Effects of Y2O3 addition on microstructures and mechanical properties of WC–Co functionally graded cemented carbides

Effects of Y2O3 addition on microstructures and mechanical properties of WC–Co functionally graded cemented carbides

Int. Journal of Refractory Metals and Hard Materials 50 (2015) 53–58 Contents lists available at ScienceDirect Int. Journal of Refractory Metals and...

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Int. Journal of Refractory Metals and Hard Materials 50 (2015) 53–58

Contents lists available at ScienceDirect

Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM

Effects of Y2O3 addition on microstructures and mechanical properties of WC–Co functionally graded cemented carbides Yong Liu a,⁎, Xiaofeng Li a, Jianhua Zhou a,b, Kun Fu a,b, Wei Wei a, Meng Du a, Xinfu Zhao a a b

The State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China Central South Kaida Powder Metallurgy Co., Ltd, Changsha 410083, PR China

a r t i c l e

i n f o

Article history: Received 15 August 2014 Received in revised form 6 November 2014 Accepted 16 November 2014 Available online 18 November 2014 Keywords: Functionally graded cemented carbides Microstructure Grain growth Gradient layer Mechanical properties

a b s t r a c t Functionally graded cemented carbides (FGCCs) have an excellent combination of high hardness and high toughness, and the mechanical properties are strongly influenced by the graded structures. In this work, the effects of the addition of Y2O3 on the microstructures and mechanical properties of FGCCs were studied. The FGCCs were prepared by pre-sintering and carburizing of carbon-deficient WC–Co cemented carbides. It was found that the addition of Y2O3 does not change much the microstructures, but refines the grain size of WC in the gradient layer during the pre-sintering and the carburization processes. Moreover, the thickness of the gradient layer in the Y2O3-added FGCCs is almost doubled compared with that in the Y2O3-free FGCCs. Chemical analyses indicated that Y is mainly distributed in the Co phase. Therefore, Y has a strong effect on suppressing the abnormal grain growth of WC grains by influencing the dissolution behaviors of alloying elements in liquid Co during the presintering and the carburizing processes. The increased thickness of the graded layer is proposed to be due to the refinement of microstructures and the increased diffusion channels for carbon through the Co phase. The FGCCs with the addition of Y2O3 exhibit a much higher transverse rupture strength than the Y-free FGCCs due to thicker gradient layer, finer microstructures and solution strengthening of Y in the Co phase. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Cemented carbides have been used as tooling materials for cutting, drilling, and molding in many industry fields because of their excellent mechanical properties [1,2]. Up to now, cemented carbides are undergoing more and more severe working conditions, and must meet higher needs with the development of modern manufacturing industries [2]. Many efforts have been paid towards researching new types of cemented carbides [2,3]. Functionally graded cemented carbides (FGCCs) were firstly proposed by Sandvik in the 1980s [4–6], which provide a feasible solution to the trade-off between the hardness and the fracture toughness by varying the cobalt content from the surface to the interior [4–7]. The formation of the graded structure involved the pre-sintering of a carbon-deficient cemented carbide and subsequent carburization. Firstly, carbon-deficient cemented carbides were sintered, and an eta phase-containing structure (WC + Co + η) was obtained. Then, the pre-sintered samples were carburized. During the carburization, carbon diffuses from the surface of the sample into the inner part, and reacts with the η phase to form new WC and Co. The reactions result in the formation of a sandwich-like graded structure. Fang et al. [8–10] investigated the effects of the carbon gradient and processing ⁎ Corresponding author at: State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, Hunan, PR China. E-mail address: [email protected] (Y. Liu).

http://dx.doi.org/10.1016/j.ijrmhm.2014.11.004 0263-4368/© 2014 Elsevier Ltd. All rights reserved.

parameters on the overall kinetics at different sintering temperatures, and found that the formation of the cobalt gradient depended on the differences in the chemical potential of carbon and grain sizes of WC during liquid phase sintering. Liu et al. [7,11] developed FGCCs by presintering carbon-deficient compacts and subsequent carburizing, and proposed that the formation mechanism of the graded structure could be attributed to the Ostwald ripening induced by carbon diffusion. Yuan et al. [12] used the Taguchi method to formulate the experimental layout, and showed that the order of the parameters influencing both the thickness of the gradient layer and the cobalt gradient were determined by the volume fraction of methane, time and flow rate of mixed gases. However, the formation of FGCCs often need long time in the sintering or carburizing processes, so the grain growth of WC is inevitable [7–11], which may hinder the carbonizing process and reduce mechanical properties. It was pointed out that the Ostwald ripening is the main reason for the coarsening of WC in cemented carbides [13–15]. So many studies focused on hindering the grain growth of WC by suppressing the Ostwald ripening process [15–17], including adjusting the sintering parameters, as well as adding growth inhibitors such as VC, Cr2C3, and NbC [16]. VC and Cr2C3 are the mostly used grain growth inhibitors for conventional cemented carbides by far, due to their high solubility and mobility in the cobalt phase at relatively low temperatures [18,19]. Although it is well known that these grain growth inhibitors are efficient, there are still some uncertainties on the working mechanisms [20]. The rare-earth

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(RE) element-doped cemented carbides have been widely investigated since the 1990s, because RE elements can hinder the grain growth and enhance mechanical properties effectively [21–24]. Xiong et al. [21] found that RE additives in WC–8%Co cemented carbides increases the content of fcc Co and improves the bending strength and the impact resistance. Liu et al. [23] added 1.5 wt.% RE oxides (Y2O3, La2O3 and CeO2) in WC–10Co cemented carbides, and obtained a much finer grain size and better mechanical properties as compared with the RE-free ones. Zhang et al. [24] found that the addition of La restricts the dissolution of WC in the Co phase, and hinders the grain growth. The aim of this work was focused on the addition of rare earth oxide in WC–Co FGCCs, in order to investigate the effects on microstructures and mechanical properties.

3. Results 3.1. Microstructures Fig. 1(a) shows the morphology of the gradient structure of FGCC with the addition of Y2O3 after the carburization at 1420 °C for 80 min. There are evidently three layers: the outer layer with a low content of Co, the middle layer with a high content of Co, and the inner part with a nominal content of Co. Fig. 1(b) shows that the content of Co changes with the distance from the surface of FGCC. The contents of Co first decrease in the outer layer, then increase in the middle layer, and finally stabilize in the inner part. The thickness of the outer layer with a high content of Co in Y-free FGCC appears to be about 1 mm, while that in the Y-containing FGCC is observed to be about 2 mm. Meanwhile, the highest content of Co measured in the middle layer of the Y-free FGCC is about 7 wt.%, while that of the Y-containing FGCC is approximately 8 wt.%. In the inner part, the content of Co in both FGCCs is maintained at 6 wt.% approximately. Fig. 2 shows the microstructures of FGCCs after the carburization at 1420 °C for 80 min. Fig. 2(a) indicates that the outer layer consists of WC and Co binary phases, and no η phase can be found. The growth of WC grains is obvious, and some abnormally large grains can be observed. The middle layer is also composed of WC and Co binary phases in Fig. 2(b), but the content of the Co phase significantly increases. Meanwhile, the grain size of WC is slightly smaller than that in the outer layer. The inner part (Fig. 2(c)), is composed of three phases: the WC phase (the bright phase), the η phase (the gray phase), and the Co phase (the dark phase). A large amount of the η phase appears so that the contents of the Co and WC phases reduce. For the FGCC with 0.5% Y2O3, the number of abnormally coarse grains decreases, and the WC grains are much smaller in Fig. 2(d) than those in Fig. 2(a). There are more Co phases in the Y-containing FGCC in Fig. 2(e) than in the Y-free FGCC in Fig. 2(b). Compared with the Yfree FGCC in Fig. 2(c), the microstructures in the Y-containing FGCC does not change much in Fig. 2(f), except for the facts that the amount and the size of the η phase are slightly larger, and the WC grain size is slightly smaller. Fig. 3 shows the dependence of the thickness of the gradient layer and the grain size of WC in the surface layer on the carburization time. Both the thickness of the gradient layer and the grain size of WC increase with the carburization time. The thickness of the gradient layer of the Y-containing FGCC is larger than that of the Y-free FGCC, and

2. Experimental Cemented carbides with a nominal composition of WC–6 wt.% Co were used. Pure W powder was added to adjust the total carbon content of the mixed powders, which was measured as 5.32 wt.%. Y2O3 (99.5%), with an average particle size of 0.5 μm, was added in the mixed powders before the milling process. Mechanical milling was conducted in gasoline, with 2 wt.% paraffin as the binder. The ball-to-powder weight ratio was 4:1 and the rotation speed of the mill was 400 rpm. After a milling for 36 h, the powders were dried in a vacuum oven at 80 °C, and then pressed at 200 MPa. Sintering was performed at 1430 °C for 60 min in a sinter-HIP furnace under an argon pressure of 6 MPa. The carbon-deficient WC–Co cemented carbides were covered by graphite powders, and carburized at 1420 °C for 40–120 min in a high temperature furnace under hydrogen atmosphere. The details of the processing of FGCCs can be found in our previous work [7]. The compositional distributions in the gradient layers were measured using the electron probe micro analysis (EPMA). Scanning electron microscopy (SEM; Nove nano 230) was used for the observation of the microstructures. The thickness of gradient layers was measured by the software of Image-Pro Plus on optical microscopy images. Transmission electron microscopy (TEM; JEM-2100F) was employed for detailed microstructural analyses at an accelerating voltage of 200 kV. The hardness of FGCCs was measured by Rockwell hardness (HRA), and the transverse rupture strength was tested by three-point bending on polished samples of a size of 5 mm × 5 mm × 35 mm, using an Instron Mechanical Test Machine.

(a)

(b)

9

Y-Free Y-Containing

Co content(wt.%)

8

7

6

5

4

3 0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

d(mm)

Fig. 1. Morphology and Co profile in FGCCs with and without the addition of Y2O3 after carburization at 1420 °C for 80 min: (a) Morphology of FGCC with the addition of Y2O3; and (b) Co profiles of FGCCs with and without Y2O3.

Y. Liu et al. / Int. Journal of Refractory Metals and Hard Materials 50 (2015) 53–58

(a)

(b)

(c)

(d)

(e)

(f)

55

Fig. 2. SEM morphology(BEI) of the FGCCs with and without Y2O3 after carburization at 1420 °C for 80 min: (a) Outer layer of the Y-free FGCC; (b) middle layer of the Y-free FGCC; (c) inner part of the Y-free FGCC;(d) outer layer of the Y-containing FGCC; (e) middle layer of the Y-containing FGCC; and (f) inner part of the Y-containing FGCC.

increases much faster in Fig. 3(a). At the carburization times of 80 min and 100 min, the thickness of the gradient layer in the Y-containing FGCC is nearly twice that of the Y-free FGCC. At the carburization time of 120 min, the inner part of the Y-containing FGCCs disappears, which means that the carburizing reaction has completed. In Fig. 3(b), the initial grain size of WC in the Y-containing FGCC is smaller than that of the Y-free FGCC, but it increases more quickly. At the carburization time of 120 min, the grain sizes of WC become almost the same between two FGCCs. Fig. 4 indicates the TEM image of the outer layer in the Y-containing FGCC after the carbonization at 1420 °C for 80 min. It can be found the grown-up WC grains and the flow of the Co phase to the inner part of the cemented carbide. No precipitated phases can be found both in the WC and Co phases, however, the content of Y is higher in Co than in WC, indicated by EDS in the inset of Fig. 4.

(a) 3.0

Y-Free Y-Containing

Gradient layer thickness(mm)

2.5

2.0

1.5

1.0

0.5

0.0 0

20

40

60

80

100

120

3.2. Mechanical properties

Carburization time(min)

Our previous work [7,11] found that mechanical properties of carbon-deficient cemented carbides are very low due to the existence

(b) 2.0

Y-Free Y-Containing

1.9

Grain Size (µm)

1.8

WC

1.7

1.6

1.5

Co

• WC

1.4

• 1.3 0

20

40

60

80

100

120

Carbonization time (min) Fig. 3. Characteristics of the gradient structure of FGCCs vs. carburization time. (a) Thickness of gradient layer; (b) grain size of the surface layer.

Fig. 4. TEM image of the outer layer of the Y-containing FGCC after carbonization at 1420 °C for 80 min.

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of the η phase, and will increase after the formation of gradient layer. Fig. 6 shows the relationship between mechanical properties of FGCCs and the carburization time. The Y-containing FGCCs exhibit higher transverse rupture strength and slightly higher hardness than those of the Y-free FGCCs. Both the hardness of the two FGCCs increases with the carburization time, and reaches almost the same value at the carburization time of 100 min. Both the transverse rupture strength of the two FGCCs also increase with the carburization time, but the strength of the Y-containing FGCCs becomes higher than that of the Y-free FGCCs. At a carburization time of 80 min, an increase of 30% in the strength of the Y-containing FGCC than that of the Y-free FGCC can be seen in Fig. 5(b). After a carburization time of 100 min, the strength of the Y-containing FGCC drops a little possibly due to the coarsening of WC grains. Fig. 7 shows the fractographs of the FGCCs after the transverse rupture. The corresponding high magnification images are inserted into the micrographs. In the outer layer (Fig. 6(a) and (d)), with a lower content of Co, the plastic deformation of the Co phase can only be found along the WC grain boundaries. The inner part is completely of a transgranular fracture mode. All the fracture initiates along the grain boundaries of the WC and η phases, and very few plastic deformation of the Co phase can be observed (Fig. 6(c) and (f)). The middle layer shows more plastic deformation of the Co phase, indicating a good toughness in Fig. 6(b) and (e). There is no significant difference between the fractographs of the two FGCCs in all layers, except for the amount of plastic deformation of the Co phase. The Y-containing FGCCs have higher contents of plastic deformation of the Co phase in the outer and middle layers than the Yfree FGCCs.

(a) Y-Free Y-Containing

Hardness(HRA)

4.1. Effect of Y in FGCCs The effects and the distributions of grain growth inhibitors in cemented carbides are complicated. It is generally believed that the suppression of the grain growth is dependent upon the availability of inhibiting elements at the WC–Co interface during the sintering [25, 26]. The inhibiting elements affect the adsorption/desorption processes of WC grains in the Co phase, and prevent the WC grains from connecting and congregating together. The existence of RE elements in WC/Co cemented carbides is yet not well understood. Some work indicated that the RE elements exist at the WC–Co interface [23,24]. Others showed that the RE elements form stable oxide and sulfide compounds during the sintering process, and even RE2C3 may be formed by the reacting of RE elements with free graphite [21,22]. In this work, no obvious precipitate phases form in WC and Co phases, hence, Y might mainly dissolve in the Co phase, since it has an enough high solubility in liquid Co. During the sintering process, Y may inhibit the dissolution and reprecipitation process of WC due to its solubility in liquid Co, and then hinder the growth of the WC grains. Lifshitz and Wagner found that the growth of solids in liquids can be controlled by the diffusion of atoms in the matrix and the reaction at the solid/liquid interface [27, 28], which is called as LSW theory. Greenwood simplified the LSW model as [29,30]: 3

3

dt −d0 ¼

64 DCV m t 9 RT

ð1Þ

where dt and d0 are the diameters of the final and the initial grains, respectively, t is the carburizing time and D is the diffusion coefficient, C is the solute solubility of the average sized grain, and Vm is the volume fraction of solid phase. The Eq. (1) can be rewritten as:

92

3

3

dt −d0 ¼ Kt

ð2Þ

91

90

40

60

80

100

120

Carburization time(min)

(b) Y-Free Y-Containing

2800

Transverse rupture strength(MPa)

4. Discussions

2400

2000

where K is the kinetic index of the grain growth. In this work, Fig. 3(b) shows the increase of the grain size of WC in the surface layer with the carburization time, and the kinetic index of the grain growth can be calculated in Fig. 7. Although the Y-containing FGCC has a higher kinetic index than Y-free FGCC, the average grain size is always below the Y-free FGCC. There may be two reasons, one is that the initial grain size of the Y-containing FGCC is smaller and more homogeneous, and the other is that Y is a large atom, and may suppress other atoms successively getting into the liquid Co structure in local areas. In Y-containing Fe melt, the viscosity is obviously increased, which is beneficial for the refinement of the solidified microstructures [31]. During the pre-sintering process, the amount of liquid Co phase is reduced by the formation of the η phase, so the effect of Y on the grain refinement is more obvious. However, during the carburization process, the η phase reacts with carbon to form more liquid Co. The dissolution and precipitation of WC are enhanced, and the effect of Y in suppressing the grain growth of WC in liquid Co is weakened. The kinetics of the formation of the gradient layer is highly dependent on the microstructures. The fine WC skeleton provides more diffusion channels for carbon from the outside to the inside of FGCCs, while the coarse WC skeleton can block the diffusion of carbon and hinder the formation of the gradient layer.

1600

4.2. Mechanical properties 40

60

80

100

120

Carburization time(min) Fig. 5. Mechanical properties of FGCCs vs. carburization time. (a) Hardness; (b) transverse rupture strength.

The relationship of the graded structure on the mechanical properties of FGCCs has been discussed by the authors in Ref. [7]. The hardness depends on the content of WC grains in the surface. The higher the WC content, the higher the hardness. But there is a limit of the increment of

Y. Liu et al. / Int. Journal of Refractory Metals and Hard Materials 50 (2015) 53–58

(b)

(a)

(c)

1µm

1µm

(d)

57

(e)

1µm

(f)

1µm

1µm

1µm

Fig. 6. Fractographs of FGCCs carburized at 1420 °C for 80 min. (a) Outer layer of the Y-free FGCC; (b) middle layer of the Y-free FGCC; (c) inner layer of the Y-free FGCC; (d) outer layer of the Y-containing FGCC; (e) middle layer of the Y-containing FGCC; and (f) inner part of the Y-containing FGCC.

the surface hardness. The coarsening of WC grains may decrease the hardness, as shown in the case of long time of carburization. Since Y refines the WC grain size, the Y-containing FGCCs have a slightly higher hardness than the Y-free FGCC. The transverse rupture strength may be more complicated. The hard surface of FGCCs can suppress the formation of cracks, and the intermediate Co-rich layer can release the stress concentration, and prevent the propagation of cracks by the plastic deformation of the Co phase. Therefore, FGCCs usually have a higher strength than the conventional cemented carbides. The η phase in the inner part may degrade the strength by the formation of internal cracks, so the thicker the gradient layer, the less the η phase, and the higher the strength of FGCCs. Also, the rupture strength depends on the grain size of WC. The Y-containing FGCC has a much higher strength than the Yfree FGCC, largely depending on the thicker graded layer and fine WC grain size. Coarse WC grain usually leads to a drop of strength, as shown in Fig. 5(b). On the other hand, the dissolution of Y in the Co phase may also induce a solution strengthening effect, and enhance the strength.

5. Conclusions (1) The addition of Y2O3 enhances the thickness of the gradient layer in FGCCs. At the carburization time of 80 min and 100 min, the thickness of the gradient layer increases by twice, compared with the Y-free FGCCs. (2) The addition of Y2O3 effectively refines the grain size of WC in the gradient layer by suppressing the abnormal grain growth of WC during the pre-sintering and the carburizing processes. (3) The increased thickness of the gradient layer is proposed to be due to the refinement of microstructures and increased diffusion channels for carbon through the Co phase. (4) FGCCs with the addition of Y2O3 exhibit much higher transverse rupture strength than the Y-free FGCCs, due to thicker gradient layer, finer microstructures and solution strengthening of Y in the Co phase.

References 0.70 0.65

Y-Free Y-Containing

0.60

ln d(µm)

0.55 0.50 0.45 0.40 0.35 0.30 3.6

3.8

4.0

4.2

4.4

4.6

4.8

ln t(min) Fig. 7. Kinetics of grain growth of WC in FGCCs in the carburizing process.

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