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Effects of NbC additions on the microstructure and properties of non-uniform structure WC-Co cemented carbides
Author’s Accepted Manuscript Effects of NbC additions on the microstructure and properties of non-uniform structure WC-Co cemented carbides Yang Gao, Ming-Yuan Yan, Bing-Hui Luo, Sheng Ouyang, Wei Chen, Zhen-hai Bai, Hui-bo Jing, Wen-Wen Zhang www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)30094-1 http://dx.doi.org/10.1016/j.msea.2017.01.077 MSA34639
To appear in: Materials Science & Engineering A Received date: 10 December 2016 Revised date: 19 January 2017 Accepted date: 21 January 2017 Cite this article as: Yang Gao, Ming-Yuan Yan, Bing-Hui Luo, Sheng Ouyang, Wei Chen, Zhen-hai Bai, Hui-bo Jing and Wen-Wen Zhang, Effects of NbC additions on the microstructure and properties of non-uniform structure WC-Co cemented carbides, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.01.077 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of NbC additions on the microstructure and properties of non-uniform structure WC-Co cemented carbides Yang Gao, Ming-Yuan Yan*, Bing-Hui Luo*, Sheng Ouyang, Wei Chen, Zhen-hai Bai, Hui-bo Jing, Wen-Wen Zhang School of Materials Science and Engineering, Central South University, Changsha 410083, P. R. China [email protected] [email protected] *
Corresponding authors.
Abstract In this work, the effects of NbC additions on microstructure and properties of non-uniform structure WC-7Co cemented carbides were investigated X-ray diffractometer, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), mechanical properties tester and electrochemical workstation, respectively. The results show that WC phase can be partially dissolved into the NbC to form a (Nb,W)C solid solution. According to EPMA analysis, the amount of W atoms, dissolving into the NbC grains, increases with the NbC addition. Moreover, when the content of NbC is beyond 1%, the WC-Co cemented carbides with non-uniform structure are formed with significant reduction of average grain size of WC. With NbC addition increasing from 0 to 2 wt.%, the hardness is increased from 1475 to 1570 MPa while the fracture
toughness decreased from 12.1 to 10.3 MPa m1/2. However, with the further addition of NbC, the hardness slightly decreased. With NbC addition between 0 and 1 wt.%, the TRS is gradually decreased from 2982 MPa to 2745 MPa, while, as the NbC content exceeds 1 wt.%, the TRS leveled off. Because of the decrease of grain size and the (Nb,W)C phase formation, caused by NbC addition, the crack defection was weakened, which led to the decrease of fracture toughness. Meanwhile, the corrosion resistance of non-uniform structure WC-Co cemented carbides can be significantly improved by adding NbC to the material due to increased -Co in binder phase.
Keywords Cemented carbides; NbC; Microstructure; Mechanical properties; Corrosion resistance
1. Introduction Conventional WC-Co cemented carbides with high hardness, strength, toughness, and good wear resistance, has been widely used in cutting, machining, drilling, mining tools, wear resistant parts, etc [1-4]. For the precision in machining work, the cemented carbides with a combination of higher hardness, wear resistance and toughness is required. However, the hardness and fracture toughness are trade-off for uniform structure cemented carbides [5-6]. The grain size is a critical for the properties of cemented carbides. The cemented carbides with coarse grain possess great toughness and thermal conductivity, while the cemented carbides with fine grain are of excellent wear resistance and hardness [6]. So currently, non-uniform structure cemented carbides are being developed to obtain a better combination of wear resistance and toughness which is unobtainable for the conventional uniform microstructure [6-8]. During liquid phase
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sintering, grain coarsening occurs caused by small grains dissolution and large grains growth [9-11]. On the grain growth, the powder particle size distribution has a strong impact, that is, the broader is the particle size distribution range, the more abnormally the grains grow. The key step of preparing non-uniform structure cemented carbides is maintaining a fine grain structure in order to avoid the structure homogenization during sintering process, and the addition of grain growth inhibitors is an effective way to prevent grain growth and structure homogenization [12]. The mechanical properties of WC–Co cemented carbides are usually affected by adding different grain growth inhibitors. Generally, the overall inhibition effectiveness is in the order of VC>Cr3C2> NbC>TaC>TiC [13]. However, would result in the embrittlement of WC-Co cemented carbide and the weakening of mechanical properties. The introduction of Cr3C2 nearly exhibits no effect on improving high temperature properties. TiC is the worst one of all the inhibitors mentioned above. So, it seems that NbC and TaC are the best grain growth inhibitors for WC-Co cemented carbides, as a result of their unique physical and chemical characteristics. However, in comparison with NbC, TaC is more expensive, which restrains the application and popularity in engineering. Moreover, NbC presents higher hardness of 19.60 GPa, lower density of 7.79 g/cm3 and higher melting temperature of 3600 °C, compared to WC [14-17]. NbC is, therefore, a very promising grain growth inhibitor for cemented carbides as evidenced by increased interest in the research of non-uniform structure cemented carbides containing NbC in recent years. The hardness, toughness as well as transverse rupture strength of WC–Co cemented carbide can be strongly affected by the NbC contents [16-18]. Wei Zhou [18] et al studied the fracture behavior and found that when the addition of NbC was up to 3wt%, the brittle (W,Nb)C solid solution increased considerably, and this led to a low stress brittle fracture. Da Silva et al [19]
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have verified that NbC can effectively inhibit WC grain growth and increase the hardness in WC–Co cemented carbide. Li [20] et al declared that the addition of NbC refined the grain size, increased the transverse rupture strength and improved the high temperature hardness of WC-Co functionally graded cemented carbides. Acchar et al [21] also investigated the microstructure and mechanical properties of uniform structure WC-Co reinforced With 2 wt.% NbC. However, little effort has been made so far in studying the effects of NbC additions on the microstructure, mechanical properties and corrosion behavior of non-uniform structure WC-Co cemented carbides. The WC-Co cemented carbides containing NbC can be used to cut metal, plastic and wood. But, during cutting, the alloys are usually exposed to aggressive environments (e.g. water-based coolants, humid environment and the processed materials) that may contain acid medium. For simulating the acid environment experienced by the alloys used in cutting applications, the HCl solution was chosen for this investigation. In this study, non-uniform structure cemented carbides were sintered of WC powder (coarse/fine weight ratio is 1:1), Co powder and certain amount of NbC via low pressure sintering using the powder metallurgy technique. The effects of various NbC contents on the microstructure, mechanical properties, corrosion behavior of non-uniform structure WC-7Co cemented carbides were investigated.
2. Experimental procedure 2.1 Preparation of cemented carbides The characteristics of the raw powders used in the paper are listed in Table 1. The WC-1 and WC-2 powders were produced by Zhuzhou Cemented Carbide Group Co. Ltd. of China. The
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Co and NbC powders were provided by Hunan Metallurgy material institute. Based on our previous work [22], when the weight ratio of coarse to fine WC is 1:1, the WC-Co cemented carbides show a relatively good combination between hardness and toughness. In this research, the nominal compositions of non-uniform structure cemented carbides are given in Table 2. The WC powder (coarse/fine weight ratio is 1:1), Co powder and a given amount of NbC powders were milled in a stainless steel tumbler under argon atmosphere in ethanol for 30 h. The cemented carbide balls, with a diameter of 8 mm, were used as milling bodies. The ball-to powder weight ratio was 3:1 and the rotation rate of the mill was 238 rpm. After the milling, 2 wt.% paraffin was added as pressing aid, milled slurry was dried under vacuum conditions at 70 °C for 2 h, granulated and sieved. Then the milled powders were pressed into a cylindrical specimen under a uniaxial pressure of 200 MPa. The pressed specimens were sintered in a SIP-500/10MP industrial sinter-HIP furnace with the following cycle: (a) Heated from room temperature to 450 °C with a heating rate of 2.22 °C/min and held at 450 °C for 150 min under hydrogen; (b) Heated from 450 °C to 800 °C with a heating rate of 3.89 °C/min and held at 800 °C for 30 min, and then heated from 800 °C to 1220 °C with a heating rate of about 4.44°C/min and held at 1220 °C for 40 min. All this step is done under vacuum; (c) Heated to sintering temperature (1450 °C) with a heating rate of 2.89 °C/min, when the temperature rose to 1450 °C, at which temperature it would be held for 60 min, 5 MPa argon pressure would be exerted and maintained till it is cooled down to room temperature; (d) At last, the furnace cooled down .from sintering temperature to room temperature.
Raw powders WC-1 WC-2 NbC
Table 1 General characteristics of the raw powders. Purity (wt.%) Carbon content Oxygen content (wt.%) (wt.%) 99.9 6.13 0.01 99.9 6.13 0.01 99.8 0.2 5
Particle size (µm) 1.0 3.5 1.48
Co
98.5
-
0.4
1.31
Table 2 Nominal compositions of non-uniform structure cemented carbides (wt.%) and volume fraction of Co in the alloys (vol.%). Volume Samples Co WC-1 WC-2 NbC fraction of Co Alloy 1 7 46.5 46.5 0 11.65 Alloy 2 7 46.25 46.25 0.5 11.60 Alloy 3 7 46 46 1 11.55 Alloy 4 7 45.5 45.5 2 11.44 Alloy 5 7 44.5 44.5 4 11.22
2.2 Properties testing The coercive force (Hc) and magnetic saturation (CoM) of the specimens were measured with the ZDHC40 coercive meter and ZDMA6530 magnetic saturation induction tester, respectively. The hardness was measured by the Vickers hardness tester (HVS-50) with a load of 30 kgf and dwell time of 15 s. For each of the properties five replicates were performed to ensure reliability of the results generated. The fracture toughness can be obtained by Palmqvist indentation method. The crack length the tip of the indentation was measured the applied Vickers indentation load of 30 kg, The fracture toughness (KIC) of the sintered specimens could be calculated by the following equation [23]： K IC 0.15 HV 30 / l
(1)
where HV is the Vickers hardness, ∑l is the sum of the crack length. The TRS (transverse rupture strength) was measured using rectangular specimens with dimensions of 5.25 mm×6.5 mm×20 mm by Istron3369 materials test system. The values of TRS can be obtained according to international standard BS EN ISO 3327: 2009 using Type B test pieces as follows [24]: 6
R
3FL 2bh 2
(2)
where R is the TRS, F is the load required to fracture the test specimen, L is the distance between supports, b is the width of test beam width of test piece, and h is the height of test piece. 2.3 Microstructure and phase constitution testing The specimens were ground and polished for elemental mapping and microstructure analyses, which was carried out by Sirion-200 scanning electron microscopy (SEM) with Gensis60 X-ray energy dispersive spectroscopy (EDS). The WC mean grain size (d) of the specimens was measured by the linear intercept method on the 10k- magnification SEM images. The line scan of the specimens from one WC to another WC via Co binder phase was measured using electron probe microanalysis (EPMA, JXA–8230) with tungsten filament. The Nb, W and C contents in the (Nb,W)C phase were also obtained by EPMA. The phase constitution analysis of the specimens was detected by D/max-2500 X-ray diffractometry (XRD) with Cu Ka radiation (λ=0.15406 nm), obtained from a tube operated at 250 mA , 40 kV. The scan range was between 30° and 80° (2). The X-ray diffraction data was carried out with a step of 0.02° and a count time of 10 s. Meanwhile, the lattice constant of cubic NbC phase was determined by external standard method calibration. 2.4 Corrosion tests The polarization curves were measured using three electrode testing system in the IM6e electrochemical workstation. The three electrode experimental set-up was comprised of the specimen as working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum auxiliary electrode. The test area of the specimen was 1 cm2 and the corrosion behavior was studied in 1 mol/L HCl solution at room temperature. Potentiodynamic polarization curves 7
were carried out from -0.6 V to 1.0 V with a scanning speed of 2 mV/s. The corrosion potential (Ecorr) and corrosion current (Icorr) of the specimens were analyzed by corr-view software.
3. Results and discussion 3.1 Phase constitution and microstructure analysis Fig. 1 shows the X-ray diffraction patterns and partially enlarged patterns of WC-7Co cemented carbides with various NbC additions. As shown in Fig. 1(a), only the diffraction peaks of WC hard phase, cubic NbC and fcc Co binder phase can be identified in the specimens. The diffraction peaks of fcc Co become stronger with increasing NbC content. The hcp Co in binder phase can hardly be detected due to its quite small amount. It is known that there are two allotropes of cobalt, hcp ε-Co, stable at temperatures below about 417 °C, and fcc -Co, stable at higher temperatures. Some studies have demonstrated that the Nb dissolved in binder phase results in a strong resistance to martensitic phase transformation from α-Co to ε-Co [16]. Therefore, with the increasing of Nb dissolved in binder phase, the content of fcc-Co is enhanced in the specimens. This would explain why the diffraction peaks of fcc Co become stronger with increasing NbC content. According to the partially enlarged XRD diffraction patterns in Fig. 1(b), it can be seen that diffraction peaks of NbC gradually shift toward the high angle direction with the increasing of NbC content. The shifting of peak position from 40.29° to 40.48° should be attributed to the dissolution of W atoms of smaller radius into the cubic NbC lattice.
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Fig. 1 (a) XRD patterns of WC-7Co cemented carbides with various NbC additions and (b) partially enlarged XRD patterns of each alloy in the ranges of 2 from 40° and 41°. The variation of the lattice constant of cubic NbC phase in the alloys with the addition of NbC is plotted in Fig. 2. As shown, the lattice constant is gradually reduced with increasing the NbC addition. The decrease of lattice constant is due to increasing dissolution of W atoms in NbC lattice and the formation of a (Nb,W)C solid solution.
Fig. 2 The variation of lattice constant of the cubic NbC phase in WC-Co cemented carbides with NbC content. The solubility of WC in cubic NbC can reach to 38 wt.% at the temperature of 1300 °C [25]. However, the solubility of NbC in Co binder is very low at room temperature [19]. Therefore NbC precipitation will appear on cooling from the sintering temperature. Furthermore,
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WC phase can be partially dissolved into the NbC to form a (Nb,W)C solid solution during sintering process. The composition of (Nb,W)C solid solution of the alloys are conducted by quantitative EPMA analysis. The W, Nb and C contents in (Nb,W)C solid solution are shown in Table 3. It can be seen from Table 3 that the amount of W atoms, dissolving into the NbC grains, increases with the NbC addition. Therefore, the results in Table 3 well explain the evolution of the peak position shifting and lattice constant. Table 3 Nb, W, and C contents in the (Nb,W)C phase obtained by EPMA (wt.%). Elements Alloy 2 Alloy 3 Alloy 4 Alloy 5 Nb 78.155 75.315 70.52 63.174 W 13.645 15.996 20.561 26.606 C 8.20 8.689 8.919 10.22
Figure 3 shows the effect of NbC content on the microstructure of WC-7Co cemented carbides. To identify the WC, (Nb,W)C and Co phases in the alloys, a SEM elemental mapping analysis is exerted on the microstructure in Fig. 3(d), as shown in Fig 4. According to the elemental mapping (in Fig. 4), the phases can be confirmed that the light gray phase and the black gray phase are WC and (Nb,W)C cubic phase respectively, and the black phase is Co binder phase, as shown in Fig. 3. As shown in Fig. 3(a), the amount of small WC grains decrease with the amount of coarse WC grains increasing. Alloy 1 is of the largest average grain size (1.38 µm) compared with the other alloys. Moreover, in Fig. 3(a) the abnormal grain growth has been observed and the largest abnormal growth grain size is about 14 µm. The addition of 0.5% NbC has very little effect on the average WC grain size (1.30 µm). Meanwhile, in Fig. 3(b) the abnormal WC growths also can be observed and the abnormal growth grain size is close to 7.2 µm. When NbC addition is more than 1%, abnormal WC grains almost disappear. However, black grey (Nb,W)C phase appears and the amount of (Nb,W) C increases with the increasing NbC content. When the content of NbC excesses 2%, the average grain size of WC remains 10
unaltered compared with the sample with 1% NbC content.
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Fig. 3 SEM micrograph of WC-7Co cemented carbides with different NbC additions: (a) 0; (b) 0.5%; (c) 1%; (d) 2%; NbC (e) 4%.
Fig. 4 SEM elemental mapping of Fig. 3(d) (alloy 4): showing W (yellow-green); Nb (green); Co (blue); and C (red). The WC grain size distribution of WC-7Co cemented carbides with different NbC additions is presented in Fig. 5. As shown in Fig. 5(a), the overall grain size is uniformly distributed in spite of several abnormal growth grains (see Fig. 3a), and the proportion of fine structure grains (by number) is lowest when compared with the other four alloys with NbC additions. It can be seen that the average grain size of these alloys shows a negative dependency on the content of
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the NbC, and a clear decrease in the average grain size occurs when the NbC content is higher than 0.5 %. As shown, a platform gradually appears at the peak of the grain size distribution, and its width tends to increase with the NbC content. This indicates that a certain percentage of coarse grain and fine grain co-exist in these alloys. When the content of NbC exceeds 1%, non-uniform structure WC-Co cemented carbides are observed with obvious reduction of average grain size of WC.
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Fig. 5 Grain size distribution of WC-7Co cemented carbides with different NbC additions: (a) 0; (b) 0.5%; (c) 1%; (d) 2%; NbC (e) 4%. Table 4 shows the grain size, coercive force (Hc) and magnetic saturation (CoM) of WC-Co cemented carbide with NbC addition. As shown in Table 4, the variation of mean grain size is consistent with the microstructural change (shown in Fig. 3). It also can be clearly seen that the coercive force increases with the decreasing WC mean grain size. For constant binder phase content, coercive force is associated with the mean WC grain size of cemented carbides, which is in consistent with the results described in the literature [26]. With the increasing addition of NbC, the magnetic saturation gradually increases to a constant. It is known that magnetic saturation is associated with the solubility of W in Co binder phase [27]. A higher magnetic saturation means a lower amount of tungsten in the binder. Therefore, the result above indicates that the W dissolved in the binder phase is gradually reduced. This phenomenon described above will be discussed in the following. Table 4 The grain sizes and magnetic properties of WC-Co cemented carbides for different alloys. Samples Mean grain size (µm) Hc (kA/m) CoM (%) Alloy 1 1.38 13.02 6.62 Alloy 2 1.30 14.87 6.71 Alloy 3 1.13 14.91 6.70 Alloy 4 1.11 14.97 6.71 Alloy 5 1.09 14.32 6.70
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Figure 6 shows the SEM micrograph of Alloy 3, and the variation of Co, W, C and Nb concentrations from one WC to the other WC via Co binder phase. It can be seen that NbC uniformly dissolves into the Co binder and the segregation of NbC is not found at the WC/Co interface.
Fig. 6 (a) SEM micrograph of alloy 3 and (b) EPMA line scan along the red line in (a) from one WC to the other WC via Co binder phase. The above WC grain size distribution characteristics can be interpreted as an Ostwald ripening process during the liquid phase sintering of WC-Co cemented carbides. It is well known that the reduction in surface energy of the solid particles is the driving force for small grain dissolving and large grain growth [28]. The growth rate of WC grains is linearly proportional to the driving force (Gv) of coarsening for a grain of size (R) [11], which is given as 1 1 Gv 2 R r
(3)
where γ is a solid/liquid interfacial energy and r is a critical size that is not dissolving and growing. The dissolution of NbC into liquid Co phase is prior to that of WC, resulting from the higher eutectic temperature of 1340°C between WC and Co [29], compared with that of 1300°C between NbC and Co [30]. And, the NbC dissolved in the binder phase (shown in Fig. 6) leads to the decrease of solid/liquid interface energy and the driving force, which inhibits the dissolution
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and precipitation processes of WC grains. This process can be well described by the simplified schematic diagram (in Fig. 7). In addition, that can well explain the increasing of magnetic saturation by adding the NbC. Meanwhile, with the addition of NbC, initial fine and coarse WC grains can be kept after cooling from liquid phase sintering temperature.
Fig. 7 Simplified schematic diagram of the dissolution and reprecipitation processes of: (a) WC-Co; (b) WC-Co containing NbC. 3.2 Properties of non-uniform structure WC-Co cemented carbides with NbC additions The effects of NbC content on the hardness and fracture toughness of non-uniform structure WC-Co cemented carbides are given in Fig. 8. The relationship between hardness and fracture toughness can be well demonstrated in Fig. 8. As shown, there is an antagonistic correlation existing between the hardness and fracture toughness. With NbC adding from 0 to and 2 wt.%, the hardness increases from 1475 to 1570 MPa, whereas the fracture toughness decreases from 12.1 to 10.3 MPa m1/2. The hardness and fracture toughness reach closely their extreme value when the NbC addition is 2 wt.%. When NbC addition is more than 2%, the hardness slightly decreases. The results in Fig 3 and Fig 5 show that the mean grain size decreases with the increasing NbC content. According to Hall-Petch formula, small WC grain size leads to high
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hardness. However, when the content of NbC exceeds 2 wt.%, the hardness slightly decreases. The decreasing hardness should be attributed to the decreasing WC hard phase content and the coarsening of (Nb,W)C particles with increasing NbC (shown in Fig. 3 and Table 3).
Fig. 8 The hardness and fracture toughness of WC-Co cemented carbides with different NbC contents. Figure 9 presents the effect of NbC content on the TRS of WC-Co cemented carbides. It can be seen from Fig. 9 that the TRS generally decreases with the increasing NbC content. With NbC addition increasing from 0 to 1 wt.%, the TRS decreases from 2982 MPa to 2745 MPa. Then, as the NbC content exceeds 1 wt.%, the TRS levels off. Since the solubility of NbC in Co binder is very low at room temperature [19], (Nb,W)C phase is gradually formed and the grain coarsens with the NbC addition. It is apparent that the (Nb,W)C phase is more brittle than the WC hard phase. Compared with WC phase, (Nb,W)C has poorer wettability with Co [18], so there is a reduction in interface strength with Co phase when the NbC content increases. According to the discussion above, the decrease of TRS value is mainly caused by the emerging coarse (Nb,W)C phase and the weak phase interface.
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Fig. 9 The TRS of non-uniform structure WC-Co cemented carbides with different NbC contents. Crack propagation morphology of WC-7Co cemented carbides with NbC additions are shown in Fig. 10. As shown in Fig. 10 (a) and (b), with NbC addition increasing from 0 to 0.5 wt.%, the cracks mainly propagate intergranularly along WC/WC and WC/Co grain boundaries, and the crack deflection can be obviously observed. Meanwhile, as shown in Fig. 3, the content of (Nb,W)C phase gradually increases with the addition of NbC. Compared to WC grain, it is easier for crack to pass through (Nb,W)C particles due to its high brittleness. At higher NbC contents transgranular fracture of the (Nb,W)C grains is observed (shown in Fig. 10 (d) and (e)), limiting the contribution of crack deflection as a toughening mechanisms. Thus, the (Nb,W)C solid solution will degrade the crack propagation resistance. As shown in Fig. 10 (b)-(d), with the increasing NbC content, crack deflection is inhibited and the crack path is gradually straightened. It is known that, when the cobalt content keeps constant, the fracture toughness also decreases with the decreasing of WC grain size [26]. Furthermore, there are only three independent slip systems in the hcp WC crystal. So, the cemented carbides with coarse grain have more crack defection than the materials with fine grain [6-7, 18, 26], which are consistent with the above results shown in Fig. 10. During the crack propagation, the crack defection leads to an enlargement of fracture area, which will enhance the energy consumption [18]. Because of the 18
grain size decrease and the (Nb,W)C phase formation, caused by NbC addition, the crack defection is weakened, which leads to the decreasing of fracture toughness.
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Fig. 10 Crack propagation morphology of WC-7Co cemented carbides with different NbC additions: (a) 0; (b) 0.5%; (c) 1%; (d) 2%; (e) 4%. 3.3 Effect of NbC additions on the corrosion properties of WC-Co cemented carbides Figure 11 presents the polarization curve of WC-Co cemented carbides with NbC addition in 1 mol/L HCl solution. The corresponding electrochemical corrosion results of WC-Co cemented carbides with NbC addition are listed in Table 5. As shown, with the increasing NbC content, the corrosion current (Icorr) decreases from 4.56×10-4 to 2.81×10-6 A.cm-2 and the corrosion potential (Ecorr) increases from -0.246 to -0.012 V. Fig. 11 and Table 5 show the information that a minor NbC addition between 0 and 2 wt.% brings little improvement to corrosion resistance. However, when NbC addition is more than 2%, the corrosion current is reduced at least two orders of magnitude. It has been found that corrosion resistance of cemented carbides strongly depends on binder composition [31-32], binder content [33] and binder structure [34-36]. Generally, binder phase is selectively dissolved from the cemented carbides in an acid corrosive environment while WC remains almost unaffected [33, 37]. Hence, the corrosion resistance of the cemented carbides is mainly determined by the corrosion behavior of the binder phase [37]. A higher content of -Co phase would improve the corrosion resistance of
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WC-Co cemented carbides as the cemented carbides containing fcc -Co shows a better corrosion behavior than the hcp ε-Co due to their higher thermodynamic stability in an acid corrosive solution [36]. The above XRD results shown in Fig. 1 indicate that the additions of NbC in the alloys gradually increase the content of fcc -Co due to the suppression effect of martensitic phase transformation. Therefore, the corrosion resistance of WC-Co cemented carbides is improved with the increasing of NbC. Meanwhile, some studies indicate that the additions of refractory metal carbides (e.g., Cr3C2, TiC and TaC) also enhance the corrosion resistance of cemented carbide [27, 38]. According to the results and discussions above, the corrosion resistance of non-uniform structure WC-Co cemented carbides can be significantly improved by adding NbC to the alloys due to an increase in -Co phase content.
Fig. 11 Polarization curve of WC-Co cemented carbides for different alloys. Table 5 The electrochemical corrosive results of WC-Co cemented carbides for different alloys. Samples Icorr(A.cm-2) Ecorr(V) -4 Alloy 1 4.56×10 -0.246 Alloy 2 3.42×10-4 -0.202 -4 Alloy 3 1.92×10 -0.129 Alloy 4 5.96×10-6 -0.032 -6 Alloy 5 2.81×10 -0.012
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4. Conclusions In this work, non-uniform structure WC-Co cemented carbides with NbC additions have been successfully prepared. The effects of various NbC additions on the microstructure, mechanical properties, corrosion behavior of the alloys investigated, from which the following conclusions can be drawn: 1) The amount of W atoms dissolving into the NbC grains increased with the NbC additions. The NbC addition also increased the coercive force and decreased the average WC grain size of cemented carbides. When the content of NbC was higher than 1%, non-uniform structure WC-Co cemented carbides were observed, and abnormal WC grains almost disappeared, and a certain percentage of coarse-grain and fine grain co-exist.
2) With NbC adding from 0 to 2 wt.%, the hardness increased while the fracture toughness decreased, but with further addition of NbC, the hardness slightly decreased. With the increase of NbC content, the TRS gradually decreased and leveled off. 3) With the NbC content increasing from 0 to 4 wt.%, the corrosion current (Icorr) decreased from 4.56×10-4 to 2.81×10-6 A.cm-2, while the corrosion potential (Ecorr) increased from -0.246 to -0.012 V. The corrosion resistance of non-uniform structure WC-Co cemented carbides can be significantly improved by NbC addition to the alloys due to increased -Co in binder phase.
Acknowledgement The authors gratefully acknowledge the financial support provided by Nonferrous Research Foundation of Hunan Province (grant no. 20120619). The authors also gratefully acknowledge Mr. Gang Liu (Zhu Zhou OKE Cemented Carbide Co.,Ltd) for his contribution on the materials
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preparation and professor Ke-jian He (Advanced Research Centre, Central South University) and professor Yong Du (State Key Laboratory of Powder Metallurgy, Central South University) for their helpful discussion.
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PII: DOI: Reference:
S0921-5093(17)30094-1 http://dx.doi.org/10.1016/j.msea.2017.01.077 MSA34639
To appear in: Materials Science & Engineering A Received date: 10 December 2016 Revised date: 19 January 2017 Accepted date: 21 January 2017 Cite this article as: Yang Gao, Ming-Yuan Yan, Bing-Hui Luo, Sheng Ouyang, Wei Chen, Zhen-hai Bai, Hui-bo Jing and Wen-Wen Zhang, Effects of NbC additions on the microstructure and properties of non-uniform structure WC-Co cemented carbides, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.01.077 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of NbC additions on the microstructure and properties of non-uniform structure WC-Co cemented carbides Yang Gao, Ming-Yuan Yan*, Bing-Hui Luo*, Sheng Ouyang, Wei Chen, Zhen-hai Bai, Hui-bo Jing, Wen-Wen Zhang School of Materials Science and Engineering, Central South University, Changsha 410083, P. R. China [email protected] [email protected] *
Corresponding authors.
Abstract In this work, the effects of NbC additions on microstructure and properties of non-uniform structure WC-7Co cemented carbides were investigated X-ray diffractometer, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), mechanical properties tester and electrochemical workstation, respectively. The results show that WC phase can be partially dissolved into the NbC to form a (Nb,W)C solid solution. According to EPMA analysis, the amount of W atoms, dissolving into the NbC grains, increases with the NbC addition. Moreover, when the content of NbC is beyond 1%, the WC-Co cemented carbides with non-uniform structure are formed with significant reduction of average grain size of WC. With NbC addition increasing from 0 to 2 wt.%, the hardness is increased from 1475 to 1570 MPa while the fracture
toughness decreased from 12.1 to 10.3 MPa m1/2. However, with the further addition of NbC, the hardness slightly decreased. With NbC addition between 0 and 1 wt.%, the TRS is gradually decreased from 2982 MPa to 2745 MPa, while, as the NbC content exceeds 1 wt.%, the TRS leveled off. Because of the decrease of grain size and the (Nb,W)C phase formation, caused by NbC addition, the crack defection was weakened, which led to the decrease of fracture toughness. Meanwhile, the corrosion resistance of non-uniform structure WC-Co cemented carbides can be significantly improved by adding NbC to the material due to increased -Co in binder phase.
Keywords Cemented carbides; NbC; Microstructure; Mechanical properties; Corrosion resistance
1. Introduction Conventional WC-Co cemented carbides with high hardness, strength, toughness, and good wear resistance, has been widely used in cutting, machining, drilling, mining tools, wear resistant parts, etc [1-4]. For the precision in machining work, the cemented carbides with a combination of higher hardness, wear resistance and toughness is required. However, the hardness and fracture toughness are trade-off for uniform structure cemented carbides [5-6]. The grain size is a critical for the properties of cemented carbides. The cemented carbides with coarse grain possess great toughness and thermal conductivity, while the cemented carbides with fine grain are of excellent wear resistance and hardness [6]. So currently, non-uniform structure cemented carbides are being developed to obtain a better combination of wear resistance and toughness which is unobtainable for the conventional uniform microstructure [6-8]. During liquid phase
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sintering, grain coarsening occurs caused by small grains dissolution and large grains growth [9-11]. On the grain growth, the powder particle size distribution has a strong impact, that is, the broader is the particle size distribution range, the more abnormally the grains grow. The key step of preparing non-uniform structure cemented carbides is maintaining a fine grain structure in order to avoid the structure homogenization during sintering process, and the addition of grain growth inhibitors is an effective way to prevent grain growth and structure homogenization [12]. The mechanical properties of WC–Co cemented carbides are usually affected by adding different grain growth inhibitors. Generally, the overall inhibition effectiveness is in the order of VC>Cr3C2> NbC>TaC>TiC [13]. However, would result in the embrittlement of WC-Co cemented carbide and the weakening of mechanical properties. The introduction of Cr3C2 nearly exhibits no effect on improving high temperature properties. TiC is the worst one of all the inhibitors mentioned above. So, it seems that NbC and TaC are the best grain growth inhibitors for WC-Co cemented carbides, as a result of their unique physical and chemical characteristics. However, in comparison with NbC, TaC is more expensive, which restrains the application and popularity in engineering. Moreover, NbC presents higher hardness of 19.60 GPa, lower density of 7.79 g/cm3 and higher melting temperature of 3600 °C, compared to WC [14-17]. NbC is, therefore, a very promising grain growth inhibitor for cemented carbides as evidenced by increased interest in the research of non-uniform structure cemented carbides containing NbC in recent years. The hardness, toughness as well as transverse rupture strength of WC–Co cemented carbide can be strongly affected by the NbC contents [16-18]. Wei Zhou [18] et al studied the fracture behavior and found that when the addition of NbC was up to 3wt%, the brittle (W,Nb)C solid solution increased considerably, and this led to a low stress brittle fracture. Da Silva et al [19]
3
have verified that NbC can effectively inhibit WC grain growth and increase the hardness in WC–Co cemented carbide. Li [20] et al declared that the addition of NbC refined the grain size, increased the transverse rupture strength and improved the high temperature hardness of WC-Co functionally graded cemented carbides. Acchar et al [21] also investigated the microstructure and mechanical properties of uniform structure WC-Co reinforced With 2 wt.% NbC. However, little effort has been made so far in studying the effects of NbC additions on the microstructure, mechanical properties and corrosion behavior of non-uniform structure WC-Co cemented carbides. The WC-Co cemented carbides containing NbC can be used to cut metal, plastic and wood. But, during cutting, the alloys are usually exposed to aggressive environments (e.g. water-based coolants, humid environment and the processed materials) that may contain acid medium. For simulating the acid environment experienced by the alloys used in cutting applications, the HCl solution was chosen for this investigation. In this study, non-uniform structure cemented carbides were sintered of WC powder (coarse/fine weight ratio is 1:1), Co powder and certain amount of NbC via low pressure sintering using the powder metallurgy technique. The effects of various NbC contents on the microstructure, mechanical properties, corrosion behavior of non-uniform structure WC-7Co cemented carbides were investigated.
2. Experimental procedure 2.1 Preparation of cemented carbides The characteristics of the raw powders used in the paper are listed in Table 1. The WC-1 and WC-2 powders were produced by Zhuzhou Cemented Carbide Group Co. Ltd. of China. The
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Co and NbC powders were provided by Hunan Metallurgy material institute. Based on our previous work [22], when the weight ratio of coarse to fine WC is 1:1, the WC-Co cemented carbides show a relatively good combination between hardness and toughness. In this research, the nominal compositions of non-uniform structure cemented carbides are given in Table 2. The WC powder (coarse/fine weight ratio is 1:1), Co powder and a given amount of NbC powders were milled in a stainless steel tumbler under argon atmosphere in ethanol for 30 h. The cemented carbide balls, with a diameter of 8 mm, were used as milling bodies. The ball-to powder weight ratio was 3:1 and the rotation rate of the mill was 238 rpm. After the milling, 2 wt.% paraffin was added as pressing aid, milled slurry was dried under vacuum conditions at 70 °C for 2 h, granulated and sieved. Then the milled powders were pressed into a cylindrical specimen under a uniaxial pressure of 200 MPa. The pressed specimens were sintered in a SIP-500/10MP industrial sinter-HIP furnace with the following cycle: (a) Heated from room temperature to 450 °C with a heating rate of 2.22 °C/min and held at 450 °C for 150 min under hydrogen; (b) Heated from 450 °C to 800 °C with a heating rate of 3.89 °C/min and held at 800 °C for 30 min, and then heated from 800 °C to 1220 °C with a heating rate of about 4.44°C/min and held at 1220 °C for 40 min. All this step is done under vacuum; (c) Heated to sintering temperature (1450 °C) with a heating rate of 2.89 °C/min, when the temperature rose to 1450 °C, at which temperature it would be held for 60 min, 5 MPa argon pressure would be exerted and maintained till it is cooled down to room temperature; (d) At last, the furnace cooled down .from sintering temperature to room temperature.
Raw powders WC-1 WC-2 NbC
Table 1 General characteristics of the raw powders. Purity (wt.%) Carbon content Oxygen content (wt.%) (wt.%) 99.9 6.13 0.01 99.9 6.13 0.01 99.8 0.2 5
Particle size (µm) 1.0 3.5 1.48
Co
98.5
-
0.4
1.31
Table 2 Nominal compositions of non-uniform structure cemented carbides (wt.%) and volume fraction of Co in the alloys (vol.%). Volume Samples Co WC-1 WC-2 NbC fraction of Co Alloy 1 7 46.5 46.5 0 11.65 Alloy 2 7 46.25 46.25 0.5 11.60 Alloy 3 7 46 46 1 11.55 Alloy 4 7 45.5 45.5 2 11.44 Alloy 5 7 44.5 44.5 4 11.22
2.2 Properties testing The coercive force (Hc) and magnetic saturation (CoM) of the specimens were measured with the ZDHC40 coercive meter and ZDMA6530 magnetic saturation induction tester, respectively. The hardness was measured by the Vickers hardness tester (HVS-50) with a load of 30 kgf and dwell time of 15 s. For each of the properties five replicates were performed to ensure reliability of the results generated. The fracture toughness can be obtained by Palmqvist indentation method. The crack length the tip of the indentation was measured the applied Vickers indentation load of 30 kg, The fracture toughness (KIC) of the sintered specimens could be calculated by the following equation [23]： K IC 0.15 HV 30 / l
(1)
where HV is the Vickers hardness, ∑l is the sum of the crack length. The TRS (transverse rupture strength) was measured using rectangular specimens with dimensions of 5.25 mm×6.5 mm×20 mm by Istron3369 materials test system. The values of TRS can be obtained according to international standard BS EN ISO 3327: 2009 using Type B test pieces as follows [24]: 6
R
3FL 2bh 2
(2)
where R is the TRS, F is the load required to fracture the test specimen, L is the distance between supports, b is the width of test beam width of test piece, and h is the height of test piece. 2.3 Microstructure and phase constitution testing The specimens were ground and polished for elemental mapping and microstructure analyses, which was carried out by Sirion-200 scanning electron microscopy (SEM) with Gensis60 X-ray energy dispersive spectroscopy (EDS). The WC mean grain size (d) of the specimens was measured by the linear intercept method on the 10k- magnification SEM images. The line scan of the specimens from one WC to another WC via Co binder phase was measured using electron probe microanalysis (EPMA, JXA–8230) with tungsten filament. The Nb, W and C contents in the (Nb,W)C phase were also obtained by EPMA. The phase constitution analysis of the specimens was detected by D/max-2500 X-ray diffractometry (XRD) with Cu Ka radiation (λ=0.15406 nm), obtained from a tube operated at 250 mA , 40 kV. The scan range was between 30° and 80° (2). The X-ray diffraction data was carried out with a step of 0.02° and a count time of 10 s. Meanwhile, the lattice constant of cubic NbC phase was determined by external standard method calibration. 2.4 Corrosion tests The polarization curves were measured using three electrode testing system in the IM6e electrochemical workstation. The three electrode experimental set-up was comprised of the specimen as working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum auxiliary electrode. The test area of the specimen was 1 cm2 and the corrosion behavior was studied in 1 mol/L HCl solution at room temperature. Potentiodynamic polarization curves 7
were carried out from -0.6 V to 1.0 V with a scanning speed of 2 mV/s. The corrosion potential (Ecorr) and corrosion current (Icorr) of the specimens were analyzed by corr-view software.
3. Results and discussion 3.1 Phase constitution and microstructure analysis Fig. 1 shows the X-ray diffraction patterns and partially enlarged patterns of WC-7Co cemented carbides with various NbC additions. As shown in Fig. 1(a), only the diffraction peaks of WC hard phase, cubic NbC and fcc Co binder phase can be identified in the specimens. The diffraction peaks of fcc Co become stronger with increasing NbC content. The hcp Co in binder phase can hardly be detected due to its quite small amount. It is known that there are two allotropes of cobalt, hcp ε-Co, stable at temperatures below about 417 °C, and fcc -Co, stable at higher temperatures. Some studies have demonstrated that the Nb dissolved in binder phase results in a strong resistance to martensitic phase transformation from α-Co to ε-Co [16]. Therefore, with the increasing of Nb dissolved in binder phase, the content of fcc-Co is enhanced in the specimens. This would explain why the diffraction peaks of fcc Co become stronger with increasing NbC content. According to the partially enlarged XRD diffraction patterns in Fig. 1(b), it can be seen that diffraction peaks of NbC gradually shift toward the high angle direction with the increasing of NbC content. The shifting of peak position from 40.29° to 40.48° should be attributed to the dissolution of W atoms of smaller radius into the cubic NbC lattice.
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Fig. 1 (a) XRD patterns of WC-7Co cemented carbides with various NbC additions and (b) partially enlarged XRD patterns of each alloy in the ranges of 2 from 40° and 41°. The variation of the lattice constant of cubic NbC phase in the alloys with the addition of NbC is plotted in Fig. 2. As shown, the lattice constant is gradually reduced with increasing the NbC addition. The decrease of lattice constant is due to increasing dissolution of W atoms in NbC lattice and the formation of a (Nb,W)C solid solution.
Fig. 2 The variation of lattice constant of the cubic NbC phase in WC-Co cemented carbides with NbC content. The solubility of WC in cubic NbC can reach to 38 wt.% at the temperature of 1300 °C [25]. However, the solubility of NbC in Co binder is very low at room temperature [19]. Therefore NbC precipitation will appear on cooling from the sintering temperature. Furthermore,
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WC phase can be partially dissolved into the NbC to form a (Nb,W)C solid solution during sintering process. The composition of (Nb,W)C solid solution of the alloys are conducted by quantitative EPMA analysis. The W, Nb and C contents in (Nb,W)C solid solution are shown in Table 3. It can be seen from Table 3 that the amount of W atoms, dissolving into the NbC grains, increases with the NbC addition. Therefore, the results in Table 3 well explain the evolution of the peak position shifting and lattice constant. Table 3 Nb, W, and C contents in the (Nb,W)C phase obtained by EPMA (wt.%). Elements Alloy 2 Alloy 3 Alloy 4 Alloy 5 Nb 78.155 75.315 70.52 63.174 W 13.645 15.996 20.561 26.606 C 8.20 8.689 8.919 10.22
Figure 3 shows the effect of NbC content on the microstructure of WC-7Co cemented carbides. To identify the WC, (Nb,W)C and Co phases in the alloys, a SEM elemental mapping analysis is exerted on the microstructure in Fig. 3(d), as shown in Fig 4. According to the elemental mapping (in Fig. 4), the phases can be confirmed that the light gray phase and the black gray phase are WC and (Nb,W)C cubic phase respectively, and the black phase is Co binder phase, as shown in Fig. 3. As shown in Fig. 3(a), the amount of small WC grains decrease with the amount of coarse WC grains increasing. Alloy 1 is of the largest average grain size (1.38 µm) compared with the other alloys. Moreover, in Fig. 3(a) the abnormal grain growth has been observed and the largest abnormal growth grain size is about 14 µm. The addition of 0.5% NbC has very little effect on the average WC grain size (1.30 µm). Meanwhile, in Fig. 3(b) the abnormal WC growths also can be observed and the abnormal growth grain size is close to 7.2 µm. When NbC addition is more than 1%, abnormal WC grains almost disappear. However, black grey (Nb,W)C phase appears and the amount of (Nb,W) C increases with the increasing NbC content. When the content of NbC excesses 2%, the average grain size of WC remains 10
unaltered compared with the sample with 1% NbC content.
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Fig. 3 SEM micrograph of WC-7Co cemented carbides with different NbC additions: (a) 0; (b) 0.5%; (c) 1%; (d) 2%; NbC (e) 4%.
Fig. 4 SEM elemental mapping of Fig. 3(d) (alloy 4): showing W (yellow-green); Nb (green); Co (blue); and C (red). The WC grain size distribution of WC-7Co cemented carbides with different NbC additions is presented in Fig. 5. As shown in Fig. 5(a), the overall grain size is uniformly distributed in spite of several abnormal growth grains (see Fig. 3a), and the proportion of fine structure grains (by number) is lowest when compared with the other four alloys with NbC additions. It can be seen that the average grain size of these alloys shows a negative dependency on the content of
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the NbC, and a clear decrease in the average grain size occurs when the NbC content is higher than 0.5 %. As shown, a platform gradually appears at the peak of the grain size distribution, and its width tends to increase with the NbC content. This indicates that a certain percentage of coarse grain and fine grain co-exist in these alloys. When the content of NbC exceeds 1%, non-uniform structure WC-Co cemented carbides are observed with obvious reduction of average grain size of WC.
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Fig. 5 Grain size distribution of WC-7Co cemented carbides with different NbC additions: (a) 0; (b) 0.5%; (c) 1%; (d) 2%; NbC (e) 4%. Table 4 shows the grain size, coercive force (Hc) and magnetic saturation (CoM) of WC-Co cemented carbide with NbC addition. As shown in Table 4, the variation of mean grain size is consistent with the microstructural change (shown in Fig. 3). It also can be clearly seen that the coercive force increases with the decreasing WC mean grain size. For constant binder phase content, coercive force is associated with the mean WC grain size of cemented carbides, which is in consistent with the results described in the literature [26]. With the increasing addition of NbC, the magnetic saturation gradually increases to a constant. It is known that magnetic saturation is associated with the solubility of W in Co binder phase [27]. A higher magnetic saturation means a lower amount of tungsten in the binder. Therefore, the result above indicates that the W dissolved in the binder phase is gradually reduced. This phenomenon described above will be discussed in the following. Table 4 The grain sizes and magnetic properties of WC-Co cemented carbides for different alloys. Samples Mean grain size (µm) Hc (kA/m) CoM (%) Alloy 1 1.38 13.02 6.62 Alloy 2 1.30 14.87 6.71 Alloy 3 1.13 14.91 6.70 Alloy 4 1.11 14.97 6.71 Alloy 5 1.09 14.32 6.70
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Figure 6 shows the SEM micrograph of Alloy 3, and the variation of Co, W, C and Nb concentrations from one WC to the other WC via Co binder phase. It can be seen that NbC uniformly dissolves into the Co binder and the segregation of NbC is not found at the WC/Co interface.
Fig. 6 (a) SEM micrograph of alloy 3 and (b) EPMA line scan along the red line in (a) from one WC to the other WC via Co binder phase. The above WC grain size distribution characteristics can be interpreted as an Ostwald ripening process during the liquid phase sintering of WC-Co cemented carbides. It is well known that the reduction in surface energy of the solid particles is the driving force for small grain dissolving and large grain growth [28]. The growth rate of WC grains is linearly proportional to the driving force (Gv) of coarsening for a grain of size (R) [11], which is given as 1 1 Gv 2 R r
(3)
where γ is a solid/liquid interfacial energy and r is a critical size that is not dissolving and growing. The dissolution of NbC into liquid Co phase is prior to that of WC, resulting from the higher eutectic temperature of 1340°C between WC and Co [29], compared with that of 1300°C between NbC and Co [30]. And, the NbC dissolved in the binder phase (shown in Fig. 6) leads to the decrease of solid/liquid interface energy and the driving force, which inhibits the dissolution
15
and precipitation processes of WC grains. This process can be well described by the simplified schematic diagram (in Fig. 7). In addition, that can well explain the increasing of magnetic saturation by adding the NbC. Meanwhile, with the addition of NbC, initial fine and coarse WC grains can be kept after cooling from liquid phase sintering temperature.
Fig. 7 Simplified schematic diagram of the dissolution and reprecipitation processes of: (a) WC-Co; (b) WC-Co containing NbC. 3.2 Properties of non-uniform structure WC-Co cemented carbides with NbC additions The effects of NbC content on the hardness and fracture toughness of non-uniform structure WC-Co cemented carbides are given in Fig. 8. The relationship between hardness and fracture toughness can be well demonstrated in Fig. 8. As shown, there is an antagonistic correlation existing between the hardness and fracture toughness. With NbC adding from 0 to and 2 wt.%, the hardness increases from 1475 to 1570 MPa, whereas the fracture toughness decreases from 12.1 to 10.3 MPa m1/2. The hardness and fracture toughness reach closely their extreme value when the NbC addition is 2 wt.%. When NbC addition is more than 2%, the hardness slightly decreases. The results in Fig 3 and Fig 5 show that the mean grain size decreases with the increasing NbC content. According to Hall-Petch formula, small WC grain size leads to high
16
hardness. However, when the content of NbC exceeds 2 wt.%, the hardness slightly decreases. The decreasing hardness should be attributed to the decreasing WC hard phase content and the coarsening of (Nb,W)C particles with increasing NbC (shown in Fig. 3 and Table 3).
Fig. 8 The hardness and fracture toughness of WC-Co cemented carbides with different NbC contents. Figure 9 presents the effect of NbC content on the TRS of WC-Co cemented carbides. It can be seen from Fig. 9 that the TRS generally decreases with the increasing NbC content. With NbC addition increasing from 0 to 1 wt.%, the TRS decreases from 2982 MPa to 2745 MPa. Then, as the NbC content exceeds 1 wt.%, the TRS levels off. Since the solubility of NbC in Co binder is very low at room temperature [19], (Nb,W)C phase is gradually formed and the grain coarsens with the NbC addition. It is apparent that the (Nb,W)C phase is more brittle than the WC hard phase. Compared with WC phase, (Nb,W)C has poorer wettability with Co [18], so there is a reduction in interface strength with Co phase when the NbC content increases. According to the discussion above, the decrease of TRS value is mainly caused by the emerging coarse (Nb,W)C phase and the weak phase interface.
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Fig. 9 The TRS of non-uniform structure WC-Co cemented carbides with different NbC contents. Crack propagation morphology of WC-7Co cemented carbides with NbC additions are shown in Fig. 10. As shown in Fig. 10 (a) and (b), with NbC addition increasing from 0 to 0.5 wt.%, the cracks mainly propagate intergranularly along WC/WC and WC/Co grain boundaries, and the crack deflection can be obviously observed. Meanwhile, as shown in Fig. 3, the content of (Nb,W)C phase gradually increases with the addition of NbC. Compared to WC grain, it is easier for crack to pass through (Nb,W)C particles due to its high brittleness. At higher NbC contents transgranular fracture of the (Nb,W)C grains is observed (shown in Fig. 10 (d) and (e)), limiting the contribution of crack deflection as a toughening mechanisms. Thus, the (Nb,W)C solid solution will degrade the crack propagation resistance. As shown in Fig. 10 (b)-(d), with the increasing NbC content, crack deflection is inhibited and the crack path is gradually straightened. It is known that, when the cobalt content keeps constant, the fracture toughness also decreases with the decreasing of WC grain size [26]. Furthermore, there are only three independent slip systems in the hcp WC crystal. So, the cemented carbides with coarse grain have more crack defection than the materials with fine grain [6-7, 18, 26], which are consistent with the above results shown in Fig. 10. During the crack propagation, the crack defection leads to an enlargement of fracture area, which will enhance the energy consumption [18]. Because of the 18
grain size decrease and the (Nb,W)C phase formation, caused by NbC addition, the crack defection is weakened, which leads to the decreasing of fracture toughness.
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Fig. 10 Crack propagation morphology of WC-7Co cemented carbides with different NbC additions: (a) 0; (b) 0.5%; (c) 1%; (d) 2%; (e) 4%. 3.3 Effect of NbC additions on the corrosion properties of WC-Co cemented carbides Figure 11 presents the polarization curve of WC-Co cemented carbides with NbC addition in 1 mol/L HCl solution. The corresponding electrochemical corrosion results of WC-Co cemented carbides with NbC addition are listed in Table 5. As shown, with the increasing NbC content, the corrosion current (Icorr) decreases from 4.56×10-4 to 2.81×10-6 A.cm-2 and the corrosion potential (Ecorr) increases from -0.246 to -0.012 V. Fig. 11 and Table 5 show the information that a minor NbC addition between 0 and 2 wt.% brings little improvement to corrosion resistance. However, when NbC addition is more than 2%, the corrosion current is reduced at least two orders of magnitude. It has been found that corrosion resistance of cemented carbides strongly depends on binder composition [31-32], binder content [33] and binder structure [34-36]. Generally, binder phase is selectively dissolved from the cemented carbides in an acid corrosive environment while WC remains almost unaffected [33, 37]. Hence, the corrosion resistance of the cemented carbides is mainly determined by the corrosion behavior of the binder phase [37]. A higher content of -Co phase would improve the corrosion resistance of
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WC-Co cemented carbides as the cemented carbides containing fcc -Co shows a better corrosion behavior than the hcp ε-Co due to their higher thermodynamic stability in an acid corrosive solution [36]. The above XRD results shown in Fig. 1 indicate that the additions of NbC in the alloys gradually increase the content of fcc -Co due to the suppression effect of martensitic phase transformation. Therefore, the corrosion resistance of WC-Co cemented carbides is improved with the increasing of NbC. Meanwhile, some studies indicate that the additions of refractory metal carbides (e.g., Cr3C2, TiC and TaC) also enhance the corrosion resistance of cemented carbide [27, 38]. According to the results and discussions above, the corrosion resistance of non-uniform structure WC-Co cemented carbides can be significantly improved by adding NbC to the alloys due to an increase in -Co phase content.
Fig. 11 Polarization curve of WC-Co cemented carbides for different alloys. Table 5 The electrochemical corrosive results of WC-Co cemented carbides for different alloys. Samples Icorr(A.cm-2) Ecorr(V) -4 Alloy 1 4.56×10 -0.246 Alloy 2 3.42×10-4 -0.202 -4 Alloy 3 1.92×10 -0.129 Alloy 4 5.96×10-6 -0.032 -6 Alloy 5 2.81×10 -0.012
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4. Conclusions In this work, non-uniform structure WC-Co cemented carbides with NbC additions have been successfully prepared. The effects of various NbC additions on the microstructure, mechanical properties, corrosion behavior of the alloys investigated, from which the following conclusions can be drawn: 1) The amount of W atoms dissolving into the NbC grains increased with the NbC additions. The NbC addition also increased the coercive force and decreased the average WC grain size of cemented carbides. When the content of NbC was higher than 1%, non-uniform structure WC-Co cemented carbides were observed, and abnormal WC grains almost disappeared, and a certain percentage of coarse-grain and fine grain co-exist.
2) With NbC adding from 0 to 2 wt.%, the hardness increased while the fracture toughness decreased, but with further addition of NbC, the hardness slightly decreased. With the increase of NbC content, the TRS gradually decreased and leveled off. 3) With the NbC content increasing from 0 to 4 wt.%, the corrosion current (Icorr) decreased from 4.56×10-4 to 2.81×10-6 A.cm-2, while the corrosion potential (Ecorr) increased from -0.246 to -0.012 V. The corrosion resistance of non-uniform structure WC-Co cemented carbides can be significantly improved by NbC addition to the alloys due to increased -Co in binder phase.
Acknowledgement The authors gratefully acknowledge the financial support provided by Nonferrous Research Foundation of Hunan Province (grant no. 20120619). The authors also gratefully acknowledge Mr. Gang Liu (Zhu Zhou OKE Cemented Carbide Co.,Ltd) for his contribution on the materials
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preparation and professor Ke-jian He (Advanced Research Centre, Central South University) and professor Yong Du (State Key Laboratory of Powder Metallurgy, Central South University) for their helpful discussion.
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