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Densification and microstructure evolution of W-TiC-Y2O3 during spark plasma sintering
International Journal of Refractory Metals & Hard Materials 79 (2019) 95–101
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International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Densification and microstructure evolution of W-TiC-Y2O3 during spark plasma sintering
T
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Yu-Fen Zhoua, Zhi-Yuan Zhaoa, Xiao-Yue Tana, Lai-Ma Luoa,c, , Yue Xua,c, Xiang Zana,c, Qiu Xub, ⁎ Kazutoshi Tokunagad, Xiao-Yong Zhua,c, Yu-Cheng Wua,c,e, a
School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China Institute for Integrated Radiation and Nuclear Science, Kyoto University, Osaka-fu 590-0494, Japan c Laboratory of Nonferrous Metal Material and Processing Engineering of Anhui Province, Hefei 230009, China d Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan e National-Local Joint Engineering Research Centre of Nonferrous Metals and Processing Technology, Hefei 230009, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Spark plasma sintering (SPS) W-TiC-Y2O3 composite Densification process Surface diffusion Grain boundary diffusion
Spark plasma sintering (SPS) is one of the methods used to achieve the low-temperature densification of refractory metal materials. In this study, powder prepared through a wet chemical method was consolidated via SPS at 1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C to obtain a high-performance W-TiC-Y2O3 composite material. Densification was studied by analyzing the densification curve and changes in the microstructure of the samples. This process could be divided into three stages: the bonding stage, the sintering neck growth stage, and the shrinkage and spherification stage of closed pores. Surface diffusion and grain boundary diffusion played different roles in densification. The density, grain size, and Vickers hardness of the tungsten material increased significantly as temperature increased. This study evaluated the sintering process and provided a basis for obtaining high-performance tungsten materials through SPS.
1. Introduction Tungsten is considered a promising plasma facing material for future nuclear fusion devices. However, in actual situations, tungsten is associated with severe problems, such as low-temperature brittleness, recrystallization embrittlement, radiation embrittlement and high ductile–brittle transition temperature [1,2]. To overcome these embrittlement issues, researchers investigated grain refinement, secondphase addition, and deformation [3–5]. Among them, the addition of second phases widely distributed in a tungsten matrix, such as Y2O3 [6,7] and TiC [8,9], can effectively improve the mechanical properties of tungsten and its alloys at increased temperatures. The addition of rare earth oxides helps improve the high temperature and radiation resistance of tungsten alloys [10]. The addition of carbide can also enhance the strength and plasticity of tungsten materials [11]. For refractory tungsten metals, compact materials are usually obtained through powder metallurgy [12–14], which requires high sintering temperatures, long soaking times, and microstructures with coarse grains. However, these conditions are not conducive to improving the mechanical properties of tungsten-based composite materials. Thus, efficient sintering techniques, such as spark-plasma
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sintering (SPS) [15,16], hot isostatic pressing (HIP) [17], and hot pressing (HP) [18], have been developed to consolidate tungsten. Pressureless sintering is not chosen because it produces materials that require processing deformation to obtain a composite material with a small grain size. SPS is a widely published technique characterized by low sintering temperature and short sintering time [19,20], which are effective for the sintering densification of refractory metals [21] and ceramics [22]. In comparison with HP and HIP, SPS generates a spark discharge that can clean the surface of particles and provide favorable current for powder compacts, thereby acting as a heat source through the Joule effect. As such, the heating rate can reach 50–1000 °C/min. The heating caused by the Joule effect during SPS is caused by the absence of insulators and heating elements with a large heat capacity, and the direct heating of a graphite die by an electric current, resulting in rapid heating and cooling, which contribute to rapid sintering [23,24]. Powder particles do not grow too violently, and fine crystals and highdensity materials are consequently obtained. Osman El-Atwani [25] studied the sinterability of tungsten powder at different temperatures and pressures through SPS, and confirmed that a high external pressure applied during the SPS of powders favors the production of high-density
Corresponding authors at: School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China. E-mail addresses: [email protected] (L.-M. Luo), [email protected] (Y.-C. Wu).
https://doi.org/10.1016/j.ijrmhm.2018.11.014 Received 26 September 2018; Received in revised form 17 November 2018; Accepted 24 November 2018 Available online 26 November 2018 0263-4368/ © 2018 Elsevier Ltd. All rights reserved.
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consolidated samples at a low sintering temperature. The sintering densification of SPS is related to heating rate, pressure, soaking time, temperature, and other parameters. In this study, we added two kinds of second phases (TiC and Y2O3) to a tungsten matrix to obtain a tungsten-based composite material with a greatly improved comprehensive performance. The composite material was sintered at different temperatures through SPS, and the sintering behavior and densification of the W-TiC-Y2O3 composite were studied.
2. Experiments 2.1. Powder preparation The appropriate amounts of ammonium tungstate ((NH4)6H2W12O40%XH2O, AMT), yttrium nitrate (Y(NO3)3%6H2O), and TiC particles were mixed in deionized water under ultrasound conditions. The TiC powders (Shanghai Chao'er Nano Technology Co., Ltd.) had an average grain size of 50 nm and purity of 99.9. A certain amount of oxalic acid (C2H2O4%2H2O) as a precipitating agent during precursor preparation was added to the mixed liquid. The precursor powder was obtained by drying the solution, which was reduced in a hydrogen atmosphere to obtain W-1vol%TiC-1vol%Y2O3 (W-TiC-Y2O3) composite powder. As shown in Fig. 1, the composite powders with a particle size of 900 nm approximately.
Fig. 2. Sintering curve of the samples during SPS at 1800 °C for 5 min.
2.3. Characterization 2.3.1. Density measurement In order to avoid the influence of pores on the sample surfaces on this experimental result, the actual density of the composite could be calculated as follows:
M = ρv where M is mass, v is volume. In order to remove the influence of tungsten carbide on the measurement results, the surfaces of samples were ground off at least 1 mm and then polished, cleaned with alcohol, and blow dried. All samples were cylindrical with the same size. Every sample was weighed 12 times with an analytical balance, and the maximum and minimum values were determined from the obtained values. The average of the remaining values was calculated to obtain the mass of the sample. The diameter and thickness of the sample were measured with a Vernier caliper and a spiral micrometer. The measured data were substituted into the equation to obtain the density of the material. The theoretical density used to calculate the relative density of W-TiC-Y2O3 is 18.9644 cm3/g.
2.2. Consolidation Approximately 25 g of the reduced powder was placed in a 20 mminternal diameter graphite module which was previously lined with a thick graphitic sheet for easy removal. The module was fixed in an SPS apparatus (LABOX-350, Sinter Land Inc., Japan). After a vacuum level of 5–10 Pa was achieved, a uniaxial pressure of 14 MPa was applied to the powder bed before sintering was carried out. The sample was heated at 800 °C at a rate of 100 °C/min and dwelled for 4 min to relieve the residual gas in this powder. The uniaxial pressure was increased to 50 MPa within 1 min, and then kept constant in the subsequent sintering. The entire sintering process was conducted under vacuum conditions to prevent sample oxidation. Simultaneously, the sample was heated to the target temperatures (1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C) and dwelled for 5 min at a heating rate of 100 °C/min. In the cooling step, the uniaxial pressure was released to 25 MPa as the temperature was decreased to 800 °C and allowed to cool at room temperature. A representative sintering program at 1800 °C is shown in Fig. 2.
2.3.2. Microstructure observation The surface of the sample was polished and etched before the microstructure was observed. The etchant was prepared with a mixture of heated hydrogen peroxide and ammonia at a ratio of 1:1. The etchant was cleaned off the surface of the sample with alcohol and dried. Afterward, its surface structure was observed using a field emission scanning electron microscope (SU8020, Hitachi, Japan). The fracture surface of sample could be observed directly using a scanning electron microscope. 2.3.3. Vickers hardness The sample was polished and subjected to Vickers hardness test (HV-(10Z), Shangcai, Shanghai) by applying a load of 300 gf for 10 s. During the measurement, 12 different positions on the surface of the sample were selected for repeated testing. The maximum and minimum values were removed, and the 10 remaining positions were averaged as the microhardness of the composite. 3. Results 3.1. Sintering process at 1800 °C Fig. 3 shows the sintering process curve at a sintering temperature of 1800 °C. The displacement showed a negative value because the mold was subjected to a uniaxial pressure during sintering. The curve
Fig. 1. SEM image of the composite powder. 96
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Fig. 3. (a) Displacement-temperature curve and (b) vacuum-temperature curve at a sintering temperature of 1800 °C.
displacements at the three temperature points, namely, 1100 °C, 1350 °C, and 1800 °C, changed significantly, so the samples were obtained by sintering at the three temperatures. In Fig. 3(a), the displacement increased positively before 800 °C was reached because the temperature increased rapidly at the beginning of sintering and the powder expanded thermally. This phenomenon could be illustrated by the change in the corresponding temperature curve in Fig. 3(b), the vacuum level increased significantly from the initial value of 8 Pa to 170 Pa approximately. It might also be related to the behavior of swelling. At 800 °C, the displacement increased rapidly because the axial pressure increased from 14 MPa to 50 MPa. The powder compact began to shrink from 800 °C to 1800 °C. In Fig. 3(b), the vacuum level achieved after 800 °C was > 8 Pa, indicating that the sample exhausted gas outward and tended to be densified. The process could be divided into three stages in Fig. 3(a). At the first stage (800 °C–1100 °C), the displacement increased relatively more slowly than that at the second stage (1100 °C–1350 °C). At the third stage, the displacement growth flattened (1350 °C–1800 °C). At the cooling step, the sudden decrease in temperature caused the sample to shrink, so the displacement continued to increase. The displacement at 800 °C suddenly changed because the applied pressure decreased by half. In general, the curves exhibited a well-known behavior, that is, density increased as temperature increased, thereby causing the densification to accelerate at the beginning of sintering and to decelerate densification after a certain period.
Fig. 4. Physical map of the samples obtained during sintering at different temperatures. Table 1 Density, grain size, and Vickers hardness (HV) of the samples at five different sintering temperatures.
3.2. Sintering temperature selection The shrinkage process of the sample could be divided into three stages. To study the densification behavior at each stage, we selected the interrupted sintering temperature for sintering, including 1200 °C and 1600 °C. The sintering temperatures corresponding to the samples from left to right in Fig. 4 were 1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C, and their densities were 11.946, 13.696, 15.556, 17.454, 17.762 cm3/g, respectively (Table 1). Intuitively, the higher the sintering temperature was, the thinner the sample would be. In other words, an increase in temperature was conducive to densification.
Temperature (°C)
Density/relative density (cm3/g)/ (%)
Grain size (μm)
HV
1100 1200 1350 1600 1800
11.946/63 13.696/72 15.556/82 17.454/92 17.762/94
1 1.3 1.8 5 8
118.29 161.33 233.31 346.73 363.64
(Fig. 5(a)), so the surface of the sample was not flat (Fig. 6(a)), and the Vickers hardness (HV) of the sample was low, (118.29 HV0.3; Table 1). The particle size distribution was bimodal, and the average particle size was approximately 1 μm (Table 1). At 1200 °C, the bimodal grain distribution still existed, but the small particles with an average particle size of approximately 1.3 μm gradually disappeared (Table 1). The number of the sintering neck increased, and some of the particles became irregular in shape (Fig. 5(b)), suggesting the contact between the particles (Fig. 6(b)). When the temperature reached 1350 °C, the relative density of the sample increased to 82%. The particle shape completely changed from spherical to irregular, and the small particles and the small sintering necks disappeared. As such, the particles of approximately 1.8 μm in size were uniformly distributed (Fig. 5(c);
3.3. Microstructure characterization The obtained fracture surfaces and metallographs of five sintered samples at different temperatures were analyzed. At 1100 °C, the relative density of the sample was 63%. The fracture surface revealed that most of the particles were spherical, and a few sintering necks existed 97
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Fig. 5. SEM images of the fracture surfaces of the samples sintered at (a) 1100 °C; (b) 1200 °C; (c) 1350 °C; (d)1600 °C, and (e) 1800 °C.
Fig. 6. SEM images of the metallographic surfaces of the samples sintered at (a) 1100 °C; (b) 1200 °C; (c) 1350 °C; (d) 1600 °C, and (e) 1800 °C.
second phase was largely distributed at the grain boundary with a size of about 2 μm. In Fig. 6(e), one part of the pores existed between the grains, and the other part of the pores coexisted with the second phase distributed at the grain boundary with a grain size of approximately 8 μm (Table 1). The observations indicated that the densification behavior of the composite was mainly reflected in the growth of the sintering neck at lower temperature, and this process was gradually replaced by the reduction of pores at higher temperatures [26]. The grain size and HV of the samples increased during densification.
Table 1). The sintering neck was formed between the particles at 1350 °C, resulting in the presence of pores. Numerous connected pores were found on the surface of the sample because of a low sample density (Fig. 6(c)). At 1600 °C, the sintering neck further enlarged, and obvious river patterns, characteristic of transgranular fracture could be observed from the fracture surface of the sintered sample (Fig. 5(d)). As such, the HV value of the samples significantly increased as temperature increased (Table 1). The second phase and the pores coexisted at the grain boundary, so porosity decreased, and the grain size measured approximately 5 μm (Fig. 6(d); Table 1). When sintering was performed at 1800 °C, the relative density of the sample reached 94%. The number of transgranular fractures increased (Fig. 5(e)), and numerous pits were formed because of the shedding of the second phase, indicating that the 98
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4. Discussion
to discharging the gas in the powder compact in Process 1. Immediately increasing the pressure to 50 MPa causes the displacement to increase rapidly. Next, the increment of displacement is caused by the shrinkage of the powder compact, which is the same as that in Process 2. In Fig. 7(b), the powder compact releases more gas before the dwell stage in Process 1, indicating that the initial pressure influences the densification of the powder compact [38]. Zhao-Hui Zhang et al. [39] found that residual gas may be closed in the powder compact because of plastic deformation that results from the discharge effect and the initial pressure. Once a pore is formed, its removal in the late sintering process is difficult. Fig. 8 shows the microstructure of the compacts obtained through the two sintering processes, and the compact density obtained using Process 2 is 17.146 cm3/g. The initial pressure is high, the volume of the pore is great, and the compact density is low. The compact density does not reach the ideal value, and Process 1 is not the best sintering method for the densification of tungsten powder. Previous studies have shown that the relative density of W-5 wt% Y2O3 and W-0.5 wt% TaC obtained by different SPS processes were close to 100%, and grain sizes were 3–4 μm [40,41]. There are many factors influencing the densification of tungsten-base materials, so we need to constantly explore to obtain high-performance tungsten-based materials [42].
4.1. Densification The obtained sintering curve suggests that the isothermal sintering of the powder compact can be divided into three stages [27,28]. Different changes occur in the powder compact at each stage in densification not only at a macro level but also at a micro level. Therefore, the densification mechanism during sintering should be further analyzed to study the densification of W-TiC-Y2O3 composite. At the initial stage of sintering (800–1100 °C), the powder particles tend to bond or form a mass automatically because of a great deal of gas is discharged from the powder (Fig. 3(b)). The local overheating of the powder, causing the contact surfaces of the particles melt and deform [29–31]. This phenomenon is related to current because it rapidly increases to a certain value at the beginning of sintering, thereby promoting the formation of the sintering necks between particles [32], and would lead to coarsening. This phenomenon is also called the bond stage. At this stage, the relative density is < 65%, and the grain boundary migration is pinned by the open pore channels in the porous sample, so surface diffusion plays an important role to promote densification by coarsening [33]. In Fig. 5(a), the sintering necks begin to form between coarsened particles in the area of the weak contact surfaces. Therefore, the hardness and density of the composite at a sintering temperature of 1100 °C are low. At the late stage of sintering (1100–1350 °C), the initial coarsening of particles results in the formation of particle boundaries and the contact area between particles becomes larger, narrowing the distance between the particles to increase the particle size and the growth of the sintering neck. This event is called the sintering neck growth stage. At this stage, the relative density is obviously increased, so grain boundary diffusion has a significant effect on densification [34]. More vacancies aggregate in the sintering neck region compared with those in the region that is far from the sintering neck. Consequently, the vacancies in the sintering neck region and pores are easily diffused or absorbed by adjacent grain boundaries, including the grain boundaries of the second phase, and the pores distributed near the grain boundary preferentially decrease or disappear. This phenomenon helps form a stable grain boundary and contributes to the reduction of pores and shrinkage of the sintered sample [35,36]. At 1600 °C, the grain boundaries form and interlace with pores (Fig. 6(d)). Sintering densification is a complex process, different densification mechanisms are dominant at different stages. At the final stage of sintering (1350–1800 °C), the density does not further increase. Although grain boundary migration can eliminate pores, it also causes micropores to aggregate and form large pores, which hinder grain boundary migration. Therefore, the effect of surface diffusion is obviously at the final stage [37], leading to the growth of grains and the shrinkage and spherification of closed pores by coarsening at the final stage (Fig. 6(e)). It is also called the shrinkage and spherification stage of closed pores.
5. Summary and outlook The sintering densification process of tungsten powder prepared using wet chemical method during SPS was investigated. Five samples were obtained at sintering temperatures of 1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C under the maximum pressure of 50 MPa. The density, grain size, and HV of the composite increased with temperature. The densification of the composite accelerated at the initial stage of sintering but decelerated after a certain period. The analysis of the sintering curve and microstructure of the samples revealed that the sintering process could be divided into three stages: the bonding stage (initial sintering stage), the sintering neck growth stage (late sintering stage), and the shrinkage and spherification stage of closed pores (final sintering stage). The densification of the powder at different sintering stages was influenced by different mechanisms. At the initial stage of sintering, surface diffusion played an important role to promote densification by coarsening. At the late stage of sintering, grain boundary diffusion led to the shrinkage of the compact by promoting the growth of the sintering neck. As sintering continued, the grain size increased to a certain extent through surface diffusion. The few remaining pores hindered the movement of the grain boundaries, so the density of the composite no longer increased at the final stage of sintering. In SPS, densification is affected by various parameters, including temperature, pressure, holding time, and heating rate. In our study, increasing the temperature and decreasing the initial pressure during sintering densification of the powder compact were effective. In future work, sintering should be further improved, and the holding time and heating rate also should be considered to obtain an almost completely densified compact.
4.2. Evaluation of the sintering process SPS remarkably influences the comprehensive properties of the composite. In this study, a low initial pressure was applied to the powder compact during sintering to promote the removal of gas from the powder at the initial stage and improve its densification. To verify the correctness of this idea, we slightly changed the sintering process. For comparison, the pressure of the sample at a sintering temperature of 1600 °C is directly increased to 50 MPa at the beginning, and the heating rate remains unchanged and without holding 5 min at 800 °C. For the convenience of description, this process is named Process 2, and the other one is named Process 1. Fig. 7(a) illustrates the obvious difference between the two sintering processes. At the beginning of sintering, which is the exhaust behavior of the powder compact mentioned above. Moreover, the behavior of holding 5 min at 800 °C is beneficial
Acknowledgements This work is supported by National Magnetic Confinement Fusion Program (Grant No. 2014GB121001), National Natural Science Foundation of China (Grant No. 51574101 and 51474083), the Fundamental Research Funds for the Central Universities (PA2018GDQT0010), the Foundation of Laboratory of Nonferrous Metal Material and Processing Engineering of Anhui Province (15CZS08031), and the 111 Project (B18018).
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Fig. 7. Comparison of Process 1 and Process 2. (a) Time–displacement curve and (b) time–vacuum curve.
Fig. 8. SEM images of the metallographic surfaces of the samples obtained using different sintering processes: (a) Process 2 and (b) Process 1.
References
[14] A. Mondal, A. Upadhyaya, D. Agrawal, Effect of heating mode and sintering temperature on the consolidation of 90W-7Ni-3Fe alloys, J. Alloys Compd. 509 (2011) 301–310. [15] G. Lee, J. Mckittrick, E. Ivanov, E.A. Olevsky, Densification mechanism and mechanical properties of tungsten powder consolidated by spark plasma sintering, Int. J. Refract. Met. Hard Mater. 61 (2016) 22–29. [16] Z.P. Gao, G. Viola, B. Milsom, et al., Kinetics of densification and grain growth of pure tungsten during spark plasma sintering, Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 43 (2012) 1608–1614. [17] A. Muñoz, J. Martínez, M.A. Monge, B. Savoini, R. Pareja, A. Radulescu, SANS evidence for the dispersion of nanoparticles in W–1Y2O3 and W–1La2O3 processed by hot isostatic pressing, Int. J. Refract. Met. H. 33 (2012) 6–9. [18] S. Miao, Z.M. Xie, L.F. Zeng, et al., Mechanical properties, thermal stability and microstructure of fine-grained W-0.5wt.%TaC alloys fabricated by an optimized multi-step process, Nucl. Mater. Energy 13 (2017) 12–20. [19] Z.A. Munir, U. Anselmi-Tamburini, M. Ohyanagi, The effect of electric field and pressure on the synthesis and consolidation of materials: a review of the spark plasma sintering method, J. Mater. Sci. 41 (2006) 763–777. [20] W. Li, E.A. Olevsky, J. Mckittrick, et al., Densification mechanisms of spark plasma sintering: multi-step pressure dilatometry, J. Mater. Sci. 47 (2012) 7036–7046. [21] M. Omori, Sintering, consolidation, reaction and crystal growth by the spark plasma system (SPS), Mater. Sci. Eng. A 287 (2000) 183–188. [22] R. Chaim, Densification mechanisms in spark plasma sintering of nanocrystalline ceramics, Mater. Sci. Eng. A 443 (2007) 25–32. [23] M. Gendre, A. Maître, G. Trolliard, A study of the densification mechanisms during spark plasma sintering of zirconium (oxy-) carbide, Acta Mater. 58 (2010) 2598–2609. [24] S.H. Deng, R.D. Li, T.C. Yuan, et al., Direct current-enhanced densification kinetics during spark plasma sintering of tungsten powder, Scr. Mater. 143 (2018) 25–29. [25] O. El-Atwani, D.V. Quach, M. Efe, et al., Multimodal grain size distribution and high hardness in fine grained tungsten fabricated by spark plasma sintering, Mater. Sci. Eng. A 528 (2011) 5670–5677. [26] K. Hu, X.Q. Li, C. Yang, Y.Y. Li, Densification and microstructure evolution during SPS consolidation process in W-Ni-Fe system, Trans. Nonferrous Metals Soc. China
[1] F. Ferroni, X.O. Yi, K. Arakawa, et al., High temperature annealing of ion irradiated tungsten, Acta Mater. 90 (2015) 380–393. [2] M.R. Gilbert, J.-C. Sublet, Neutroneinduced transmutation effects in W and W-alloys in a fusion environment, Nucl. Fusion 51 (2011) 043005. [3] H.Y. Guo, M. Xia, L.C. Chan, et al., Nanostructured laminar tungsten alloy with improved ductility by surface mechanical attrition treatment, Sci. Rep. 7 (2017) 1351. [4] S. Wang, J. Zhang, L.M. Luo, et al., Properties of Lu2O3 doped tungsten and thermal shock performance, Powder Technol. 301 (2016) 65–69. [5] S.T. Lang, Q.Z. Yan, N.B. Sun, et al., Microstructure, basic thermal–mechanical and Charpy impact properties of W-0.1wt.% TiC alloy via chemical method, J. Alloys Compd. 660 (2016) 184–192. [6] M. Battabyal, R. Schäublin, P. Spätig, et al., Microstructure and mechanical properties of a W-2wt.%Y2O3 composite produced by sintering and hot forging, J. Nucl. Mater. 442 (2013) S225–S228. [7] M. Battabyal, R. Schäublin, P. Spätig, N. Baluc, W-2wt.%Y2O3, composite: microstructure and mechanical properties, Mater. Sci. Eng. A 538 (2012) 53–57. [8] H. Kurishita, S. Matsuo, H. Arakawa, et al., Development of re-crystallized W–1.1%TiC with enhanced room-temperature ductility and radiation performance, J. Nucl. Mater. 398 (2010) 87–92. [9] H. Kurishita, Y. Amano, S. Kobayashi, et al., Development of ultra-fine grained W–TiC and their mechanical properties for fusion applications, J. Nucl. Mater. 367 (2007) 1453–1457. [10] R. Liu, X.P. Wang, T. Hao, et al., Characterization of ODS-tungsten microwavesintered from sol–gel prepared nano-powders, J. Nucl. Mater. 450 (2014) 69–74. [11] X.Y. Tan, P. Li, L.M. Luo, et al., Effect of second-phase particles on the properties of W-based materials under high-heat loading, Nucl. Mater. Energy 9 (2016) 399–404. [12] C. Selcuk, N. Morley, J.V. Wood, Effect of post-sintering heat treatment on porous tungsten in relation to sintering swelling, Powder Metall. 47 (2004) 267–272. [13] A. Mondal, A. Upadhyaya, D. Agrawal, Effect of heating mode on sintering of tungsten, Int. J. Refract. Met. Hard Mater. 28 (2010) 597–600.
100
International Journal of Refractory Metals & Hard Materials 79 (2019) 95–101
Y.-F. Zhou et al.
tungsten powder, J. Am. Ceram. Soc. 95 (2012) 2458–2464. [35] H. Djohari, J.J. Derby, Transport mechanisms and densification during sintering: II. Grain boundaries, Chem. Eng. Sci. 64 (2009) 3810–3816. [36] H. Djohari, J.I. Martínez-Herrera, J.J. Derby, Transport mechanisms and densification during sintering: I. Viscous flow versus vacancy diffusion, Chem. Eng. Sci. 64 (2009) 3799–3809. [37] S.H. Deng, T.C. Yuan, R.D. Li, et al., Spark plasma sintering of pure tungsten powder: densification kinetics and grain growth, Powder Technol. 310 (2017) 264–271. [38] S. Diouf, A. Molinari, Densification mechanisms in spark plasma sintering: effect of particle size and pressure, Powder Technol. 221 (2012) 220–227. [39] Z.H. Zhang, F.C. Wang, L. Wang, S.K. Li, Ultrafine-grained copper prepared by spark plasma sintering process, Mater. Sci. Eng. A 476 (2008) 201–205. [40] Y. Kim, K.H. Lee, E.P. Kim, et al., Fabrication of high temperature oxides dispersion strengthened tungsten composites by spark plasma sintering process, Int. J. Refract. Met. Hard Mater. 27 (2009) 842–846. [41] S. Miao, Z.M. Xie, X.D. Wang, et al., Effect of hot rolling and annealing on the mechanical properties and thermal conductivity of W-0.5 wt.% TaC alloys, Int. J. Refract. Met. Hard Mater. 56 (2016) 8–17. [42] J.S. Lin, L.M. Luo, K. Huang, et al., Preparation and properties of fine-grained and ultrafine-grained W-TiC-Y2O3 composite, Fusion Eng. Des. 126 (2018) 147–155.
21 (2011) 493–501. [27] K. Matsui, H. Yoshida, Y. Ikuhara, Isothermal sintering effects on phase separation and grain growth in yttria-stabilized tetragonal zirconia polycrystal, J. Am. Ceram. Soc. 92 (2009) 467–475. [28] A.K. Nanda Kumar, M. Watabe, K. Kurokawa, The sintering kinetics of ultrafine tungsten carbide powders, Ceram. Int. 37 (2011) 2643–2654. [29] S. Diouf, C. Menapace, A. Molinari, Study of effect of particle size on densification of copper during spark plasma sintering, Powder Metall. 55 (2012) 228–234. [30] X.Y. Song, X.M. Liu, J.X. Zhang, Neck formation and self-adjusting mechanism of neck growth of conducting powders in spark plasma sintering, J. Am. Ceram. Soc. 89 (2006) 494–500. [31] X.M. Liu, X.Y. Song, S.X. Zhao, J.X. Zhang, Spark plasma sintering densification mechanism for cemented carbides with different WC particle sizes, J. Am. Ceram. Soc. 93 (2010) 3153–3158. [32] N. Toyofuku, T. Kuramoto, T. Imai, et al., Effect of pulsed DC current on neck growth between tungsten wires and tungsten plates during the initial stage of sintering by the spark plasma sintering method, J. Mater. Sci. 47 (2012) 2201–2205. [33] Z.Z. Fang, H.T. Wang, V. Kuman, Coarsening, densification, and grain growth during sintering of nano-sized powders–a perspective, Int. J. Refract. Met. Hard Mater. 62 (2017) 110–117. [34] H.T. Wang, Z.Z. Fang, Kinetic analysis of densification behavior of nano-sized
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Densification and microstructure evolution of W-TiC-Y2O3 during spark plasma sintering
T
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Yu-Fen Zhoua, Zhi-Yuan Zhaoa, Xiao-Yue Tana, Lai-Ma Luoa,c, , Yue Xua,c, Xiang Zana,c, Qiu Xub, ⁎ Kazutoshi Tokunagad, Xiao-Yong Zhua,c, Yu-Cheng Wua,c,e, a
School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China Institute for Integrated Radiation and Nuclear Science, Kyoto University, Osaka-fu 590-0494, Japan c Laboratory of Nonferrous Metal Material and Processing Engineering of Anhui Province, Hefei 230009, China d Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan e National-Local Joint Engineering Research Centre of Nonferrous Metals and Processing Technology, Hefei 230009, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Spark plasma sintering (SPS) W-TiC-Y2O3 composite Densification process Surface diffusion Grain boundary diffusion
Spark plasma sintering (SPS) is one of the methods used to achieve the low-temperature densification of refractory metal materials. In this study, powder prepared through a wet chemical method was consolidated via SPS at 1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C to obtain a high-performance W-TiC-Y2O3 composite material. Densification was studied by analyzing the densification curve and changes in the microstructure of the samples. This process could be divided into three stages: the bonding stage, the sintering neck growth stage, and the shrinkage and spherification stage of closed pores. Surface diffusion and grain boundary diffusion played different roles in densification. The density, grain size, and Vickers hardness of the tungsten material increased significantly as temperature increased. This study evaluated the sintering process and provided a basis for obtaining high-performance tungsten materials through SPS.
1. Introduction Tungsten is considered a promising plasma facing material for future nuclear fusion devices. However, in actual situations, tungsten is associated with severe problems, such as low-temperature brittleness, recrystallization embrittlement, radiation embrittlement and high ductile–brittle transition temperature [1,2]. To overcome these embrittlement issues, researchers investigated grain refinement, secondphase addition, and deformation [3–5]. Among them, the addition of second phases widely distributed in a tungsten matrix, such as Y2O3 [6,7] and TiC [8,9], can effectively improve the mechanical properties of tungsten and its alloys at increased temperatures. The addition of rare earth oxides helps improve the high temperature and radiation resistance of tungsten alloys [10]. The addition of carbide can also enhance the strength and plasticity of tungsten materials [11]. For refractory tungsten metals, compact materials are usually obtained through powder metallurgy [12–14], which requires high sintering temperatures, long soaking times, and microstructures with coarse grains. However, these conditions are not conducive to improving the mechanical properties of tungsten-based composite materials. Thus, efficient sintering techniques, such as spark-plasma
⁎
sintering (SPS) [15,16], hot isostatic pressing (HIP) [17], and hot pressing (HP) [18], have been developed to consolidate tungsten. Pressureless sintering is not chosen because it produces materials that require processing deformation to obtain a composite material with a small grain size. SPS is a widely published technique characterized by low sintering temperature and short sintering time [19,20], which are effective for the sintering densification of refractory metals [21] and ceramics [22]. In comparison with HP and HIP, SPS generates a spark discharge that can clean the surface of particles and provide favorable current for powder compacts, thereby acting as a heat source through the Joule effect. As such, the heating rate can reach 50–1000 °C/min. The heating caused by the Joule effect during SPS is caused by the absence of insulators and heating elements with a large heat capacity, and the direct heating of a graphite die by an electric current, resulting in rapid heating and cooling, which contribute to rapid sintering [23,24]. Powder particles do not grow too violently, and fine crystals and highdensity materials are consequently obtained. Osman El-Atwani [25] studied the sinterability of tungsten powder at different temperatures and pressures through SPS, and confirmed that a high external pressure applied during the SPS of powders favors the production of high-density
Corresponding authors at: School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China. E-mail addresses: [email protected] (L.-M. Luo), [email protected] (Y.-C. Wu).
https://doi.org/10.1016/j.ijrmhm.2018.11.014 Received 26 September 2018; Received in revised form 17 November 2018; Accepted 24 November 2018 Available online 26 November 2018 0263-4368/ © 2018 Elsevier Ltd. All rights reserved.
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consolidated samples at a low sintering temperature. The sintering densification of SPS is related to heating rate, pressure, soaking time, temperature, and other parameters. In this study, we added two kinds of second phases (TiC and Y2O3) to a tungsten matrix to obtain a tungsten-based composite material with a greatly improved comprehensive performance. The composite material was sintered at different temperatures through SPS, and the sintering behavior and densification of the W-TiC-Y2O3 composite were studied.
2. Experiments 2.1. Powder preparation The appropriate amounts of ammonium tungstate ((NH4)6H2W12O40%XH2O, AMT), yttrium nitrate (Y(NO3)3%6H2O), and TiC particles were mixed in deionized water under ultrasound conditions. The TiC powders (Shanghai Chao'er Nano Technology Co., Ltd.) had an average grain size of 50 nm and purity of 99.9. A certain amount of oxalic acid (C2H2O4%2H2O) as a precipitating agent during precursor preparation was added to the mixed liquid. The precursor powder was obtained by drying the solution, which was reduced in a hydrogen atmosphere to obtain W-1vol%TiC-1vol%Y2O3 (W-TiC-Y2O3) composite powder. As shown in Fig. 1, the composite powders with a particle size of 900 nm approximately.
Fig. 2. Sintering curve of the samples during SPS at 1800 °C for 5 min.
2.3. Characterization 2.3.1. Density measurement In order to avoid the influence of pores on the sample surfaces on this experimental result, the actual density of the composite could be calculated as follows:
M = ρv where M is mass, v is volume. In order to remove the influence of tungsten carbide on the measurement results, the surfaces of samples were ground off at least 1 mm and then polished, cleaned with alcohol, and blow dried. All samples were cylindrical with the same size. Every sample was weighed 12 times with an analytical balance, and the maximum and minimum values were determined from the obtained values. The average of the remaining values was calculated to obtain the mass of the sample. The diameter and thickness of the sample were measured with a Vernier caliper and a spiral micrometer. The measured data were substituted into the equation to obtain the density of the material. The theoretical density used to calculate the relative density of W-TiC-Y2O3 is 18.9644 cm3/g.
2.2. Consolidation Approximately 25 g of the reduced powder was placed in a 20 mminternal diameter graphite module which was previously lined with a thick graphitic sheet for easy removal. The module was fixed in an SPS apparatus (LABOX-350, Sinter Land Inc., Japan). After a vacuum level of 5–10 Pa was achieved, a uniaxial pressure of 14 MPa was applied to the powder bed before sintering was carried out. The sample was heated at 800 °C at a rate of 100 °C/min and dwelled for 4 min to relieve the residual gas in this powder. The uniaxial pressure was increased to 50 MPa within 1 min, and then kept constant in the subsequent sintering. The entire sintering process was conducted under vacuum conditions to prevent sample oxidation. Simultaneously, the sample was heated to the target temperatures (1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C) and dwelled for 5 min at a heating rate of 100 °C/min. In the cooling step, the uniaxial pressure was released to 25 MPa as the temperature was decreased to 800 °C and allowed to cool at room temperature. A representative sintering program at 1800 °C is shown in Fig. 2.
2.3.2. Microstructure observation The surface of the sample was polished and etched before the microstructure was observed. The etchant was prepared with a mixture of heated hydrogen peroxide and ammonia at a ratio of 1:1. The etchant was cleaned off the surface of the sample with alcohol and dried. Afterward, its surface structure was observed using a field emission scanning electron microscope (SU8020, Hitachi, Japan). The fracture surface of sample could be observed directly using a scanning electron microscope. 2.3.3. Vickers hardness The sample was polished and subjected to Vickers hardness test (HV-(10Z), Shangcai, Shanghai) by applying a load of 300 gf for 10 s. During the measurement, 12 different positions on the surface of the sample were selected for repeated testing. The maximum and minimum values were removed, and the 10 remaining positions were averaged as the microhardness of the composite. 3. Results 3.1. Sintering process at 1800 °C Fig. 3 shows the sintering process curve at a sintering temperature of 1800 °C. The displacement showed a negative value because the mold was subjected to a uniaxial pressure during sintering. The curve
Fig. 1. SEM image of the composite powder. 96
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Fig. 3. (a) Displacement-temperature curve and (b) vacuum-temperature curve at a sintering temperature of 1800 °C.
displacements at the three temperature points, namely, 1100 °C, 1350 °C, and 1800 °C, changed significantly, so the samples were obtained by sintering at the three temperatures. In Fig. 3(a), the displacement increased positively before 800 °C was reached because the temperature increased rapidly at the beginning of sintering and the powder expanded thermally. This phenomenon could be illustrated by the change in the corresponding temperature curve in Fig. 3(b), the vacuum level increased significantly from the initial value of 8 Pa to 170 Pa approximately. It might also be related to the behavior of swelling. At 800 °C, the displacement increased rapidly because the axial pressure increased from 14 MPa to 50 MPa. The powder compact began to shrink from 800 °C to 1800 °C. In Fig. 3(b), the vacuum level achieved after 800 °C was > 8 Pa, indicating that the sample exhausted gas outward and tended to be densified. The process could be divided into three stages in Fig. 3(a). At the first stage (800 °C–1100 °C), the displacement increased relatively more slowly than that at the second stage (1100 °C–1350 °C). At the third stage, the displacement growth flattened (1350 °C–1800 °C). At the cooling step, the sudden decrease in temperature caused the sample to shrink, so the displacement continued to increase. The displacement at 800 °C suddenly changed because the applied pressure decreased by half. In general, the curves exhibited a well-known behavior, that is, density increased as temperature increased, thereby causing the densification to accelerate at the beginning of sintering and to decelerate densification after a certain period.
Fig. 4. Physical map of the samples obtained during sintering at different temperatures. Table 1 Density, grain size, and Vickers hardness (HV) of the samples at five different sintering temperatures.
3.2. Sintering temperature selection The shrinkage process of the sample could be divided into three stages. To study the densification behavior at each stage, we selected the interrupted sintering temperature for sintering, including 1200 °C and 1600 °C. The sintering temperatures corresponding to the samples from left to right in Fig. 4 were 1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C, and their densities were 11.946, 13.696, 15.556, 17.454, 17.762 cm3/g, respectively (Table 1). Intuitively, the higher the sintering temperature was, the thinner the sample would be. In other words, an increase in temperature was conducive to densification.
Temperature (°C)
Density/relative density (cm3/g)/ (%)
Grain size (μm)
HV
1100 1200 1350 1600 1800
11.946/63 13.696/72 15.556/82 17.454/92 17.762/94
1 1.3 1.8 5 8
118.29 161.33 233.31 346.73 363.64
(Fig. 5(a)), so the surface of the sample was not flat (Fig. 6(a)), and the Vickers hardness (HV) of the sample was low, (118.29 HV0.3; Table 1). The particle size distribution was bimodal, and the average particle size was approximately 1 μm (Table 1). At 1200 °C, the bimodal grain distribution still existed, but the small particles with an average particle size of approximately 1.3 μm gradually disappeared (Table 1). The number of the sintering neck increased, and some of the particles became irregular in shape (Fig. 5(b)), suggesting the contact between the particles (Fig. 6(b)). When the temperature reached 1350 °C, the relative density of the sample increased to 82%. The particle shape completely changed from spherical to irregular, and the small particles and the small sintering necks disappeared. As such, the particles of approximately 1.8 μm in size were uniformly distributed (Fig. 5(c);
3.3. Microstructure characterization The obtained fracture surfaces and metallographs of five sintered samples at different temperatures were analyzed. At 1100 °C, the relative density of the sample was 63%. The fracture surface revealed that most of the particles were spherical, and a few sintering necks existed 97
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Fig. 5. SEM images of the fracture surfaces of the samples sintered at (a) 1100 °C; (b) 1200 °C; (c) 1350 °C; (d)1600 °C, and (e) 1800 °C.
Fig. 6. SEM images of the metallographic surfaces of the samples sintered at (a) 1100 °C; (b) 1200 °C; (c) 1350 °C; (d) 1600 °C, and (e) 1800 °C.
second phase was largely distributed at the grain boundary with a size of about 2 μm. In Fig. 6(e), one part of the pores existed between the grains, and the other part of the pores coexisted with the second phase distributed at the grain boundary with a grain size of approximately 8 μm (Table 1). The observations indicated that the densification behavior of the composite was mainly reflected in the growth of the sintering neck at lower temperature, and this process was gradually replaced by the reduction of pores at higher temperatures [26]. The grain size and HV of the samples increased during densification.
Table 1). The sintering neck was formed between the particles at 1350 °C, resulting in the presence of pores. Numerous connected pores were found on the surface of the sample because of a low sample density (Fig. 6(c)). At 1600 °C, the sintering neck further enlarged, and obvious river patterns, characteristic of transgranular fracture could be observed from the fracture surface of the sintered sample (Fig. 5(d)). As such, the HV value of the samples significantly increased as temperature increased (Table 1). The second phase and the pores coexisted at the grain boundary, so porosity decreased, and the grain size measured approximately 5 μm (Fig. 6(d); Table 1). When sintering was performed at 1800 °C, the relative density of the sample reached 94%. The number of transgranular fractures increased (Fig. 5(e)), and numerous pits were formed because of the shedding of the second phase, indicating that the 98
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4. Discussion
to discharging the gas in the powder compact in Process 1. Immediately increasing the pressure to 50 MPa causes the displacement to increase rapidly. Next, the increment of displacement is caused by the shrinkage of the powder compact, which is the same as that in Process 2. In Fig. 7(b), the powder compact releases more gas before the dwell stage in Process 1, indicating that the initial pressure influences the densification of the powder compact [38]. Zhao-Hui Zhang et al. [39] found that residual gas may be closed in the powder compact because of plastic deformation that results from the discharge effect and the initial pressure. Once a pore is formed, its removal in the late sintering process is difficult. Fig. 8 shows the microstructure of the compacts obtained through the two sintering processes, and the compact density obtained using Process 2 is 17.146 cm3/g. The initial pressure is high, the volume of the pore is great, and the compact density is low. The compact density does not reach the ideal value, and Process 1 is not the best sintering method for the densification of tungsten powder. Previous studies have shown that the relative density of W-5 wt% Y2O3 and W-0.5 wt% TaC obtained by different SPS processes were close to 100%, and grain sizes were 3–4 μm [40,41]. There are many factors influencing the densification of tungsten-base materials, so we need to constantly explore to obtain high-performance tungsten-based materials [42].
4.1. Densification The obtained sintering curve suggests that the isothermal sintering of the powder compact can be divided into three stages [27,28]. Different changes occur in the powder compact at each stage in densification not only at a macro level but also at a micro level. Therefore, the densification mechanism during sintering should be further analyzed to study the densification of W-TiC-Y2O3 composite. At the initial stage of sintering (800–1100 °C), the powder particles tend to bond or form a mass automatically because of a great deal of gas is discharged from the powder (Fig. 3(b)). The local overheating of the powder, causing the contact surfaces of the particles melt and deform [29–31]. This phenomenon is related to current because it rapidly increases to a certain value at the beginning of sintering, thereby promoting the formation of the sintering necks between particles [32], and would lead to coarsening. This phenomenon is also called the bond stage. At this stage, the relative density is < 65%, and the grain boundary migration is pinned by the open pore channels in the porous sample, so surface diffusion plays an important role to promote densification by coarsening [33]. In Fig. 5(a), the sintering necks begin to form between coarsened particles in the area of the weak contact surfaces. Therefore, the hardness and density of the composite at a sintering temperature of 1100 °C are low. At the late stage of sintering (1100–1350 °C), the initial coarsening of particles results in the formation of particle boundaries and the contact area between particles becomes larger, narrowing the distance between the particles to increase the particle size and the growth of the sintering neck. This event is called the sintering neck growth stage. At this stage, the relative density is obviously increased, so grain boundary diffusion has a significant effect on densification [34]. More vacancies aggregate in the sintering neck region compared with those in the region that is far from the sintering neck. Consequently, the vacancies in the sintering neck region and pores are easily diffused or absorbed by adjacent grain boundaries, including the grain boundaries of the second phase, and the pores distributed near the grain boundary preferentially decrease or disappear. This phenomenon helps form a stable grain boundary and contributes to the reduction of pores and shrinkage of the sintered sample [35,36]. At 1600 °C, the grain boundaries form and interlace with pores (Fig. 6(d)). Sintering densification is a complex process, different densification mechanisms are dominant at different stages. At the final stage of sintering (1350–1800 °C), the density does not further increase. Although grain boundary migration can eliminate pores, it also causes micropores to aggregate and form large pores, which hinder grain boundary migration. Therefore, the effect of surface diffusion is obviously at the final stage [37], leading to the growth of grains and the shrinkage and spherification of closed pores by coarsening at the final stage (Fig. 6(e)). It is also called the shrinkage and spherification stage of closed pores.
5. Summary and outlook The sintering densification process of tungsten powder prepared using wet chemical method during SPS was investigated. Five samples were obtained at sintering temperatures of 1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C under the maximum pressure of 50 MPa. The density, grain size, and HV of the composite increased with temperature. The densification of the composite accelerated at the initial stage of sintering but decelerated after a certain period. The analysis of the sintering curve and microstructure of the samples revealed that the sintering process could be divided into three stages: the bonding stage (initial sintering stage), the sintering neck growth stage (late sintering stage), and the shrinkage and spherification stage of closed pores (final sintering stage). The densification of the powder at different sintering stages was influenced by different mechanisms. At the initial stage of sintering, surface diffusion played an important role to promote densification by coarsening. At the late stage of sintering, grain boundary diffusion led to the shrinkage of the compact by promoting the growth of the sintering neck. As sintering continued, the grain size increased to a certain extent through surface diffusion. The few remaining pores hindered the movement of the grain boundaries, so the density of the composite no longer increased at the final stage of sintering. In SPS, densification is affected by various parameters, including temperature, pressure, holding time, and heating rate. In our study, increasing the temperature and decreasing the initial pressure during sintering densification of the powder compact were effective. In future work, sintering should be further improved, and the holding time and heating rate also should be considered to obtain an almost completely densified compact.
4.2. Evaluation of the sintering process SPS remarkably influences the comprehensive properties of the composite. In this study, a low initial pressure was applied to the powder compact during sintering to promote the removal of gas from the powder at the initial stage and improve its densification. To verify the correctness of this idea, we slightly changed the sintering process. For comparison, the pressure of the sample at a sintering temperature of 1600 °C is directly increased to 50 MPa at the beginning, and the heating rate remains unchanged and without holding 5 min at 800 °C. For the convenience of description, this process is named Process 2, and the other one is named Process 1. Fig. 7(a) illustrates the obvious difference between the two sintering processes. At the beginning of sintering, which is the exhaust behavior of the powder compact mentioned above. Moreover, the behavior of holding 5 min at 800 °C is beneficial
Acknowledgements This work is supported by National Magnetic Confinement Fusion Program (Grant No. 2014GB121001), National Natural Science Foundation of China (Grant No. 51574101 and 51474083), the Fundamental Research Funds for the Central Universities (PA2018GDQT0010), the Foundation of Laboratory of Nonferrous Metal Material and Processing Engineering of Anhui Province (15CZS08031), and the 111 Project (B18018).
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Fig. 7. Comparison of Process 1 and Process 2. (a) Time–displacement curve and (b) time–vacuum curve.
Fig. 8. SEM images of the metallographic surfaces of the samples obtained using different sintering processes: (a) Process 2 and (b) Process 1.
References
[14] A. Mondal, A. Upadhyaya, D. Agrawal, Effect of heating mode and sintering temperature on the consolidation of 90W-7Ni-3Fe alloys, J. Alloys Compd. 509 (2011) 301–310. [15] G. Lee, J. Mckittrick, E. Ivanov, E.A. Olevsky, Densification mechanism and mechanical properties of tungsten powder consolidated by spark plasma sintering, Int. J. Refract. Met. Hard Mater. 61 (2016) 22–29. [16] Z.P. Gao, G. Viola, B. Milsom, et al., Kinetics of densification and grain growth of pure tungsten during spark plasma sintering, Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 43 (2012) 1608–1614. [17] A. Muñoz, J. Martínez, M.A. Monge, B. Savoini, R. Pareja, A. Radulescu, SANS evidence for the dispersion of nanoparticles in W–1Y2O3 and W–1La2O3 processed by hot isostatic pressing, Int. J. Refract. Met. H. 33 (2012) 6–9. [18] S. Miao, Z.M. Xie, L.F. Zeng, et al., Mechanical properties, thermal stability and microstructure of fine-grained W-0.5wt.%TaC alloys fabricated by an optimized multi-step process, Nucl. Mater. Energy 13 (2017) 12–20. [19] Z.A. Munir, U. Anselmi-Tamburini, M. Ohyanagi, The effect of electric field and pressure on the synthesis and consolidation of materials: a review of the spark plasma sintering method, J. Mater. Sci. 41 (2006) 763–777. [20] W. Li, E.A. Olevsky, J. Mckittrick, et al., Densification mechanisms of spark plasma sintering: multi-step pressure dilatometry, J. Mater. Sci. 47 (2012) 7036–7046. [21] M. Omori, Sintering, consolidation, reaction and crystal growth by the spark plasma system (SPS), Mater. Sci. Eng. A 287 (2000) 183–188. [22] R. Chaim, Densification mechanisms in spark plasma sintering of nanocrystalline ceramics, Mater. Sci. Eng. A 443 (2007) 25–32. [23] M. Gendre, A. Maître, G. Trolliard, A study of the densification mechanisms during spark plasma sintering of zirconium (oxy-) carbide, Acta Mater. 58 (2010) 2598–2609. [24] S.H. Deng, R.D. Li, T.C. Yuan, et al., Direct current-enhanced densification kinetics during spark plasma sintering of tungsten powder, Scr. Mater. 143 (2018) 25–29. [25] O. El-Atwani, D.V. Quach, M. Efe, et al., Multimodal grain size distribution and high hardness in fine grained tungsten fabricated by spark plasma sintering, Mater. Sci. Eng. A 528 (2011) 5670–5677. [26] K. Hu, X.Q. Li, C. Yang, Y.Y. Li, Densification and microstructure evolution during SPS consolidation process in W-Ni-Fe system, Trans. Nonferrous Metals Soc. China
[1] F. Ferroni, X.O. Yi, K. Arakawa, et al., High temperature annealing of ion irradiated tungsten, Acta Mater. 90 (2015) 380–393. [2] M.R. Gilbert, J.-C. Sublet, Neutroneinduced transmutation effects in W and W-alloys in a fusion environment, Nucl. Fusion 51 (2011) 043005. [3] H.Y. Guo, M. Xia, L.C. Chan, et al., Nanostructured laminar tungsten alloy with improved ductility by surface mechanical attrition treatment, Sci. Rep. 7 (2017) 1351. [4] S. Wang, J. Zhang, L.M. Luo, et al., Properties of Lu2O3 doped tungsten and thermal shock performance, Powder Technol. 301 (2016) 65–69. [5] S.T. Lang, Q.Z. Yan, N.B. Sun, et al., Microstructure, basic thermal–mechanical and Charpy impact properties of W-0.1wt.% TiC alloy via chemical method, J. Alloys Compd. 660 (2016) 184–192. [6] M. Battabyal, R. Schäublin, P. Spätig, et al., Microstructure and mechanical properties of a W-2wt.%Y2O3 composite produced by sintering and hot forging, J. Nucl. Mater. 442 (2013) S225–S228. [7] M. Battabyal, R. Schäublin, P. Spätig, N. Baluc, W-2wt.%Y2O3, composite: microstructure and mechanical properties, Mater. Sci. Eng. A 538 (2012) 53–57. [8] H. Kurishita, S. Matsuo, H. Arakawa, et al., Development of re-crystallized W–1.1%TiC with enhanced room-temperature ductility and radiation performance, J. Nucl. Mater. 398 (2010) 87–92. [9] H. Kurishita, Y. Amano, S. Kobayashi, et al., Development of ultra-fine grained W–TiC and their mechanical properties for fusion applications, J. Nucl. Mater. 367 (2007) 1453–1457. [10] R. Liu, X.P. Wang, T. Hao, et al., Characterization of ODS-tungsten microwavesintered from sol–gel prepared nano-powders, J. Nucl. Mater. 450 (2014) 69–74. [11] X.Y. Tan, P. Li, L.M. Luo, et al., Effect of second-phase particles on the properties of W-based materials under high-heat loading, Nucl. Mater. Energy 9 (2016) 399–404. [12] C. Selcuk, N. Morley, J.V. Wood, Effect of post-sintering heat treatment on porous tungsten in relation to sintering swelling, Powder Metall. 47 (2004) 267–272. [13] A. Mondal, A. Upadhyaya, D. Agrawal, Effect of heating mode on sintering of tungsten, Int. J. Refract. Met. Hard Mater. 28 (2010) 597–600.
100
International Journal of Refractory Metals & Hard Materials 79 (2019) 95–101
Y.-F. Zhou et al.
tungsten powder, J. Am. Ceram. Soc. 95 (2012) 2458–2464. [35] H. Djohari, J.J. Derby, Transport mechanisms and densification during sintering: II. Grain boundaries, Chem. Eng. Sci. 64 (2009) 3810–3816. [36] H. Djohari, J.I. Martínez-Herrera, J.J. Derby, Transport mechanisms and densification during sintering: I. Viscous flow versus vacancy diffusion, Chem. Eng. Sci. 64 (2009) 3799–3809. [37] S.H. Deng, T.C. Yuan, R.D. Li, et al., Spark plasma sintering of pure tungsten powder: densification kinetics and grain growth, Powder Technol. 310 (2017) 264–271. [38] S. Diouf, A. Molinari, Densification mechanisms in spark plasma sintering: effect of particle size and pressure, Powder Technol. 221 (2012) 220–227. [39] Z.H. Zhang, F.C. Wang, L. Wang, S.K. Li, Ultrafine-grained copper prepared by spark plasma sintering process, Mater. Sci. Eng. A 476 (2008) 201–205. [40] Y. Kim, K.H. Lee, E.P. Kim, et al., Fabrication of high temperature oxides dispersion strengthened tungsten composites by spark plasma sintering process, Int. J. Refract. Met. Hard Mater. 27 (2009) 842–846. [41] S. Miao, Z.M. Xie, X.D. Wang, et al., Effect of hot rolling and annealing on the mechanical properties and thermal conductivity of W-0.5 wt.% TaC alloys, Int. J. Refract. Met. Hard Mater. 56 (2016) 8–17. [42] J.S. Lin, L.M. Luo, K. Huang, et al., Preparation and properties of fine-grained and ultrafine-grained W-TiC-Y2O3 composite, Fusion Eng. Des. 126 (2018) 147–155.
21 (2011) 493–501. [27] K. Matsui, H. Yoshida, Y. Ikuhara, Isothermal sintering effects on phase separation and grain growth in yttria-stabilized tetragonal zirconia polycrystal, J. Am. Ceram. Soc. 92 (2009) 467–475. [28] A.K. Nanda Kumar, M. Watabe, K. Kurokawa, The sintering kinetics of ultrafine tungsten carbide powders, Ceram. Int. 37 (2011) 2643–2654. [29] S. Diouf, C. Menapace, A. Molinari, Study of effect of particle size on densification of copper during spark plasma sintering, Powder Metall. 55 (2012) 228–234. [30] X.Y. Song, X.M. Liu, J.X. Zhang, Neck formation and self-adjusting mechanism of neck growth of conducting powders in spark plasma sintering, J. Am. Ceram. Soc. 89 (2006) 494–500. [31] X.M. Liu, X.Y. Song, S.X. Zhao, J.X. Zhang, Spark plasma sintering densification mechanism for cemented carbides with different WC particle sizes, J. Am. Ceram. Soc. 93 (2010) 3153–3158. [32] N. Toyofuku, T. Kuramoto, T. Imai, et al., Effect of pulsed DC current on neck growth between tungsten wires and tungsten plates during the initial stage of sintering by the spark plasma sintering method, J. Mater. Sci. 47 (2012) 2201–2205. [33] Z.Z. Fang, H.T. Wang, V. Kuman, Coarsening, densification, and grain growth during sintering of nano-sized powders–a perspective, Int. J. Refract. Met. Hard Mater. 62 (2017) 110–117. [34] H.T. Wang, Z.Z. Fang, Kinetic analysis of densification behavior of nano-sized
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