety of different technologies [5]. It is raising prospects for producing nanocrystalline tungsten carbide with superior mechanical properties. The production of bulk nanocrystalline cemented tungsten carbide, however, remains a technological challenge. The goal of sintering nanocrystalline powders is not only to achieve full densification but also to retain nanoscaled grain sizes. This is difficult because of rapid grain growth at high temperatures. In a broad definition, nanocrystalline bulk materials are those with grain sizes less than 100 nm. In
∗ Corresponding author. Tel.: +86 431 85262220; fax: +86 431 85262220. E-mail address: [email protected] (X. Ma). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.10.075
published literature, the finest grain sizes of sintered WC–Co using nanocrystalline powders are reported to be 70–200 nm or larger, even when grain growth inhibitors and pressure assisted sintering techniques are used [6–8]. Clearly controlling grain growth during sintering and producing bulk nanocrystalline materials have emerged as a critical technological challenge for the development of bulk materials with a nanoscaled grain structure. During sintering, solution reprecipitation processes result in WC grain growth and formation of low energy prismatic interfaces, due to energy reasons. Such prismatic grains (truncated trigonal prisms) contain sharp edges. Sharp edges lead to tensile stress concentrations on loading, and as a result, promote crack initiation and propagation. More rounded WC grains in the sintered microstructure should result in a tougher material with reduced sensitivity to cracking, especially where cracks originate due to thermal shock. It has been reported that the rounded WC grains were successfully prepared with the rounded WC powders and the growth inhibitor were used to avoid extensive particle shape transformations during liquid phase sintering [9]. The resulting “rounded” WC grains show superior mechanical properties compared with normal WC grains. The aim of this study is to prepare nickel bonded nanoscaled hard material (W0.6 Al0.4 )C0.5 –Ni by mechanical alloying and hotpressing sintering. This was achieved by using the nanocrystalline “rounded” (W0.6 Al0.4 )C0.5 as starting powders and hot-pressing sintering to preserve the rounded shape of the (W0.6 Al0.4 )C0.5 powder in nanoscaled. The microstructure and the mechanical properties of the novel hard material are also discussed.
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2. Experimental details 2.1. Preparation Elemental powders of nickel (58 m, 99.6 wt%), carbon (<300 mesh, 99%purity) tungsten (<200 mesh, 99%purity) and aluminum (<200 mesh, 99%purity) were used as the raw materials. The powders were weighed in an atomic ratio of W:Al:C = 6:4:5, then the powders were sealed under argon atmosphere in a steel vial with ball-to-powder weight ratios 15:1. The ball-milling experiments were carried out at room temperature, using a GN2 high-energy ball mill at rotation speed of 1280 r/min. Then the obtained nanostructred (W0.6 Al0.4 )C0.5 mixed with nickel powder under argon atmosphere and ball-milled for 2 h at rotation speed of 1280 r/min. The mixture powders were pressed into cuboids of 40 mm × 10 mm × 10 mm at a compaction pressure of 350 MPa and the specimens were reduced in H2 for 1 h. Then the specimens were sintered in a vacuum hot-pressing sintering furnace with the following cycle: (a) heated from room temperature to sintering temperature (1300–1400 ◦ C) with a heating rate of about 120 ◦ C min−1 ; (b) kept the sintering temperature for the desired duration (10–20 min); (c) cooled down from the sintering temperature to 600 ◦ C at about 400 ◦ C min−1 , and then furnace cooled from 600 ◦ C to room temperature. After hot-pressing sintering, the bulk specimens were ground and polished. 2.2. Characterization The microstructural characterization was conducted using optical microscope (OM) and scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDX) and transmission electron microscope (TEM). The specimens for OM and SEM were prepared by the standard technique of grinding and polishing, followed by etching in a mixture of 20% NaOH solution with an equal volume 20% K3 Fe(CN)6 solution. The densities of the sintered specimens were determined by the Archimedes water immersion method. Microhardness of the hard alloy bulk bodies were measured by the Vickers micro-hardness tester (FM-700, Japan) with a load of 300 gf and dwell time of 15 s. The transverse strengths were measured by three-point bending test. Bending tests were performed on an Instron model 1125 test machine at a crosshead speed of 2 mm/min; the distance of support of bending test was 30 mm, bending specimens (4 mm × 3 mm × 35 mm) were cut from the alloy bulk bodies. All the reported data were the average of at least three test results.
Fig. 1. The XRD patterns of the mechanically alloyed W, Al and C powders after various milling time.
so it can conclude that the new phase of (W0.6 Al0.4 )C0.5 was formed completely. After another 20 h of milling, the (W0.6 Al0.4 )C0.5 peaks disappear completely, and only a halo peak corresponding to an amorphous phase is observed. Fig. 2 shows the XRD patterns of the mixture powder of (W0.6 Al0.4 )C0.5 and nickel ball-milled for 2 h. The corresponding minimum grain size calculated using the Scherrer formula is 7 nm. It also has been demonstrated that the grain size is nanometer dimensions from the TEM image in Fig. 3. It shows that the powder consisted of spherical particles with an average diameter of 8 nm and a 10% size distribution. Considering the results obtained from the X-ray examination and TEM, nanoscaled (W0.6 Al0.4 )C0.5 with carbon vacancies has been synthesized by mechanical alloying. 3.2. Structure of (W0.6 Al0.4 )C0.5 –Ni hard alloys Fig. 4 shows XRD pattern of (W0.6 Al0.4 )C0.5 –9.8 wt%Ni(10.1 vol%) bulk alloy obtained by hot-pressing at 1350 ◦ C for 18 min. Only the peaks of (W0.6 Al0.4 )C0.5 and Ni phase can be found. The results indicated that the bulk specimens obtained by hot-pressing were
3. Results and discussion 3.1. Synthesis of nanoscaled (W0.6 Al0.4 )C0.5 –Ni powders The XRD patterns of mechanically alloyed powders of W:Al:C = 6:4:5 for various milling times are presented in Fig. 1. The diffraction of the carbon powder to X-ray is too weak to be detected, so only peaks corresponding to tungsten and aluminum can be observed in the XRD pattern of the starting materials. As milling time increased, the tungsten peaks broadened, due to reduction of crystalline size and accumulation of lattice strain. After 80 h of milling, the Al peaks disappeared completely. New peaks were observed when the sample milled for 120 h. A new phase was apparently formed and identified to have hexagonal structure. The mill and ball that we used are made of stained steel, so it is no doubt that (W0.6 Al0.4 )C0.5 phase presented was produced by direct reaction during milling process. The peaks of (W0.6 Al0.4 ) are completely disappeared and only (W0.6 Al0.4 )C0.5 peaks were found after 160 h,
Fig. 2. The XRD patterns of (W0.5 Al0.5 )C0.5 and Ni mixture powders ball-milled for 2 h.
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Fig. 3. TEM micrograph of the (W0.6 Al0.4 )C0.5 obtained by mechanical alloying.
The optical microstructures of the sintered sample (W0.6 Al0.4 )C0.5 –9.8 wt%Ni etched for 2 min are shown in Fig. 5. It can be seen that the bulk specimens consist of “rounded” (W0.6 Al0.4 )C0.5 grains, and many “rounded” (W0.6 Al0.4 )C0.5 grains distributing in the bulk hard material are less than 500 nm. Fig. 6(A) and (B) are SEM micrograph from the fractured bulk (W0.6 Al0.4 )C0.5 –9.8 wt%Ni sintered at 1350 ◦ C for 18 min, low magnification and high magnification respectively. These pictures show that the bulk specimens consist of “rounded” (W0.6 Al0.4 )C0.5 grains with an average grain size of 300 nm, which is in agreement with the TEM results (Fig. 7). We can conclude that (W0.6 Al0.4 )C0.5 –Ni cemented hard alloy is greatly different from WC–Ni, in which clear coarsening of WC particle could be difficult to prevent. In hardmetals industry, it is considered that nanostructured cemented carbides as an important branch of nanocrystalline materials could offer new opportunities for achieving superior hardness and toughness combinations. However, attempts to fabricate nanostructured WC–Ni hard alloys have been frustrated by the inability to retain a nano-scale grain size in the fully sintered bulk alloys, since the presence of an unusually high surface and interface area provides a strong driving force for grain growth. So, in order to control the grain growth in nanostructured WC-based composites, one of the keys is to add some inhibitors, such as Cr2 C3, and VC, but the second-phase additive will become the center of cracking [10,11]. In (W0.6 Al0.4 )C0.5, Al was dissolved into WC lattice by replacing W. Although there is no inhibitors addition, it can easily obtain nano-scaled grain size in the fully sintered bulk alloy. And the shortly sintering time (18 min) and the ultra-pressure is also the important factor for the nanostructured sintered particles. Schubert et al. had reported that WC hard material with “rounded” particles have superior properties [9]. From Fig. 5, the shape of the (W0.6 Al0.4 )C0.5 grains are rounded other than prismatic grains, the rounded grains do not contain the sharp edge, which form local tensile stress concentrations on loading of the composite. So, the “rounded” (W0.6 Al0.4 )C0.5 grains can reduce the sensitivity to crack. 3.3. Mechanical properties
Fig. 4. The XRD patterns of (W0.4 Al0.6 )C0.5 –9.8 wt% Ni bulk alloy obtained by hotpressing sintering at 1350 ◦ C for 18 min.
identical with raw materials in composition and Al still dissolved in WC after sintered at 1350 ◦ C by reason of finding no peaks of Al and/or aluminous compounds in XRD pattern. That suggested that the novel solid solution (W0.6 Al0.4 )C0.5 had excellent thermal stability up to 1350 ◦ C during hot-pressing sintering.
Fig. 8 shows the hardness and bending strength values of the specimens obtained in this work and those of comparable cemented carbides. By using (W0.6 Al0.4 )C0.5 grain of about 1 m as the raw material, the (W0.6 Al0.4 )C0.5 –Co and (W0.6 Al0.4 )C0.5 –Ni samples were sintered at 1380 ◦ C and 1420 ◦ C by the same sintering process. In both cases, the alloys had (W0.6 Al0.4 ) C0.5 grains which are about the same in size (approximately 2 m), which is roughly an order of magnitude larger than the grain size of 300 nm obtained in this work. Both hardness and toughness values
Fig. 5. The optical microstructures of bulk sintered samples (W0.6 Al0.4 )C0.5 –9.8 wt%Ni etched for 2 min: (A) low magnification and (B) high magnification.
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Fig. 6. The SEM micrographs of the fracture surface of the (W0.6 Al0.4 )C0.5 –9.8 wt%Ni sintered by hot-pressing.
Fig. 7. TEM micrographs of bulk sintered (W0.6 Al0.4 )C0.5 –9.8 wt%Ni and its diffraction pattern.
Fig. 8. The hardness and bending strength values of the specimen obtained in this work and those of comparable cemented carbides.
for both cases are similar. These comparisons serve to demonstrate the effect of the grain size. Refinement of the (W0.6 Al0.4 )C0.5 grain size improves significantly the hardness of the cemented carbide without decreasing its bending strength. Furthermore, as can be seen from this figure, when the grains refined, the use of Ni instead of Co does not suffer from any reduction in mechanical properties. Given the advantage of higher oxidation, corrosion resistance and lower cost for the (W0.6 Al0.4 )C0.5 –Ni hard alloys, the results are significant for potential applications.
4. Conclusions In this paper, nanostructured Ni-bonded had alloys (W0.6 Al0.4 )C0.5 –Ni was successfully prepared by mechanical alloying and hot-pressing sintering. The bulk nano-scaled “rounded” hard materials have superior mechanical properties and they possess high hardness (22.7, 18.06 and 16.33 GPa), high bending strength (1810, 2144 and 2407 MPa), while the nickel content was 9.8%, 12.9% and 15.9%, respectively by weight. But it has lower density compared with the common hard material WC–Ni,
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for the density of (W0.6 Al0.4 )C0.5 (9.360 g cm−3 ) is much lower than that of WC (15.6 g cm−3 ). The particles size of the starting material appears to play an important role in determining the mechanical properties of the obtained bulk alloys. As the grain size refined, the use of Ni instead of Co does not suffer from any reduction in mechanical properties. Considering its superior mechanical character, lower density, and the simple producing technology (W0.6 Al0.4 )C0.5 –Ni hard alloys are expected to be a new cemented carbides with lower operating costs can replace the standard materials for cutting tools, wear parts, electrode materials, etc. Acknowledgment This work was supported by the National Natural Science Foundation of China with program: 20921002.
References [1] J.M. Yan, X.F. Ma, W. Zhao, H.G. Tang, C.J. Zhu, J. Solid State Chem. 177 (2004) 2265–2270. [2] H.G. Tang, X.F. Ma, W. Zhao, X.W. Yan, J.M. Yan, Mater. Res. Bull. 39 (2004) 707–713. [3] Z.H. Qiao, X.F. Ma, W. Zhao, H.G. Tang, B. Zhao, J. Alloy Compd. 462 (2008) 416–420. [4] H.C. Kim, I.J. Shon, J.K. Yoon, J.M. Doh, Z.A. Munir, Int. J. Refract. Met. H. 24 (2006) 427–431. [5] L.E. Mccandlish, B.H. Kear, B.K. Kim, Mater. Sci. Technol.-Lond. 6 (1990) 953–957. [6] Z.G. Fang, J.W. Eason, Int. J. Refract. Met. H. 13 (1995) 297–303. [7] I. Azcona, A. Ordonez, J.M. Sanchez, F. Castro, J. Mater. Sci. 37 (2002) 4189–4195. [8] L. Bartha, P. Atato, A.L. Toth, R. Porat, S. Berger, A. Rosen, J. Adv. Mater.-Covina 32 (2000) 23–26. [9] R.P. Herber, W.D. Schubert, B. Lux, Int. J. Refract. Met. H. 25 (2007) 119, 119. [10] S.G. Huang, O. Van der Biest, J. Vleugels, Mater. Sci. Eng. A: Struct. 488 (2008) 420–427. [11] L. Sun, C.C. Jia, R.J. Cao, C.G. Lin, Int. J. Refract. Met. H. 26 (2008) 357–361.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Nanostructured Ni-bonded hard materials (W0.6 Al0.4 )C0.5 –Ni prepared by mechanical alloying and hot-pressing Jianwei Liu a,b , Xianfeng Ma a,∗ , Huaguo Tang a , Wei Zhao a a b
State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China Graduate School of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 9 June 2011 Received in revised form 16 October 2011 Accepted 20 October 2011 Available online 28 October 2011 Keywords: Nanostructured materials Composites Mechanical characterization Mechanical alloying
a b s t r a c t In this paper, a novel nanostructured Ni bonded hard alloys (W0.6 Al0.4 )C0.5 –Ni with different nickel contents were prepared by mechanical alloying and hot-pressing sintering. Nanocrystalline (W0.6 Al0.4 )C0.5 powders with “rounded” particle shape were prepared by mechanical alloying. The novel materials were easy to process nanoscale sintering and remain the rounded particles in the bulk materials. The relative density of the bulk samples can reach over 98% under the hot-pressing sintering. The sintered samples have superior mechanical properties. The effect of Ni content and on the mechanical properties of bulk sintered (W0.6 Al0.4 )C0.5 –Ni were investigated. The mechanical properties of nanostructured and conventional (W0.6 Al0.4 )C0.5 –Ni cemented carbides compared with (W0.6 Al0.4 )C0.5 –Co were also investigated. The particles size of the starting material (W0.6 Al0.4 )C0.5 appears to play an important role in determining the mechanical properties of the obtained bulk alloys. As the (W0.6 Al0.4 )C0.5 grains refined, the use of Ni instead of Co does not suffer from any reduction in mechanical properties. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Due to its exceptional hardness and superior bending strength, cemented carbide (W1−x Alx )Cy (x = 0.1–0.86, y = 0.1–0.5) is becoming the most important hard material for tool-cutting and mining industry. (W1−x Alx )Cy is a solid solution of Al replacing W in WC lattice, and it is found to crystallize in the hexagonal space group P-6m2 (187), and has the WC-type structure [1,2]. In our previous work, we used cobalt as the binder phase and prepared a new hard alloys (W0.5 Al0.5 )C0.5 –Co by hot-pressing sintering [3]. It has superior mechanical properties than WC–Co system. As nickel is also a good binder metal for sintering WC [4], so it can use nickel as the binder phase to sinter (W0.6 Al0.4 )C0.5 instead of cobalt. In the past decade, nanocrystalline tungsten carbide powders have been produced by a variety of different technologies [5]. It is raising prospects for producing nanocrystalline tungsten carbide with superior mechanical properties. The production of bulk nanocrystalline cemented tungsten carbide, however, remains a technological challenge. The goal of sintering nanocrystalline powders is not only to achieve full densification but also to retain nanoscaled grain sizes. This is difficult because of rapid grain growth at high temperatures. In a broad definition, nanocrystalline bulk materials are those with grain sizes less than 100 nm. In
∗ Corresponding author. Tel.: +86 431 85262220; fax: +86 431 85262220. E-mail address: [email protected] (X. Ma). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.10.075
published literature, the finest grain sizes of sintered WC–Co using nanocrystalline powders are reported to be 70–200 nm or larger, even when grain growth inhibitors and pressure assisted sintering techniques are used [6–8]. Clearly controlling grain growth during sintering and producing bulk nanocrystalline materials have emerged as a critical technological challenge for the development of bulk materials with a nanoscaled grain structure. During sintering, solution reprecipitation processes result in WC grain growth and formation of low energy prismatic interfaces, due to energy reasons. Such prismatic grains (truncated trigonal prisms) contain sharp edges. Sharp edges lead to tensile stress concentrations on loading, and as a result, promote crack initiation and propagation. More rounded WC grains in the sintered microstructure should result in a tougher material with reduced sensitivity to cracking, especially where cracks originate due to thermal shock. It has been reported that the rounded WC grains were successfully prepared with the rounded WC powders and the growth inhibitor were used to avoid extensive particle shape transformations during liquid phase sintering [9]. The resulting “rounded” WC grains show superior mechanical properties compared with normal WC grains. The aim of this study is to prepare nickel bonded nanoscaled hard material (W0.6 Al0.4 )C0.5 –Ni by mechanical alloying and hotpressing sintering. This was achieved by using the nanocrystalline “rounded” (W0.6 Al0.4 )C0.5 as starting powders and hot-pressing sintering to preserve the rounded shape of the (W0.6 Al0.4 )C0.5 powder in nanoscaled. The microstructure and the mechanical properties of the novel hard material are also discussed.
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2. Experimental details 2.1. Preparation Elemental powders of nickel (58 m, 99.6 wt%), carbon (<300 mesh, 99%purity) tungsten (<200 mesh, 99%purity) and aluminum (<200 mesh, 99%purity) were used as the raw materials. The powders were weighed in an atomic ratio of W:Al:C = 6:4:5, then the powders were sealed under argon atmosphere in a steel vial with ball-to-powder weight ratios 15:1. The ball-milling experiments were carried out at room temperature, using a GN2 high-energy ball mill at rotation speed of 1280 r/min. Then the obtained nanostructred (W0.6 Al0.4 )C0.5 mixed with nickel powder under argon atmosphere and ball-milled for 2 h at rotation speed of 1280 r/min. The mixture powders were pressed into cuboids of 40 mm × 10 mm × 10 mm at a compaction pressure of 350 MPa and the specimens were reduced in H2 for 1 h. Then the specimens were sintered in a vacuum hot-pressing sintering furnace with the following cycle: (a) heated from room temperature to sintering temperature (1300–1400 ◦ C) with a heating rate of about 120 ◦ C min−1 ; (b) kept the sintering temperature for the desired duration (10–20 min); (c) cooled down from the sintering temperature to 600 ◦ C at about 400 ◦ C min−1 , and then furnace cooled from 600 ◦ C to room temperature. After hot-pressing sintering, the bulk specimens were ground and polished. 2.2. Characterization The microstructural characterization was conducted using optical microscope (OM) and scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDX) and transmission electron microscope (TEM). The specimens for OM and SEM were prepared by the standard technique of grinding and polishing, followed by etching in a mixture of 20% NaOH solution with an equal volume 20% K3 Fe(CN)6 solution. The densities of the sintered specimens were determined by the Archimedes water immersion method. Microhardness of the hard alloy bulk bodies were measured by the Vickers micro-hardness tester (FM-700, Japan) with a load of 300 gf and dwell time of 15 s. The transverse strengths were measured by three-point bending test. Bending tests were performed on an Instron model 1125 test machine at a crosshead speed of 2 mm/min; the distance of support of bending test was 30 mm, bending specimens (4 mm × 3 mm × 35 mm) were cut from the alloy bulk bodies. All the reported data were the average of at least three test results.
Fig. 1. The XRD patterns of the mechanically alloyed W, Al and C powders after various milling time.
so it can conclude that the new phase of (W0.6 Al0.4 )C0.5 was formed completely. After another 20 h of milling, the (W0.6 Al0.4 )C0.5 peaks disappear completely, and only a halo peak corresponding to an amorphous phase is observed. Fig. 2 shows the XRD patterns of the mixture powder of (W0.6 Al0.4 )C0.5 and nickel ball-milled for 2 h. The corresponding minimum grain size calculated using the Scherrer formula is 7 nm. It also has been demonstrated that the grain size is nanometer dimensions from the TEM image in Fig. 3. It shows that the powder consisted of spherical particles with an average diameter of 8 nm and a 10% size distribution. Considering the results obtained from the X-ray examination and TEM, nanoscaled (W0.6 Al0.4 )C0.5 with carbon vacancies has been synthesized by mechanical alloying. 3.2. Structure of (W0.6 Al0.4 )C0.5 –Ni hard alloys Fig. 4 shows XRD pattern of (W0.6 Al0.4 )C0.5 –9.8 wt%Ni(10.1 vol%) bulk alloy obtained by hot-pressing at 1350 ◦ C for 18 min. Only the peaks of (W0.6 Al0.4 )C0.5 and Ni phase can be found. The results indicated that the bulk specimens obtained by hot-pressing were
3. Results and discussion 3.1. Synthesis of nanoscaled (W0.6 Al0.4 )C0.5 –Ni powders The XRD patterns of mechanically alloyed powders of W:Al:C = 6:4:5 for various milling times are presented in Fig. 1. The diffraction of the carbon powder to X-ray is too weak to be detected, so only peaks corresponding to tungsten and aluminum can be observed in the XRD pattern of the starting materials. As milling time increased, the tungsten peaks broadened, due to reduction of crystalline size and accumulation of lattice strain. After 80 h of milling, the Al peaks disappeared completely. New peaks were observed when the sample milled for 120 h. A new phase was apparently formed and identified to have hexagonal structure. The mill and ball that we used are made of stained steel, so it is no doubt that (W0.6 Al0.4 )C0.5 phase presented was produced by direct reaction during milling process. The peaks of (W0.6 Al0.4 ) are completely disappeared and only (W0.6 Al0.4 )C0.5 peaks were found after 160 h,
Fig. 2. The XRD patterns of (W0.5 Al0.5 )C0.5 and Ni mixture powders ball-milled for 2 h.
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Fig. 3. TEM micrograph of the (W0.6 Al0.4 )C0.5 obtained by mechanical alloying.
The optical microstructures of the sintered sample (W0.6 Al0.4 )C0.5 –9.8 wt%Ni etched for 2 min are shown in Fig. 5. It can be seen that the bulk specimens consist of “rounded” (W0.6 Al0.4 )C0.5 grains, and many “rounded” (W0.6 Al0.4 )C0.5 grains distributing in the bulk hard material are less than 500 nm. Fig. 6(A) and (B) are SEM micrograph from the fractured bulk (W0.6 Al0.4 )C0.5 –9.8 wt%Ni sintered at 1350 ◦ C for 18 min, low magnification and high magnification respectively. These pictures show that the bulk specimens consist of “rounded” (W0.6 Al0.4 )C0.5 grains with an average grain size of 300 nm, which is in agreement with the TEM results (Fig. 7). We can conclude that (W0.6 Al0.4 )C0.5 –Ni cemented hard alloy is greatly different from WC–Ni, in which clear coarsening of WC particle could be difficult to prevent. In hardmetals industry, it is considered that nanostructured cemented carbides as an important branch of nanocrystalline materials could offer new opportunities for achieving superior hardness and toughness combinations. However, attempts to fabricate nanostructured WC–Ni hard alloys have been frustrated by the inability to retain a nano-scale grain size in the fully sintered bulk alloys, since the presence of an unusually high surface and interface area provides a strong driving force for grain growth. So, in order to control the grain growth in nanostructured WC-based composites, one of the keys is to add some inhibitors, such as Cr2 C3, and VC, but the second-phase additive will become the center of cracking [10,11]. In (W0.6 Al0.4 )C0.5, Al was dissolved into WC lattice by replacing W. Although there is no inhibitors addition, it can easily obtain nano-scaled grain size in the fully sintered bulk alloy. And the shortly sintering time (18 min) and the ultra-pressure is also the important factor for the nanostructured sintered particles. Schubert et al. had reported that WC hard material with “rounded” particles have superior properties [9]. From Fig. 5, the shape of the (W0.6 Al0.4 )C0.5 grains are rounded other than prismatic grains, the rounded grains do not contain the sharp edge, which form local tensile stress concentrations on loading of the composite. So, the “rounded” (W0.6 Al0.4 )C0.5 grains can reduce the sensitivity to crack. 3.3. Mechanical properties
Fig. 4. The XRD patterns of (W0.4 Al0.6 )C0.5 –9.8 wt% Ni bulk alloy obtained by hotpressing sintering at 1350 ◦ C for 18 min.
identical with raw materials in composition and Al still dissolved in WC after sintered at 1350 ◦ C by reason of finding no peaks of Al and/or aluminous compounds in XRD pattern. That suggested that the novel solid solution (W0.6 Al0.4 )C0.5 had excellent thermal stability up to 1350 ◦ C during hot-pressing sintering.
Fig. 8 shows the hardness and bending strength values of the specimens obtained in this work and those of comparable cemented carbides. By using (W0.6 Al0.4 )C0.5 grain of about 1 m as the raw material, the (W0.6 Al0.4 )C0.5 –Co and (W0.6 Al0.4 )C0.5 –Ni samples were sintered at 1380 ◦ C and 1420 ◦ C by the same sintering process. In both cases, the alloys had (W0.6 Al0.4 ) C0.5 grains which are about the same in size (approximately 2 m), which is roughly an order of magnitude larger than the grain size of 300 nm obtained in this work. Both hardness and toughness values
Fig. 5. The optical microstructures of bulk sintered samples (W0.6 Al0.4 )C0.5 –9.8 wt%Ni etched for 2 min: (A) low magnification and (B) high magnification.
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Fig. 6. The SEM micrographs of the fracture surface of the (W0.6 Al0.4 )C0.5 –9.8 wt%Ni sintered by hot-pressing.
Fig. 7. TEM micrographs of bulk sintered (W0.6 Al0.4 )C0.5 –9.8 wt%Ni and its diffraction pattern.
Fig. 8. The hardness and bending strength values of the specimen obtained in this work and those of comparable cemented carbides.
for both cases are similar. These comparisons serve to demonstrate the effect of the grain size. Refinement of the (W0.6 Al0.4 )C0.5 grain size improves significantly the hardness of the cemented carbide without decreasing its bending strength. Furthermore, as can be seen from this figure, when the grains refined, the use of Ni instead of Co does not suffer from any reduction in mechanical properties. Given the advantage of higher oxidation, corrosion resistance and lower cost for the (W0.6 Al0.4 )C0.5 –Ni hard alloys, the results are significant for potential applications.
4. Conclusions In this paper, nanostructured Ni-bonded had alloys (W0.6 Al0.4 )C0.5 –Ni was successfully prepared by mechanical alloying and hot-pressing sintering. The bulk nano-scaled “rounded” hard materials have superior mechanical properties and they possess high hardness (22.7, 18.06 and 16.33 GPa), high bending strength (1810, 2144 and 2407 MPa), while the nickel content was 9.8%, 12.9% and 15.9%, respectively by weight. But it has lower density compared with the common hard material WC–Ni,
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for the density of (W0.6 Al0.4 )C0.5 (9.360 g cm−3 ) is much lower than that of WC (15.6 g cm−3 ). The particles size of the starting material appears to play an important role in determining the mechanical properties of the obtained bulk alloys. As the grain size refined, the use of Ni instead of Co does not suffer from any reduction in mechanical properties. Considering its superior mechanical character, lower density, and the simple producing technology (W0.6 Al0.4 )C0.5 –Ni hard alloys are expected to be a new cemented carbides with lower operating costs can replace the standard materials for cutting tools, wear parts, electrode materials, etc. Acknowledgment This work was supported by the National Natural Science Foundation of China with program: 20921002.
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