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Cr3C2 doped W0.6Al0.4C0.8–Co hard alloys prepared by hot-pressing
Int. Journal of Refractory Metals and Hard Materials 48 (2015) 333–337
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Cr3C2 doped W0.6Al0.4C0.8–Co hard alloys prepared by hot-pressing Zhenye Zhao a,b, Jianwei Liu a,⁎, Huaguo Tang a,⁎, Xianfeng Ma a, Wei Zhao a a b
State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, No. 5625 Renmin Street, Changchun 130022, Jilin, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 12 May 2014 Received in revised form 2 October 2014 Accepted 7 October 2014 Available online 8 October 2014 Keywords: W0.6Al0.4C0.8–Co Cr3C2 Hot-pressing Microstructure Mechanical properties
a b s t r a c t W0.6Al0.4C0.88 wt.% Co hard alloys doped with 0.3, 0.6 and 0.9 wt.% Cr3C2 were prepared by hot-pressing. The effect of Cr3C2 content and sintering temperature on the relative density, microstructure and mechanical properties of the hard alloys was investigated. Based on the results, both the Cr3C2 content and sintering temperature had great influences on the carbide grain size. The hardness of the samples added by 0.6 wt.% Cr3C2 achieved the maximum value of 1987 kg/mm2 when it was sintered at 1300 °C. The transverse rupture strength of the samples achieved the maximum value of 1929 MPa with 0.3 wt.% Cr3C2 sintered at 1270 °C. © 2014 Published by Elsevier Ltd.
1. Introduction Previously, W1 − xAlxCy (x = 0.1–0.86, y = 0.1–0.8) was synthesized by mechanical alloying and solid state reactions [1–11]. W1 − xAlxCy is a solid solution of Al replacing W in the WC lattice with or without carbon vacancies and with the WC-type structure. It was reported that W1 − xAlxCy-Co had excellent mechanical properties [6–10]. Ultra-fine or nano-crystalline cemented tungsten carbide materials have high hardness, excellent wear resistance and better toughness than conventional materials [12–14]. However, WC grain growth occurs during sintering of the nanometer sized WC–Co raw powder mixtures through conventional sintering without any additions, especially in liquid state [13]. Therefore, many efforts have been made to obtain ultrafine or nanometer grained WC–Co hard materials. The most successful way of inhibiting the WC grain growth is the addition of grain growth inhibitors, such as chromium carbide (Cr3C2), vanadium carbide (VC), niobium carbide (NbC) or molybdenum carbide (Mo2C) to the raw powder mixtures at amounts less than 1 wt.% [15–19]. Cr3C2 and VC are the effective grain growth inhibitors due to their high solubility and mobility in the cobalt phase at lower temperatures [18]. Moreover, the grain growth can be inhibited to a certain extent by using special sintering technologies at lower sintering temperatures and/or shorter sintering times, such as in hot pressing [20], microwave sintering [21] or spark plasma sintering [22]. The contents of aluminum and carbon have influence on the mechanical properties of W1 − xAlxCy-M hard alloys [1–11]. In a previous work, W0.5Al0.5Cy–M and W0.4Al0.6Cy–M alloys have been well studied ⁎ Corresponding authors. E-mail address: [email protected] (J. Liu).
http://dx.doi.org/10.1016/j.ijrmhm.2014.10.004 0263-4368/© 2014 Published by Elsevier Ltd.
[1–9]. However, the research on W0.6Al0.4Cy–M alloys is less [10,11], so W0.6Al0.4C0.8 was chosen for study. Since W0.6Al0.4C0.8 has the same hexagonal structure with WC [10,11], so cobalt was selected as the binder, which is a good binder for WC sintering. Cr3C2 was used as a grain growth inhibitor in this work. Samples of W0.6Al0.4C0.8–Co hard alloys with 0 (for comparison), 0.3, 0.6 and 0.9 wt.% Cr3C2 contents were prepared by hot-pressing. The effect of Cr3C2 content and sintering temperature on the density, grain size, hardness and transverse rupture strength of W0.6Al0.4C0.8–Co hard alloys was investigated. 2. Experimental procedure 2.1. Preparation Powders of tungsten (1 μm, 99 wt.% purity, Ganzhou Tejing Tungsten Molybdenum Co. Ltd., China), aluminum (1 μm, 99.5 wt.% purity, Luoyang Discoverer Aluminum Co. Ltd., China), cobalt (2 μm, 99.6 wt.%, Changsha Langfeng Metallic Material Co. Ltd., China), carbon (2 μm, 99 wt.% purity, Foshanshi Nanhai Dali Yinyihui Graphite Mold Co. Ltd., China), and chromium carbide (1 μm, 99.5 wt.% purity, Zhuzhou Boda Tungsten Industry Co., Ltd, China) were used as raw materials. The alloy powders W0.6Al0.4C0.8 were prepared by mechanical alloying and solid state reaction [10,11]. The W0.6Al0.4C0.8, Co and Cr3C2 powders were mixed in a stainless steel vial under an argon atmosphere for 30 h in a high energy attrition mill at room temperature (Model: GN-2, China). The ball-to-powder mass ratio was 7.5:1, and the rotational speed was 532 rpm. Next, the mixtures of powders were pressed into a stainless steel die of 40 mm × 10 mm × 10 mm under a compaction pressure of 350 MPa and held at pressure for 1 min. The resultant composite powders were
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sintered in an inductive hot-pressing vacuum furnace (Model: TENGLONG 2000, China) with the following cycle. (a) Heated from room temperature to the sintering temperature with a heating rate of about 20 °C/min, (b) holding at the sintering temperature for 15 min, and (c) cooled down from sintering temperature to 200 °C at about 30 °C/min, and furnace-cooled from 200 °C to room temperature. During sintering, the pressure in the die was kept at 30 MPa. After sintering, the bulk specimens were ground and polished.
small effect, and within the error bars. At 1300 °C, the amount of liquid Co increased, which resulted in that more dissolved Cr3C2 in the liquid Co. The added Cr3C2 can dissolve fully in the liquid Co, and thus had little influence on the hard alloy densification process [19]. In addition, all the relative densities of the samples were over 99% when the hard alloys were sintered at 1270 and 1300 °C by hotpressing. The external pressure during hot-pressing sintering promotes densification of the hard alloys by adding a driving force [13]. The liquid binder phase can flow easily under the external pressure. It can fill the pores between the two W0.6Al0.4C0.8 grains. Therefore, the relative densities of these samples were high.
2.2. Characterization
3.2. Microstructure and grain size
The densities of the sintered hard alloys were determined by Archimedes' water immersion method. The microstructures of samples were examined using an environmental scanning electron microscope (ESEM, XL-30 ESEM FEG Scanning Electron Microscope FEI Company). The average W0.6Al0.4C0.8 grain size was determined from SEM micrographs using Image-pro Plus software. The hardnesses were measured by a Vickers micro-hardness tester (FM-700, Japan) with a load of 30 kgf and dwell time of 15 s. The reported Vickers hardness values were the average of four measurements. The transverse rupture strength was measured at room temperature by a three-point bending method on an Instron model 1125 test machine with a loading rate of 2 mm/min. The reported transverse rupture strength values were the average of three transverse rupture test results.
Fig. 2 shows the microstructures of the hard alloys with different Cr3C2 contents sintered at 1240–1300 °C. The bright and dark contrast phases are W0.6Al0.4C0.8 and Co. The Co binder phase was found to be mainly located around the W0.6Al0.4C0.8 grains. Fig. 3 shows the variation of the average W0.6Al0.4C0.8 grain size with sintering temperature and Cr3C2 content. The grain size of W0.6Al0.4C0.8 increased with increasing temperature. This was due to the amount of liquid Co increasing at higher temperatures, and the W0.6Al0.4C0.8 grains grew in a dissolution–reprecipitation process. The decreasing W0.6Al0.4C0.8 grain size increased with Cr3C2 content and demonstrated the inhibition of W0.6Al0.4C0.8 grain growth by Cr3C2. The influence of Cr3C2 on cemented carbide grain growth inhibition can be explained by Cr3C2 having higher solubility and diffusivity in the cobalt binder phase, which hindered the dissolution–reprecipitation process of the cemented carbide grains [23]. Therefore, the dissolution of cemented carbide in the binder phase was prevented and grain growth was decreased, and hence, the precipitation was hindered. The shorter sintering time may be another reason for the inhibition of the W0.6Al0.4C0.8 grain growth, which was due to the increase of sintering driving force caused by the external pressure of hot-pressing. The external pressure promotes the densification of the hard alloys [13], and so hot-pressing needed a shorter sintering time than vacuum sintering [5,6].
3. Results and discussion 3.1. Relative density The relative densities of the samples with different Cr3C2 contents sintered at 1240, 1270 and 1300 °C are shown in Fig. 1. When sintered at 1240 and 1270 °C, the relative densities of the samples slightly decreased as the Cr3C2 content increased, which was similar to the WC–Co system with Cr3C2 [19]. This may be due to the liquid content of Co being less at a lower temperature, which resulted in the lower solubility of Cr3C2 in the binder phase. The increased Cr3C2 content exceeded its saturated concentration in liquid Co and excessive Cr3C2 precipitated at the cemented carbide/binder phase boundary. It may have a great influence on the hard alloy densification process [19]. However, when sintered at 1300 °C, the relative densities of the samples ranged from 99.37% to 99.4% as the Cr3C2 content increased, a
Fig. 1. Relative density of W0.6Al0.4C0.8–Co with different Cr3C2 contents sintered at different temperatures.
3.3. Mechanical properties The hardness variation of samples with Cr3C2 is illustrated in Fig. 4, and there were peaks for all samples containing 0.6 wt.% Cr3C2 with the highest peak at 1987 kg/mm2 for the sample sintered at 1300 °C. The improvement of the hardness may be mainly due to the finer W0.6Al0.4C0.8 grain size. According to Hall–Petch relationship [24], the hardness of hard alloys is determined by the grain size, and fine microstructure leads to higher hardness. From Figs. 2 and 3, it can be seen that refined W0.6Al0.4C0.8 grains were obtained by adding Cr3C2. Therefore, the addition of Cr3C2 resulted in the hardness increased. However, the hardness values decreased as the content of Cr3C2 was over 0.6 wt.%, indicating that excessive Cr3C2 can reduce the hardness of the hard alloys. This behavior may be attributed to too much Cr3C2 in Co affecting the wetting ability of the binder phase on W0.6Al0.4C0.8 particles, weakening the bonding of binder/cemented carbide interface [25]. Therefore, when the addition was excessive, the hardness of the samples decreased. The hardness of samples sintered at 1240 °C was lower than those sintered at 1270 and 1300 °C. This may in part be due to the fact that the samples sintered at 1240 °C exhibited a significantly lower relative density. The influence of Cr3C2 content on transverse rupture strength of the samples for each sintering temperature is depicted in Fig. 5. The transverse rupture strength increased initially with peaks for all samples containing 0.3 wt.% Cr3C2 with the maximum being 1929 MPa for the sample sintered at 1270 °C. The increase in the transverse rupture strength may be due to the decrease of the grain size, but the 0.3 wt.% Cr3C2 addition did not give the lowest grain size. It was reported [24] that the total amount of the interfacial area between carbide grains
Z. Zhao et al. / Int. Journal of Refractory Metals and Hard Materials 48 (2015) 333–337
Fig. 2. SEM images of the microstructures of W0.6Al0.4C0.8–Co with different Cr3C2 contents sintered at different temperatures.
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Fig. 3. Average W0.6Al0.4C0.8 grain size of W0.6Al0.4C0.8–Co with different Cr3C2 contents sintered at different temperatures.
Fig. 5. Transverse rupture strength results of W0.6Al0.4C0.8–Co with different Cr3C2 contents.
and the binder phase increases as the grain size decreases. The increase in the fraction of the crack path through binder/cemented carbide interfaces contributes a significant amount of the fracture energy, which in turn enhances the overall transverse rupture strength. However, the transverse rupture strength values decreased as the Cr3C2 content exceeded 0.3 wt.%, which could have been due to decreased wetting ability of the binder phase on the cemented carbide particles [25]. Therefore, when the addition was excessive, the transverse rupture strength of the samples decreased. The transverse rupture strength of samples sintered at 1240 °C is lower than that of the hard alloys sintered at 1270 and 1300 °C. This was attributed to the lower relative density, since it is well known that under load, internal pores are sites of stress concentration, and they lower transverse rupture strength [26]. It can be seen that the peaks for hardness and transverse rupture strength did not occur at the same composition, nor sintering temperature. For a given binder phase content and at a nearly identical relative density, the hardness of hard alloys is mainly determined by the grain size of cemented carbides. Conversely, the transverse rupture strength is determined by the properties of the binder phase and the grain size of cemented carbides. The maximum hardness value was achieved by the sample containing 0.6 wt.% Cr3C2 sintered at 1300 °C. It may be that too much Cr3C2 in the binder phase affected the wetting ability of
the binder phase on cemented carbide particles, weakening the bonding of binder/cemented carbide interface [25]. Therefore, the maximum transverse rupture strength value was achieved by the sample containing 0.3 wt.% Cr3C2 sintered at 1270 °C.
Fig. 4. Hardness results of W0.6Al0.4C0.8–Co with different Cr3C2 contents.
4. Conclusions W0.6Al0.4C0.8–8 wt.% Co hard alloys with various Cr3C2 contents treated at different sintering temperatures were prepared by hotpressing. The addition of Cr3C2 not only had an effect on densification of the W0.6Al0.4C0.8–Co materials, but also significantly mitigated the growth of W0.6Al0.4C0.8 grains. The hardness and transverse rupture strength of the samples were improved by the addition of Cr3C2. The hardness of the sample with 0.6 wt.% Cr3C2 and sintered at 1300 °C had the highest value of 1987 kg/mm2. The transverse rupture strength of the samples achieved the maximum value of 1929 MPa for 0.3 wt.% Cr3C2 sintered at 1270 °C. This shows that for these alloys in terms of transverse rupture strength, higher than 0.3 wt.% Cr3C2 is detrimental. References [1] J.M. Yan, X.F. Ma, W. Zhao, H.G. Tang, C.J. Zhu, S.G. Cai, High-pressure sintering study of a novel hard material (W0.5Al0.5)C0.5 without binder metal, Int. J. Refract. Met. Hard Mater. 25 (2007) 62–66. [2] J.M. Yan, X.F. Ma, W. Zhao, H.G. Tang, C.J. Zhu, S.G. Cai, Synthesis and high-pressure sintering of (W0.5Al0.5)C, Mater. Res. Bull. 40 (2005) 701–707. [3] J.M. Yan, X.F. Ma, W. Zhao, H.G. Tang, C.J. Zhu, S.G. Cai, Crystallization and characterization of substoichiometric compound (W0.5Al0.5)C0.5 obtained by solid-state reaction, Metall. Mater. Trans. A 37A (2006) 1692–1695. [4] J.W. Liu, X.F. Ma, H.G. Tang, W. Zhao, The research on the binder phase for the (W0.5Al0.5)C0.5 system, Int. J. Refract. Met. Hard Mater. 29 (2011) 435–440. [5] Z.H. Qiao, X.F. Ma, W. Zhao, H.G. Tang, B. Zhao, Fabrication, thermal stability and mechanical properties of novel (W0.5Al0.5)C0.8-Co composite prepared by mechanical alloying and hot-pressing sintering, J. Alloys Compd. 456 (2008) 514–517. [6] Z.H. Qiao, X.F. Ma, W. Zhao, H.G. Tang, J.M. Yan, S.G. Cai, B. Zhao, Bulk ultrafine binderless (W0.4Al0.6)C prepared by high pressure sintering, J. Alloys Compd. 453 (2008) 382–385. [7] J.W. Liu, X.F. Ma, H.G. Tang, W. Zhao, Vacuum sintering of novel cemented carbide hard alloy (W0.4Al0.6)C0.5-Co, Int. J. Refract. Met. Hard Mater. 32 (2012) 7–10. [8] Z.H. Qiao, X.F. Ma, W. Zhao, H.G. Tang, S.G. Cai, B. Zhao, Fabrication, microstructure and mechanical properties of novel cemented hard alloy obtained by mechanical alloying and hot-pressing sintering, Mater. Sci. Eng. A 460–461 (2007) 46–49. [9] 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 sintering, Int. J. Refract. Met. Hard Mater. 26 (2008) 251–255. [10] 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. [11] J.W. Liu, X.F. Ma, H.G. Tang, W. Zhao, Nanostructured Ni-bonded hard materials (W0.6Al0.4)C0.5–Ni prepared by mechanical alloying and hot-pressing, Mater. Sci. Eng. A 532 (2012) 146–150.
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[19] L. Sun, T.E. Yang, C.C. Jia, J. Xiong, VC, Cr3C2 doped ultrafine WC–Co cemented carbides prepared by spark plasma sintering, Int. J. Refract. Met. Hard Mater. 29 (2011) 147–152. [20] I. Azcona, A. Ordóñez, J.M. Sánchez, F. Castro, Hot isostatic pressing of ultrafine tungsten carbide–cobalt hardmetals, J. Mater. Sci. 37 (2002) 4189–4195. [21] D. Agrawal, J. Cheng, P. Seegopual, L. Gao, Grain growth control in microwave sintering of ultrafine WC–Co composite powder compacts, Powder Metall. 43 (2000) 15–16. [22] S.I. Cha, S.H. Hong, B.K. Kim, Spark plasma sintering behavior of nanocrystalline WC– 10Co cemented carbide powders, Mater. Sci. Eng. A 351 (2003) 31–38. [23] R.K. Sadangi, L.E. McCandlish, B.H. Kear, P. Seegopaul, Grain growth inhibition in liquid phase sintered nanophase WC/Co alloys, Int. J. Powder Metall. 35 (1) (1999) 27–33. [24] T.T. Shen, D.H. Xiao, X.Q. Ou, M. Song, Y.H. He, N. Lin, D.F. Zhang, Effects of LaB6 addition on the microstructure and mechanical properties of ultrafine grained WC–10Co alloys, J. Alloys Compd. 509 (2011) 1236–1243. [25] S. Liu, Study on rare-earth doped cemented carbides in China, Int. J. Refract. Met. Hard Mater. 27 (2009) 528–534. [26] S.H. Chang, P.Y. Chang, Investigation into the sintered behavior and properties of nanostructured WC–Co–Ni–Fe hard metal alloys, Mater. Sci. Eng. A 606 (2014) 150–156.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Cr3C2 doped W0.6Al0.4C0.8–Co hard alloys prepared by hot-pressing Zhenye Zhao a,b, Jianwei Liu a,⁎, Huaguo Tang a,⁎, Xianfeng Ma a, Wei Zhao a a b
State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, No. 5625 Renmin Street, Changchun 130022, Jilin, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 12 May 2014 Received in revised form 2 October 2014 Accepted 7 October 2014 Available online 8 October 2014 Keywords: W0.6Al0.4C0.8–Co Cr3C2 Hot-pressing Microstructure Mechanical properties
a b s t r a c t W0.6Al0.4C0.88 wt.% Co hard alloys doped with 0.3, 0.6 and 0.9 wt.% Cr3C2 were prepared by hot-pressing. The effect of Cr3C2 content and sintering temperature on the relative density, microstructure and mechanical properties of the hard alloys was investigated. Based on the results, both the Cr3C2 content and sintering temperature had great influences on the carbide grain size. The hardness of the samples added by 0.6 wt.% Cr3C2 achieved the maximum value of 1987 kg/mm2 when it was sintered at 1300 °C. The transverse rupture strength of the samples achieved the maximum value of 1929 MPa with 0.3 wt.% Cr3C2 sintered at 1270 °C. © 2014 Published by Elsevier Ltd.
1. Introduction Previously, W1 − xAlxCy (x = 0.1–0.86, y = 0.1–0.8) was synthesized by mechanical alloying and solid state reactions [1–11]. W1 − xAlxCy is a solid solution of Al replacing W in the WC lattice with or without carbon vacancies and with the WC-type structure. It was reported that W1 − xAlxCy-Co had excellent mechanical properties [6–10]. Ultra-fine or nano-crystalline cemented tungsten carbide materials have high hardness, excellent wear resistance and better toughness than conventional materials [12–14]. However, WC grain growth occurs during sintering of the nanometer sized WC–Co raw powder mixtures through conventional sintering without any additions, especially in liquid state [13]. Therefore, many efforts have been made to obtain ultrafine or nanometer grained WC–Co hard materials. The most successful way of inhibiting the WC grain growth is the addition of grain growth inhibitors, such as chromium carbide (Cr3C2), vanadium carbide (VC), niobium carbide (NbC) or molybdenum carbide (Mo2C) to the raw powder mixtures at amounts less than 1 wt.% [15–19]. Cr3C2 and VC are the effective grain growth inhibitors due to their high solubility and mobility in the cobalt phase at lower temperatures [18]. Moreover, the grain growth can be inhibited to a certain extent by using special sintering technologies at lower sintering temperatures and/or shorter sintering times, such as in hot pressing [20], microwave sintering [21] or spark plasma sintering [22]. The contents of aluminum and carbon have influence on the mechanical properties of W1 − xAlxCy-M hard alloys [1–11]. In a previous work, W0.5Al0.5Cy–M and W0.4Al0.6Cy–M alloys have been well studied ⁎ Corresponding authors. E-mail address: [email protected] (J. Liu).
http://dx.doi.org/10.1016/j.ijrmhm.2014.10.004 0263-4368/© 2014 Published by Elsevier Ltd.
[1–9]. However, the research on W0.6Al0.4Cy–M alloys is less [10,11], so W0.6Al0.4C0.8 was chosen for study. Since W0.6Al0.4C0.8 has the same hexagonal structure with WC [10,11], so cobalt was selected as the binder, which is a good binder for WC sintering. Cr3C2 was used as a grain growth inhibitor in this work. Samples of W0.6Al0.4C0.8–Co hard alloys with 0 (for comparison), 0.3, 0.6 and 0.9 wt.% Cr3C2 contents were prepared by hot-pressing. The effect of Cr3C2 content and sintering temperature on the density, grain size, hardness and transverse rupture strength of W0.6Al0.4C0.8–Co hard alloys was investigated. 2. Experimental procedure 2.1. Preparation Powders of tungsten (1 μm, 99 wt.% purity, Ganzhou Tejing Tungsten Molybdenum Co. Ltd., China), aluminum (1 μm, 99.5 wt.% purity, Luoyang Discoverer Aluminum Co. Ltd., China), cobalt (2 μm, 99.6 wt.%, Changsha Langfeng Metallic Material Co. Ltd., China), carbon (2 μm, 99 wt.% purity, Foshanshi Nanhai Dali Yinyihui Graphite Mold Co. Ltd., China), and chromium carbide (1 μm, 99.5 wt.% purity, Zhuzhou Boda Tungsten Industry Co., Ltd, China) were used as raw materials. The alloy powders W0.6Al0.4C0.8 were prepared by mechanical alloying and solid state reaction [10,11]. The W0.6Al0.4C0.8, Co and Cr3C2 powders were mixed in a stainless steel vial under an argon atmosphere for 30 h in a high energy attrition mill at room temperature (Model: GN-2, China). The ball-to-powder mass ratio was 7.5:1, and the rotational speed was 532 rpm. Next, the mixtures of powders were pressed into a stainless steel die of 40 mm × 10 mm × 10 mm under a compaction pressure of 350 MPa and held at pressure for 1 min. The resultant composite powders were
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Z. Zhao et al. / Int. Journal of Refractory Metals and Hard Materials 48 (2015) 333–337
sintered in an inductive hot-pressing vacuum furnace (Model: TENGLONG 2000, China) with the following cycle. (a) Heated from room temperature to the sintering temperature with a heating rate of about 20 °C/min, (b) holding at the sintering temperature for 15 min, and (c) cooled down from sintering temperature to 200 °C at about 30 °C/min, and furnace-cooled from 200 °C to room temperature. During sintering, the pressure in the die was kept at 30 MPa. After sintering, the bulk specimens were ground and polished.
small effect, and within the error bars. At 1300 °C, the amount of liquid Co increased, which resulted in that more dissolved Cr3C2 in the liquid Co. The added Cr3C2 can dissolve fully in the liquid Co, and thus had little influence on the hard alloy densification process [19]. In addition, all the relative densities of the samples were over 99% when the hard alloys were sintered at 1270 and 1300 °C by hotpressing. The external pressure during hot-pressing sintering promotes densification of the hard alloys by adding a driving force [13]. The liquid binder phase can flow easily under the external pressure. It can fill the pores between the two W0.6Al0.4C0.8 grains. Therefore, the relative densities of these samples were high.
2.2. Characterization
3.2. Microstructure and grain size
The densities of the sintered hard alloys were determined by Archimedes' water immersion method. The microstructures of samples were examined using an environmental scanning electron microscope (ESEM, XL-30 ESEM FEG Scanning Electron Microscope FEI Company). The average W0.6Al0.4C0.8 grain size was determined from SEM micrographs using Image-pro Plus software. The hardnesses were measured by a Vickers micro-hardness tester (FM-700, Japan) with a load of 30 kgf and dwell time of 15 s. The reported Vickers hardness values were the average of four measurements. The transverse rupture strength was measured at room temperature by a three-point bending method on an Instron model 1125 test machine with a loading rate of 2 mm/min. The reported transverse rupture strength values were the average of three transverse rupture test results.
Fig. 2 shows the microstructures of the hard alloys with different Cr3C2 contents sintered at 1240–1300 °C. The bright and dark contrast phases are W0.6Al0.4C0.8 and Co. The Co binder phase was found to be mainly located around the W0.6Al0.4C0.8 grains. Fig. 3 shows the variation of the average W0.6Al0.4C0.8 grain size with sintering temperature and Cr3C2 content. The grain size of W0.6Al0.4C0.8 increased with increasing temperature. This was due to the amount of liquid Co increasing at higher temperatures, and the W0.6Al0.4C0.8 grains grew in a dissolution–reprecipitation process. The decreasing W0.6Al0.4C0.8 grain size increased with Cr3C2 content and demonstrated the inhibition of W0.6Al0.4C0.8 grain growth by Cr3C2. The influence of Cr3C2 on cemented carbide grain growth inhibition can be explained by Cr3C2 having higher solubility and diffusivity in the cobalt binder phase, which hindered the dissolution–reprecipitation process of the cemented carbide grains [23]. Therefore, the dissolution of cemented carbide in the binder phase was prevented and grain growth was decreased, and hence, the precipitation was hindered. The shorter sintering time may be another reason for the inhibition of the W0.6Al0.4C0.8 grain growth, which was due to the increase of sintering driving force caused by the external pressure of hot-pressing. The external pressure promotes the densification of the hard alloys [13], and so hot-pressing needed a shorter sintering time than vacuum sintering [5,6].
3. Results and discussion 3.1. Relative density The relative densities of the samples with different Cr3C2 contents sintered at 1240, 1270 and 1300 °C are shown in Fig. 1. When sintered at 1240 and 1270 °C, the relative densities of the samples slightly decreased as the Cr3C2 content increased, which was similar to the WC–Co system with Cr3C2 [19]. This may be due to the liquid content of Co being less at a lower temperature, which resulted in the lower solubility of Cr3C2 in the binder phase. The increased Cr3C2 content exceeded its saturated concentration in liquid Co and excessive Cr3C2 precipitated at the cemented carbide/binder phase boundary. It may have a great influence on the hard alloy densification process [19]. However, when sintered at 1300 °C, the relative densities of the samples ranged from 99.37% to 99.4% as the Cr3C2 content increased, a
Fig. 1. Relative density of W0.6Al0.4C0.8–Co with different Cr3C2 contents sintered at different temperatures.
3.3. Mechanical properties The hardness variation of samples with Cr3C2 is illustrated in Fig. 4, and there were peaks for all samples containing 0.6 wt.% Cr3C2 with the highest peak at 1987 kg/mm2 for the sample sintered at 1300 °C. The improvement of the hardness may be mainly due to the finer W0.6Al0.4C0.8 grain size. According to Hall–Petch relationship [24], the hardness of hard alloys is determined by the grain size, and fine microstructure leads to higher hardness. From Figs. 2 and 3, it can be seen that refined W0.6Al0.4C0.8 grains were obtained by adding Cr3C2. Therefore, the addition of Cr3C2 resulted in the hardness increased. However, the hardness values decreased as the content of Cr3C2 was over 0.6 wt.%, indicating that excessive Cr3C2 can reduce the hardness of the hard alloys. This behavior may be attributed to too much Cr3C2 in Co affecting the wetting ability of the binder phase on W0.6Al0.4C0.8 particles, weakening the bonding of binder/cemented carbide interface [25]. Therefore, when the addition was excessive, the hardness of the samples decreased. The hardness of samples sintered at 1240 °C was lower than those sintered at 1270 and 1300 °C. This may in part be due to the fact that the samples sintered at 1240 °C exhibited a significantly lower relative density. The influence of Cr3C2 content on transverse rupture strength of the samples for each sintering temperature is depicted in Fig. 5. The transverse rupture strength increased initially with peaks for all samples containing 0.3 wt.% Cr3C2 with the maximum being 1929 MPa for the sample sintered at 1270 °C. The increase in the transverse rupture strength may be due to the decrease of the grain size, but the 0.3 wt.% Cr3C2 addition did not give the lowest grain size. It was reported [24] that the total amount of the interfacial area between carbide grains
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Fig. 2. SEM images of the microstructures of W0.6Al0.4C0.8–Co with different Cr3C2 contents sintered at different temperatures.
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Fig. 3. Average W0.6Al0.4C0.8 grain size of W0.6Al0.4C0.8–Co with different Cr3C2 contents sintered at different temperatures.
Fig. 5. Transverse rupture strength results of W0.6Al0.4C0.8–Co with different Cr3C2 contents.
and the binder phase increases as the grain size decreases. The increase in the fraction of the crack path through binder/cemented carbide interfaces contributes a significant amount of the fracture energy, which in turn enhances the overall transverse rupture strength. However, the transverse rupture strength values decreased as the Cr3C2 content exceeded 0.3 wt.%, which could have been due to decreased wetting ability of the binder phase on the cemented carbide particles [25]. Therefore, when the addition was excessive, the transverse rupture strength of the samples decreased. The transverse rupture strength of samples sintered at 1240 °C is lower than that of the hard alloys sintered at 1270 and 1300 °C. This was attributed to the lower relative density, since it is well known that under load, internal pores are sites of stress concentration, and they lower transverse rupture strength [26]. It can be seen that the peaks for hardness and transverse rupture strength did not occur at the same composition, nor sintering temperature. For a given binder phase content and at a nearly identical relative density, the hardness of hard alloys is mainly determined by the grain size of cemented carbides. Conversely, the transverse rupture strength is determined by the properties of the binder phase and the grain size of cemented carbides. The maximum hardness value was achieved by the sample containing 0.6 wt.% Cr3C2 sintered at 1300 °C. It may be that too much Cr3C2 in the binder phase affected the wetting ability of
the binder phase on cemented carbide particles, weakening the bonding of binder/cemented carbide interface [25]. Therefore, the maximum transverse rupture strength value was achieved by the sample containing 0.3 wt.% Cr3C2 sintered at 1270 °C.
Fig. 4. Hardness results of W0.6Al0.4C0.8–Co with different Cr3C2 contents.
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