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Preparation and thermal shock characterization of yttrium doped tungsten-potassium alloy
Journal of Alloys and Compounds 686 (2016) 298e305
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation and thermal shock characterization of yttrium doped tungsten-potassium alloy Bo He a, Bo Huang a, **, Ye Xiao a, Youyun Lian b, Xiang Liu b, Jun Tang a, * a
Key Laboratory of Radiation Physics and Technology of Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China b Southwest Institute of Physics, Chengdu 610064, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 February 2016 Received in revised form 29 April 2016 Accepted 2 May 2016 Available online 3 June 2016
Novel tungsten-based W-K-Y alloys were sintered by spark plasma sintering (SPS) method using finegrained yttrium doped tungsten-potassium (W-K) powder. The relative density, microstructure, hardness and the resistance to thermal shock damage of the sintered samples were characterized. With the enhanced Y doping, the grain size decreased and the hardness increased. Thermal shock test under 0.37 GW/m2 heat load showed that low yttrium doping (0.05 wt%, 0.1 wt% and 0.5 wt%) in W-K-Y alloys can improve the resistance to thermal shock damage comparing with traditional commercial W-K, while high yttrium doping (1 wt%) easily leads to crack formation. This study will provide helpful information to optimize the preparation of tungsten-based plasma facing materials through composite tuning. © 2016 Elsevier B.V. All rights reserved.
Keywords: Potassium-doped tungsten Yttrium SPS sintering Thermal shock
1. Introduction Tungsten and tungsten-based alloys are important materials in lighting, electronics, military and in particular fusion devices due to their good thermal conductivity, high melting point, low erosion rate and low tritium retention under the extreme service environment (high thermal load, high flux H/He plasma and neutron irradiation) [1,2]. It is regarded as one of the promising candidates for plasma facing materials (PFMs) in fusion devices and fusion reactors and has been widely studied in recent years. Some properties such as its low ductile-brittle transition temperature (DBTT), irradiation embrittlement and irradiation swelling are not excellent enough serving as PFMs to date, and thus need to be further improved [3]. Especially, resistance to thermal shock is considered as one of the most important properties for PFMs, because high steady state heat load (10e20 MW/m2) will be superimposed with transient heat loads in the sub-ms and ms-range (at low and high frequencies; up to the GW/m2 range) on the first wall during plasma operation [4,5]. Such heat load may cause surface melting, cracking, recrystallization and droplet ejection on the surface of
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Huang), [email protected] (J. Tang). http://dx.doi.org/10.1016/j.jallcom.2016.05.010 0925-8388/© 2016 Elsevier B.V. All rights reserved.
PFMs, which will seriously shorten their service life [6]. Therefore, it is of great significance to develop high-thermal-shock resistant tungsten alloy as PFMs. Tungsten-potassium (W-K) alloy is a traditional material with high thermal shock resistance. The excellent mechanical properties and higher recrystallization temperature of AKS-doped tungsten (also named non-sag tungsten) was found in early 1930’s in lamp filament industry [7]. The AKS-doping means the potassium bubbles form from the decomposition of the Al-K-Si dopant particles under certain process [8]. The potassium bubbles were found to perform the similar function to other dispersion strengthening phase [7]. Some studies have been carried out on the thermal shock resistance of commercial W-K. It showed that the surface damage of commercial W-K is largely mitigated compared with pure W, displaying improvement of mechanical properties [9]. Traditional commercial W-K bulk material is fabricated through pressing, vertical sintering and swaging, and finally the potassium bubble size is tuned to 50e200 nm and mainly located on grain boundary [10]. Non-sag W-K with smaller bubble size are fabricated by rotary swaging and high temperature annealing after cold isostatic compaction and vertical sintering, but the final product is tungsten wire which is difficult to be used as PFMs. In our previous studies, SPS sintered AKS-W bulk samples with small size (20e100 nm) and intra-granular potassium bubble was obtained [8]. Besides potassium bubble strengthening, Oxide Dispersion
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Strengthening (ODS) is another common method in alloy strengthening. Especially, La2O3 [11,12] and Y2O3 are usually used for tungsten alloy strengthening [13]. The high temperature stability of Y2O3 makes it suitable for oxide dispersion strengthening in tungsten alloys. People have studied yttrium oxide doped tungsten and high temperature tensile strength and fracture strength were obviously improved [14]. Actually, low amount Y2O3 doping can effectively refine the tungsten grain, and enhance its recrystallization temperature and reduce DBTT. Considering free oxygen is regarded as the main source of crack formation to PFMs [15], yttrium can adsorb free oxygen and thus should be more suitable as the dopant rather than yttrium oxide. Veleva et al. have developed a new W-2Y material by mechanical alloying and hot isostatic pressing [16]. The yttrium dopant was finally oxidized into nano size yttrium oxide. Considering the respective advantages of potassium bubble and yttrium strengthening, there is a high interest in developing W-K-Y ternary tungsten alloy as PFM, and thus need to be systematically studied. In this work, a novel tungsten-based alloy, namely W-K-Y ingots are fabricated by powder metallurgy method to get high homogeneity and fine grain size through spark plasma sintering (SPS) technique. The effects of doping contents on microstructure, hardness, density and resistance to thermal shock were investigated. 2. Experiment details Commercial AKS tungsten powder (99.9% purity, particle size3e4 mm), yttrium powder (99.99% purity, particle size 30e70 mm) were used as the starting materials. The AKS tungsten powder is fabricated by adding aqueous solutions of Al-K-Sicontaining compounds to the tungsten blue oxide (TBO). Hydrogen reduction of the doped TBO leads to a metal powder with potassium-containing dopant phases within the tungsten grains [17]. The AKS component is shown in Table 1. Five starting powders with different yttrium doping ratio (0, 0.05 wt%, 0.1 wt%, 0.5 wt% and 1 wt%) were used to prepare bulk samples, as shown in Table 2. The powders were put into WC/8Co milling vessel with tungsten carbide balls (diameter of 5 mm and 10 mm with a mass ratio of 2:1) at the ball-to-powder weight ratio of 5:1. After 40 h high energy ball milling under rotation speed of 250 rpm and in hydrogen and argon mixed atmosphere (1:12 in volume ratio, 99.9999%), the powders were sufficiently mixed and the grain size is supposed to be refined in this process according to our previous studies [8,14]. The consolidation of the mixed powder was carried out by SPS method. The mixed powders were fed into a graphite die of 15 mm diameter and a pre-compacting of 80 MPa was done before sintering. The consolidation scheme is shown in Fig. 1. The SPS sintering machine was heated to 1750 C from room temperature at the heating rate of 100 C/min and hold for 3 min, the pressure would increase from 20 to 80 MPa with the increasing temperature in two steps. All the consolidation took about 19 min not include the cool down time and were carried out in vacuum (5 Pa). When the heating process was over, the samples were naturally cooling to room temperature and the pressure was removed at the end of the whole sintering process. Before the thermal shock tests, all the samples were polished to mirror-like to reduce the surface
Table 1 The component of commercial AKS tungsten powder.
Content (ppm)
Si
K
Al
Ni
Fe
Cr
N
O2
Co
C
185
82
30
3
28
3
13
1000
4
10
299
Table 2 The yttrium doping ratio of starting powder.
Doping percentage (wt%)
1
2
3
4
5
0
0.05%
0.1%
0.5%
1%
Fig. 1. Schematic heating cycles for sintering process.
roughness. And stress relief treatment was carried out at 1273 K in vacuum (101 Pa) to obtain better mechanical properties. Thermal shock tests were performed in the electron beam test facility EMS-60 at Southwest Institute of Physics (SWIP). Single shot and multiple shots (100) tests with different pulse duration (5 ms for single shot and 1 ms for multiple shots) were carried out. The accelerating voltage of electron beam is 120 kV and the average beam current is 90 mA. The heat load scanning area is 4 4 mm2, the heat flux could be calculated as 0.37 GW/m2 using the formula P ¼ UIa. Here, the value of electron absorption coefficient a is 0.55, which is a result of Monte Carlo Simulation. The power density corresponds to a heat flux factor FHF ¼ Pabs Dt 0:5 ¼ 26:2MW=m2 s0:5 for a single shot and FHF ¼ Pabs Dt 0:5 ¼ 11:7MW=m2 s0:5 for 100 shots. In addition, the thermal shock tests were performed at room temperature and in a vacuum degree of 102Pa. Material properties were characterized to verify that the manufacturing were successful and can be used to interpret the thermal shock tests. The density was measured by drainage method. The theoretical density of the yttrium doped AKS samples were calculated from the fraction and theoretical density of each component. The thermal shock damage and microstructure were investigated through optical microscopy and scanning electron microscopy (SEM, Hitachi S4800). Vickers hardness tests were performed under a load of 200 g and hold for 10 s.
3.. Results and discussion The SEM morphologies of initial commercial AKS powder, yttrium powder and mixed powder after 40 h ball milling are displayed in Fig. 2. Comparing Fig. 2a with Fig. 2f, the real size of powder particles are almost the same, but the surface of the ball milled powder is much rougher than that before after ball milling. As shown in Fig. 2d, the powder was crushed into flat particles after 20 h’s ball milling. With the ball milling time increasing, the powder particle size slightly decreased. The morphologies of mixed powder after 30 h and 40 h’s ball milling are nearly the same, and the extended 10 h is to fully ensure that the yttrium particles are crushed into small size. For inter-metallic powder like yttrium
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Fig. 2. SEM micrographs of (a) commercial AKS powder, (b) yttrium powder and yttrium doped AKS powder after (c) 10 h ball milling, (d) 20 h ball milling, (e) 30 h ball milling and (f) 40 h ball milling (low and high magnification).
powder we used here, the reduction of the particle size is a natural outcome of the trans-granular fracturing and cold welding [18]. In the SEM images of milled powders, no large particles were found. It is indicated that the yttrium powder has been fractured to a small size, as the original average particle size is 40e50 mm (325 mesh). In addition, almost no yttrium was found distributed among these milled powder owing to its low doping percentage and unclear morphology. As shown in the high magnification Fig. 2f, higher dislocation density occurred after milling. It is believed that the reduction of the grain size of metallic powders (tungsten powder in this work) is due to localization of plastic deformation in the form of shear bands containing a high density of dislocations. A quantitative result on grain size will be shown through XRD measurements. Taken 1.wt% yttrium doped AKS tungsten as an example, the yttrium peak disappeared after 10 h of ball milling in XRD Spectra, as shown in Fig. 3. It is obvious that the W peaks broaden after ball milling which mean grain refinement. The average grain size was calculated from XRD spectra by Scherrer formula. After 40 h of ball milling, the average grain size decreases from 700 nm to 200 nm.
Fig. 3. XRD Spectra after different mechanical alloying times.
Relative density is regarded as one of the basic properties for bulk materials fabricated through powder metallurgy technology. Generally speaking, higher relative density can lead to better thermo-mechanical properties and resistance to hydrogen retention for tungsten alloys. Several factors, such as sintering temperature, holding time and pressure are thought to largely influence the final relative density in previous studies [19]. It was suggested that excessive high sintering temperature and long holding time can lead to crystal growth, while low sintering temperature prevents the evaporation of potassium. After comparison, 1750 C is settled as the suitable sintering temperature [20]. Taking the pressure limitation and safety of graphite die as well as SPS into consideration, the optimum sintering pressure and heating rate are determined to be 80 MPa and 100 C/min, respectively. Table 3 shows the relative density of AKS-W with different yttrium doping percentage (0, 0.05 wt%, 0.1 wt%, 0.5 wt%, 1 wt%). The relative density of all the AKS-W samples are above 97.00% and highest one reaches 98.60%, which is close to the pure tungsten bulk’s relative density of 97.78%. The increasing of hardness is mainly based on the refinement of grain size and the highest one reached 505.24 HV. The room-temperature hardness can reflect the high-temperature mechanical properties of these samples at some level, as their composition and microstructure are nearly the same. The content of main chemical impurities after sintering has been reported in previous work [8]. The final content of potassium and aluminum after sintering is 60 ppm and 24 ppm, respectively. But silicon almost completely evaporates from the ingots. The surface morphologies of the sintered samples were shown in Fig. 4. Comparing Fig. 4aee, it can be deduced that the grain size becomes smaller with the increasing of yttrium doping. The grain refinement become more obvious as the doping content is largely increased, and the smallest average grain size is acquired at 1% yttrium doping among the samples. Supposing yttrium particles are oxidized during high energy ball-milling process and sintering process, the hard ceramic Y2O3 particles can effectively prevent the grain size from growing during sintering process which finally resulted in grain size refinement. Black spots can be observed in Fig. 4aee, and these black spots can be classified in two types. One type is larger, and mainly locates on grain boundaries, which can be easily found in Fig. 4c. The other is small and mainly distributes within the grain. There are three possibilities for the black spots,
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Table 3 The basic properties of SPS sintered bulk samples. Sample
W-K 0.05 wt%Y-W-K 0.1 wt%Y-W-K 0.5 wt%Y-W-K 1 wt%Y-W-K
Average grain size (mm)
Relative density (%) Single pulse
Hundred pulse
97.78 97.43 97.01 98.25 98.60
e 97.36 96.94 98.38 98.21
6.01 5.49 5.67 3.82 2.95
± ± ± ± ±
1.41 1.24 1.73 0.74 0.55
Average Vickers hardness (HV)
401.32 413.88 408.87 475.97 505.24
± ± ± ± ±
12.78 15.62 14.53 11.14 13.62
Fig. 4. Surface morphologies of (a-e) AKS-W with different yttrium doping percentage of 0, 0.05 wt%, 0.1 wt%, 0.5 wt%, and 1 wt%.
judging from optical micrographs, i.e., sintering void, dispersion phase particles and potassium bubbles. SEM micrographs and EDX spectra characterization are applied to differentiate these black spots in this work. The morphology of sintering void, yttrium particles, potassium bubbles, and relative EDX spectra were shown in Fig. 6. Comparing Fig. 5a with Fig. 5b, the size of sintering void is slightly bigger than that of the yttrium particle. The biggest observed sintering void is up to 1 mm large, while the yttrium particles is less than 1 mm. The second phase particle is yttrium or yttrium oxide dispersion phase. The potassium bubbles are much smaller than sintering void and
dispersion phase particles which are regular hexagon with a diameter 50e100 nm and mainly located in the grain, which is close to the previous report [8]. The distributions of yttrium and potassium are clearly shown on the fraction section of the AKS-W bulk samples with 1 wt% yttrium doping. Potassium bubbles disperse irregularly in grains and on grain boundaries. The bubbles located on grain boundaries are much bigger than those in grains. The EDX spectra in Fig. 6a suggested that these little bubbles are actually potassium bubbles. EDX spectra in Fig. 6b implied that the second phase particle is yttrium or its oxide. Our other work demonstrated that the second phase
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Fig. 5. SEM micrographs of (a) sintering void (b) yttrium particle (c) potassium bubbles on AKS-W bulk samples with 0.05 wt% yttrium doping.
may have different performance with yttrium oxide in improving the mechanical properties of tungsten. The pure yttrium core has higher toughness than yttrium oxide and thus brings less brittleness to the material. And the hard yttrium oxide shell plays the same role in pinning dislocation with yttrium oxide. Thermal shock resistance is an important factor to assess the property of PFMs. Pintsuk et al. investigated fully recrystallized WK in transient heat load tests at 0.32 GW/m2 (the value of absorption coefficient in their work is 0.46) for 100 shots, 1000 shots and 10,000 shots at room temperature [21]. These samples did not display obvious damage until 1000 shots. The value of cracking threshold was estimated around 0.19e0.38 GW/m2 from their work (the value was calculated by an absorption coefficient of 0.55 instead of 0.46), and it is similar to the one of pure tungsten [21]. Actually, previous work has proved that traditional vertical sintering W-K does not improve the cracking threshold of tungsten [8]. The transient heat load tests under the energy density of 0.37 GW/ m2 for 5 ms single shot were carried out to study the plasma disruption resistance of W-K-Y. In Fig. 7a the 82 ppm W-K sample showed great thermal shock resistance. The positive improvement of nano-size potassium bubbles on SPS-sintering tungsten had been study in our previous work [8]. The surface morphologies of W-K-Y bulk samples after thermal shock was shown in Fig. 7bee and the inserted SEM images are the full view of the test area. Comparing the surface morphology of these W-K-Y samples, an obvious major crack formed on the surface of yttrium 1% doped sample while on others no crack was found. At the high magnification morphology, grain boundaries can be observed on each surface of these samples, probably induced by the annealing effect of the transient heat load. Local ruptured grain boundaries could be observed on the surface of 0.1 wt% Y-W-K sample. The rupture of the grain boundaries attributed to thermal stresses induced by different thermal expansion which caused by high temperature gradient. When the thermal stresses exceed the yield strength of the material, the grain boundaries first began to rupture as they were the weakest part of the heated surface [6]. The 0.1% Y-W-K samples have lower relative density than other samples and their grain size is not refined comparing with 0.05% Y-W-K samples. It is probably due to the poor reproducibility of SPS sintering process and thus induces poor yield strength. That can explain why grain boundary rupture appeared on the surface of 0.1 wt% Y-W-K sample while others did not. For the sample with 1 wt% yttrium, the formation of major crack can mainly be attributed to induced brit-
Fig. 6. Fraction section of AKS-W bulk samples with 1 wt% yttrium doping and (a) EDX spectra of potassium bubble, (b) EDX spectra of yttrium particle of the selected area.
particles are yttrium particles covered in thin oxide layer, which will be reported later. The core-shell structure of yttrium particles
tleness due to an excess amount of the doping yttrium [16]. During one pulse, the surface temperature normally rise above DBTT and
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Fig. 7. SEM micrographs of (a-e) the surface morphology of yttrium doping AKS-W bulk samples with 0, 0.05 wt%, 0.1 wt%, 0.5 wt% and 1 wt% yttrium doping percentage after single shot transient heat load tests.
relieve thermal stresses by plastic deformation [14]. But for brittle material like 1 wt% Y-W-K, the stress relief was not possible due to poor plasticity and major crack formed. In addition, multiple (100) shots transient heat load tests under the energy density of 0.37 GW/m2 and 1 ms for each pulse were performed. The surface morphology of yttrium doping AKS-W samples are shown in Fig. 8(a-d). The inserted SEM images are the full view of the test area. A major crack formed at the loading area of the 0.5 wt% W-K-Y sample but the 1.wt% yttrium doping one did not. An anomalous particle (almost 60 mm in size) can be observed at the middle of the major crack for the sample 0.5 wt% W-K-Y (Fig. 8e). The EDX spectrum (Fig. 8g) suggests the huge particle is yttrium doping particle. The anomalous size of this particle was attributed to the unobservable ball milling process. Some starting yttrium powder stuck in the corner of the vessel and has not been crushed into small size during the whole process. Considering the different thermal expansion coefficient between the huge yttrium doping particle and tungsten, as well as the brittleness of yttrium doping particles, the reason why a major crack formed become clear. In addition, the annealing effect of 0.5 wt% W-K-Y sample could be easily observed in Fig. 8f, suggests that plastic deformation did happened at the heated area, which is further confirmed by 3D topography in Fig. 9 below. The surface roughness of the W-K-Y samples after 100 pulses transient heat load tests are shown in Fig. 9. The surface roughness
deceased with the increase of yttrium doping percentage, which indicates that the plastic deformation became weaker with the rising yttrium content. It is a predictable result of increasing yield strength by refining the grain size. The results of transient heat load under 0.37 GW/m2 for a single shot suggested that SPS sintered 82 ppm potassium doped and moderate (<0.5 wt%) yttrium doped AKS tungsten performs better thermal shock resistance than traditional commercial W-K [8], owing to the improved mechanical properties and ductility. It is implied that high yttrium doping (1 wt%) brings negative effect on the thermal shock resistance behavior. It was reported that potassium doping lowers the DBTT and increase the yield strength [8,22], and yttrium (mainly its oxide) doping brings high temperature strength and brittleness [16,23]. In this work, the core-shell-like yttrium doping plays the similar role with yttrium oxide. For transient heat load under 0.37 GW/m2 for 100 shots, the 0.5 wt% WK-Y sample cracked due to an abnormal huge yttrium particle. Comparing the surface roughness of these W-K-Y samples in multiple shots test with previous work, the plastic deformation effect are much better, indicates that SPS sintering W-K-Y alloy performs better than traditional commercial W-K [21]. Both potassium doping and yttrium doping could effectively improve the mechanical properties, but has opposite effect on the ductility of tungsten. One of the efforts in this work is to find the best yttrium doping percentage to balance the strength and
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Fig. 8. SEM micrographs of (a-d) the surface morphology of yttrium doping AKS-W bulk samples with 0.05 wt%, 0.1 wt%, 0.5 wt% and 1 wt% yttrium doping percentage after 100 pulses transient heat load tests, (e) morphology of anomalous particle for 0.5 wt% Y doping, (f) non-cracked area of 0.5 wt% W-K-Y and (g) EDX spectra of selected area.
ductility of tungsten. The results of transient heat load tests, which mainly reflect on the strength and ductility of material, is one of the most direct ways to determine the best yttrium doping percentage. Present transient heat load tests at the energy density of 0.37 GW/ m2 for a single shot 100 shots suggests that appropriate yttrium doping rate (less than 0.5 wt%) perform better thermal shock resistant than traditional commercial W-K and pure tungsten. In order to find the best binary doping content, higher heat flux transient heat load experiments will be studied to obtain the optimal yttrium content for W-K, and the micro structure optimization also need to be modulated to further improve thermal shock resistance in the future.
4. Conclusion Novel tungsten-based W-K-Y ingots were sintered by sparkplasma-sintering (SPS) method. High relative density (up to 98.60%) AKS-W bulk samples with 0.05 wt%, 0.1 wt%, 0.5 wt%, 1 wt% yttrium doping were sintered obtained. Nano size (50e100 nm) potassium bubbles are formed during this process and the size of the yttrium particles are about 500 nm. The Vickers-hardness enhances with the increasing of doping percentage as the result of refinement of grain size and the highest one reaches 505.24 HV. The samples with 0.05 wt%, 0.1 wt% and 0.5 wt% yttrium doping improved thermal shock resistance compared with traditional commercial W-K and pure tungsten. This provides a promising way to improve the properties of tungsten-based PFMs and more
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Fig. 9. (a-d) 3D topography and surface roughness of yttrium doping AKS-W bulk samples with 0.05 wt%, 0.1 wt%, 0.5 wt% and 1 wt% yttrium doping percentage after 100 pulses transient heat load tests.
detailed investigations are needed to this doping system. Acknowledgements This work was supported by the International Thermonuclear Experimental Reactor (ITER) Program Special (No. 2011GB108005), the National Natural Science Foundation of China (No. 11475118), and the National Fund of China for Fostering Talents in Basic Science (J1210004). References [1] H. Bolt, V. Barabash, W. Krauss, J. Linke, R. Neu, S. Suzuki, N. Yoshida, A.U. Team, Materials for the plasma-facing components of fusion reactors, J. Nucl. Mater 329e333 (Part A) (2004) 66e73. [2] J. Linke, Plasma facing materials and components for future fusion devicesddevelopment, characterization and performance under fusion specific loading conditions, Phys. Scr. T123 (2006) 45e53. [3] R. Liu, Y. Zhou, T. Hao, T. Zhang, X.P. Wang, C.S. Liu, Q.F. Fang, Microwave synthesis and properties of fine-grained oxides dispersion strengthened tungsten, J. Nucl. Mater 424 (2012) 171e175. [4] M. Rieth, S.L. Dudarev, S.M. Gonzalez de Vicente, J. Aktaa, et al., Recent progress in research on tungsten materials for nuclear fusion applications in Europe, J. Nucl. Mater 432 (2013) 482e500. € dig, L. Singheiser, Performance of plasma-facing ma[5] J. Linke, T. Hirai, M. Ro terials under intense thermal loads in tokamaks and stellarators, Fusion Sci. Technol. 46 (2004) 142e151. [6] A. Suslova, O. El-Atwani, S.S. Harilal, A. Hassanein, Material ejection and surface morphology changes during transient heat loading of tungsten as plasma-facing component in fusion devices, Nucl. Fusion 55 (2015) 033007. [7] P. Schade, 100years of doped tungsten wire, Int. J. Refract. Metals Hard Mater. 28 (2010) 648e660. [8] B. Huang, Y. Xiao, B. He, J. Yang, J. Liao, Y. Yang, N. Liu, Y. Lian, X. Liu, J. Tang, Effect of potassium doping on the thermal shock behavior of tungsten, Int. J. Refract. Metals Hard Mater. 51 (2015) 19e24. [9] M. Faleschini, H. Kreuzer, D. Kiener, R. Pippan, Fracture toughness investigations of tungsten alloys and SPD tungsten alloys, J. Nucl. Mater 367e370 (Part A) (2007) 800e805.
[10] O. Horacsek, L. Bartha, Development of the bubble structure from selectively deforming potassium-pores in doped tungsten wires, Int. J. Refract. Metals Hard Mater. 22 (2004) 9e15. [11] F.C. Sze, R.P. Doerner, S. Luckhardt, Investigation of plasma exposed We1% La2O3 tungsten in a high ion flux, low ion energy, low carbon impurity plasma environment for the International Thermonuclear Experimental Reactor, J. Nucl. Mater 264 (1999) 89e98. [12] X. Zhang, Q. Yan, Morphology evolution of La2O3 and crack characteristic in WeLa2O3 alloy under transient heat loading, J. Nucl. Mater 451 (2014) 283e291. €ublin, P. Sp€ [13] M. Battabyal, R. Scha atig, N. Baluc, We2wt%Y2O3 composite: microstructure and mechanical properties, Mater. Sci. Eng. A 538 (2012) 53e57. [14] N. Lemahieu, J. Linke, G. Pintsuk, G. Van Oost, M. Wirtz, Z. Zhou, Performance of yttrium doped tungsten under ‘edge localized mode’-like loading conditions, Phys. Scr. T159 (2014) 014035. [15] X.-S. Kong, Y.-W. You, Q.F. Fang, C.S. Liu, J.-L. Chen, G.N. Luo, B.C. Pan, Z. Wang, The role of impurity oxygen in hydrogen bubble nucleation in tungsten, J. Nucl. Mater 433 (2013) 357e363. €ublin, T. Plocinski, M. Walter, N. Baluc, Processing and [16] L. Veleva, R. Scha characterization of a We2Y material for fusion power reactors, Fusion Eng. Des. 86 (2011) 2450e2453. [17] O. Horacsek, L. Bartha, Influence of surface particles of AKS-doped TBO on the NS-structure of tungsten wires, Int. J. Refract. Metals Hard Mater. 20 (2002) 271e276. [18] D. Zhang, Processing of advanced materials using high-energy mechanical milling, Prog. Mater. Sci. 49 (2004) 537e560. [19] X. Shu, H. Qiu, B. Huang, Z. Gu, J. Yang, J. Liao, Y. Yang, N. Liu, J. Tang, Preparation and characterization of potassium doped tungsten, J. Nucl. Mater 440 (2013) 414e419. [20] B. Huang, B. He, Y. Xiao, R. Ang, J. Yang, J. Liao, Y. Yang, N. Liu, D. Pan, J. Tang, Microstructure and bubble formation of AleKeSi doped tungsten prepared by spark plasma sintering, Int. J. Refract. Metals Hard Mater. 54 (2016) 335e341. [21] G. Pintsuk, I. Uytdenhouwen, Thermo-mechanical and thermal shock characterization of potassium doped tungsten, Int. J. Refract. Metals Hard Mater. 28 (2010) 661e668. [22] K. Sasaki, K. Yabuuchi, S. Nogami, A. Hasegawa, Effects of temperature and strain rate on the tensile properties of potassium-doped tungsten, J. Nucl. Mater 461 (2015) 357e364. [23] L. Veleva, Z. Oksiuta, U. Vogt, N. Baluc, Sintering and characterization of WeY and WeY2O3 materials, Fusion Eng. Des. 84 (2009) 1920e1924.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation and thermal shock characterization of yttrium doped tungsten-potassium alloy Bo He a, Bo Huang a, **, Ye Xiao a, Youyun Lian b, Xiang Liu b, Jun Tang a, * a
Key Laboratory of Radiation Physics and Technology of Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China b Southwest Institute of Physics, Chengdu 610064, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 February 2016 Received in revised form 29 April 2016 Accepted 2 May 2016 Available online 3 June 2016
Novel tungsten-based W-K-Y alloys were sintered by spark plasma sintering (SPS) method using finegrained yttrium doped tungsten-potassium (W-K) powder. The relative density, microstructure, hardness and the resistance to thermal shock damage of the sintered samples were characterized. With the enhanced Y doping, the grain size decreased and the hardness increased. Thermal shock test under 0.37 GW/m2 heat load showed that low yttrium doping (0.05 wt%, 0.1 wt% and 0.5 wt%) in W-K-Y alloys can improve the resistance to thermal shock damage comparing with traditional commercial W-K, while high yttrium doping (1 wt%) easily leads to crack formation. This study will provide helpful information to optimize the preparation of tungsten-based plasma facing materials through composite tuning. © 2016 Elsevier B.V. All rights reserved.
Keywords: Potassium-doped tungsten Yttrium SPS sintering Thermal shock
1. Introduction Tungsten and tungsten-based alloys are important materials in lighting, electronics, military and in particular fusion devices due to their good thermal conductivity, high melting point, low erosion rate and low tritium retention under the extreme service environment (high thermal load, high flux H/He plasma and neutron irradiation) [1,2]. It is regarded as one of the promising candidates for plasma facing materials (PFMs) in fusion devices and fusion reactors and has been widely studied in recent years. Some properties such as its low ductile-brittle transition temperature (DBTT), irradiation embrittlement and irradiation swelling are not excellent enough serving as PFMs to date, and thus need to be further improved [3]. Especially, resistance to thermal shock is considered as one of the most important properties for PFMs, because high steady state heat load (10e20 MW/m2) will be superimposed with transient heat loads in the sub-ms and ms-range (at low and high frequencies; up to the GW/m2 range) on the first wall during plasma operation [4,5]. Such heat load may cause surface melting, cracking, recrystallization and droplet ejection on the surface of
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Huang), [email protected] (J. Tang). http://dx.doi.org/10.1016/j.jallcom.2016.05.010 0925-8388/© 2016 Elsevier B.V. All rights reserved.
PFMs, which will seriously shorten their service life [6]. Therefore, it is of great significance to develop high-thermal-shock resistant tungsten alloy as PFMs. Tungsten-potassium (W-K) alloy is a traditional material with high thermal shock resistance. The excellent mechanical properties and higher recrystallization temperature of AKS-doped tungsten (also named non-sag tungsten) was found in early 1930’s in lamp filament industry [7]. The AKS-doping means the potassium bubbles form from the decomposition of the Al-K-Si dopant particles under certain process [8]. The potassium bubbles were found to perform the similar function to other dispersion strengthening phase [7]. Some studies have been carried out on the thermal shock resistance of commercial W-K. It showed that the surface damage of commercial W-K is largely mitigated compared with pure W, displaying improvement of mechanical properties [9]. Traditional commercial W-K bulk material is fabricated through pressing, vertical sintering and swaging, and finally the potassium bubble size is tuned to 50e200 nm and mainly located on grain boundary [10]. Non-sag W-K with smaller bubble size are fabricated by rotary swaging and high temperature annealing after cold isostatic compaction and vertical sintering, but the final product is tungsten wire which is difficult to be used as PFMs. In our previous studies, SPS sintered AKS-W bulk samples with small size (20e100 nm) and intra-granular potassium bubble was obtained [8]. Besides potassium bubble strengthening, Oxide Dispersion
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Strengthening (ODS) is another common method in alloy strengthening. Especially, La2O3 [11,12] and Y2O3 are usually used for tungsten alloy strengthening [13]. The high temperature stability of Y2O3 makes it suitable for oxide dispersion strengthening in tungsten alloys. People have studied yttrium oxide doped tungsten and high temperature tensile strength and fracture strength were obviously improved [14]. Actually, low amount Y2O3 doping can effectively refine the tungsten grain, and enhance its recrystallization temperature and reduce DBTT. Considering free oxygen is regarded as the main source of crack formation to PFMs [15], yttrium can adsorb free oxygen and thus should be more suitable as the dopant rather than yttrium oxide. Veleva et al. have developed a new W-2Y material by mechanical alloying and hot isostatic pressing [16]. The yttrium dopant was finally oxidized into nano size yttrium oxide. Considering the respective advantages of potassium bubble and yttrium strengthening, there is a high interest in developing W-K-Y ternary tungsten alloy as PFM, and thus need to be systematically studied. In this work, a novel tungsten-based alloy, namely W-K-Y ingots are fabricated by powder metallurgy method to get high homogeneity and fine grain size through spark plasma sintering (SPS) technique. The effects of doping contents on microstructure, hardness, density and resistance to thermal shock were investigated. 2. Experiment details Commercial AKS tungsten powder (99.9% purity, particle size3e4 mm), yttrium powder (99.99% purity, particle size 30e70 mm) were used as the starting materials. The AKS tungsten powder is fabricated by adding aqueous solutions of Al-K-Sicontaining compounds to the tungsten blue oxide (TBO). Hydrogen reduction of the doped TBO leads to a metal powder with potassium-containing dopant phases within the tungsten grains [17]. The AKS component is shown in Table 1. Five starting powders with different yttrium doping ratio (0, 0.05 wt%, 0.1 wt%, 0.5 wt% and 1 wt%) were used to prepare bulk samples, as shown in Table 2. The powders were put into WC/8Co milling vessel with tungsten carbide balls (diameter of 5 mm and 10 mm with a mass ratio of 2:1) at the ball-to-powder weight ratio of 5:1. After 40 h high energy ball milling under rotation speed of 250 rpm and in hydrogen and argon mixed atmosphere (1:12 in volume ratio, 99.9999%), the powders were sufficiently mixed and the grain size is supposed to be refined in this process according to our previous studies [8,14]. The consolidation of the mixed powder was carried out by SPS method. The mixed powders were fed into a graphite die of 15 mm diameter and a pre-compacting of 80 MPa was done before sintering. The consolidation scheme is shown in Fig. 1. The SPS sintering machine was heated to 1750 C from room temperature at the heating rate of 100 C/min and hold for 3 min, the pressure would increase from 20 to 80 MPa with the increasing temperature in two steps. All the consolidation took about 19 min not include the cool down time and were carried out in vacuum (5 Pa). When the heating process was over, the samples were naturally cooling to room temperature and the pressure was removed at the end of the whole sintering process. Before the thermal shock tests, all the samples were polished to mirror-like to reduce the surface
Table 1 The component of commercial AKS tungsten powder.
Content (ppm)
Si
K
Al
Ni
Fe
Cr
N
O2
Co
C
185
82
30
3
28
3
13
1000
4
10
299
Table 2 The yttrium doping ratio of starting powder.
Doping percentage (wt%)
1
2
3
4
5
0
0.05%
0.1%
0.5%
1%
Fig. 1. Schematic heating cycles for sintering process.
roughness. And stress relief treatment was carried out at 1273 K in vacuum (101 Pa) to obtain better mechanical properties. Thermal shock tests were performed in the electron beam test facility EMS-60 at Southwest Institute of Physics (SWIP). Single shot and multiple shots (100) tests with different pulse duration (5 ms for single shot and 1 ms for multiple shots) were carried out. The accelerating voltage of electron beam is 120 kV and the average beam current is 90 mA. The heat load scanning area is 4 4 mm2, the heat flux could be calculated as 0.37 GW/m2 using the formula P ¼ UIa. Here, the value of electron absorption coefficient a is 0.55, which is a result of Monte Carlo Simulation. The power density corresponds to a heat flux factor FHF ¼ Pabs Dt 0:5 ¼ 26:2MW=m2 s0:5 for a single shot and FHF ¼ Pabs Dt 0:5 ¼ 11:7MW=m2 s0:5 for 100 shots. In addition, the thermal shock tests were performed at room temperature and in a vacuum degree of 102Pa. Material properties were characterized to verify that the manufacturing were successful and can be used to interpret the thermal shock tests. The density was measured by drainage method. The theoretical density of the yttrium doped AKS samples were calculated from the fraction and theoretical density of each component. The thermal shock damage and microstructure were investigated through optical microscopy and scanning electron microscopy (SEM, Hitachi S4800). Vickers hardness tests were performed under a load of 200 g and hold for 10 s.
3.. Results and discussion The SEM morphologies of initial commercial AKS powder, yttrium powder and mixed powder after 40 h ball milling are displayed in Fig. 2. Comparing Fig. 2a with Fig. 2f, the real size of powder particles are almost the same, but the surface of the ball milled powder is much rougher than that before after ball milling. As shown in Fig. 2d, the powder was crushed into flat particles after 20 h’s ball milling. With the ball milling time increasing, the powder particle size slightly decreased. The morphologies of mixed powder after 30 h and 40 h’s ball milling are nearly the same, and the extended 10 h is to fully ensure that the yttrium particles are crushed into small size. For inter-metallic powder like yttrium
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Fig. 2. SEM micrographs of (a) commercial AKS powder, (b) yttrium powder and yttrium doped AKS powder after (c) 10 h ball milling, (d) 20 h ball milling, (e) 30 h ball milling and (f) 40 h ball milling (low and high magnification).
powder we used here, the reduction of the particle size is a natural outcome of the trans-granular fracturing and cold welding [18]. In the SEM images of milled powders, no large particles were found. It is indicated that the yttrium powder has been fractured to a small size, as the original average particle size is 40e50 mm (325 mesh). In addition, almost no yttrium was found distributed among these milled powder owing to its low doping percentage and unclear morphology. As shown in the high magnification Fig. 2f, higher dislocation density occurred after milling. It is believed that the reduction of the grain size of metallic powders (tungsten powder in this work) is due to localization of plastic deformation in the form of shear bands containing a high density of dislocations. A quantitative result on grain size will be shown through XRD measurements. Taken 1.wt% yttrium doped AKS tungsten as an example, the yttrium peak disappeared after 10 h of ball milling in XRD Spectra, as shown in Fig. 3. It is obvious that the W peaks broaden after ball milling which mean grain refinement. The average grain size was calculated from XRD spectra by Scherrer formula. After 40 h of ball milling, the average grain size decreases from 700 nm to 200 nm.
Fig. 3. XRD Spectra after different mechanical alloying times.
Relative density is regarded as one of the basic properties for bulk materials fabricated through powder metallurgy technology. Generally speaking, higher relative density can lead to better thermo-mechanical properties and resistance to hydrogen retention for tungsten alloys. Several factors, such as sintering temperature, holding time and pressure are thought to largely influence the final relative density in previous studies [19]. It was suggested that excessive high sintering temperature and long holding time can lead to crystal growth, while low sintering temperature prevents the evaporation of potassium. After comparison, 1750 C is settled as the suitable sintering temperature [20]. Taking the pressure limitation and safety of graphite die as well as SPS into consideration, the optimum sintering pressure and heating rate are determined to be 80 MPa and 100 C/min, respectively. Table 3 shows the relative density of AKS-W with different yttrium doping percentage (0, 0.05 wt%, 0.1 wt%, 0.5 wt%, 1 wt%). The relative density of all the AKS-W samples are above 97.00% and highest one reaches 98.60%, which is close to the pure tungsten bulk’s relative density of 97.78%. The increasing of hardness is mainly based on the refinement of grain size and the highest one reached 505.24 HV. The room-temperature hardness can reflect the high-temperature mechanical properties of these samples at some level, as their composition and microstructure are nearly the same. The content of main chemical impurities after sintering has been reported in previous work [8]. The final content of potassium and aluminum after sintering is 60 ppm and 24 ppm, respectively. But silicon almost completely evaporates from the ingots. The surface morphologies of the sintered samples were shown in Fig. 4. Comparing Fig. 4aee, it can be deduced that the grain size becomes smaller with the increasing of yttrium doping. The grain refinement become more obvious as the doping content is largely increased, and the smallest average grain size is acquired at 1% yttrium doping among the samples. Supposing yttrium particles are oxidized during high energy ball-milling process and sintering process, the hard ceramic Y2O3 particles can effectively prevent the grain size from growing during sintering process which finally resulted in grain size refinement. Black spots can be observed in Fig. 4aee, and these black spots can be classified in two types. One type is larger, and mainly locates on grain boundaries, which can be easily found in Fig. 4c. The other is small and mainly distributes within the grain. There are three possibilities for the black spots,
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Table 3 The basic properties of SPS sintered bulk samples. Sample
W-K 0.05 wt%Y-W-K 0.1 wt%Y-W-K 0.5 wt%Y-W-K 1 wt%Y-W-K
Average grain size (mm)
Relative density (%) Single pulse
Hundred pulse
97.78 97.43 97.01 98.25 98.60
e 97.36 96.94 98.38 98.21
6.01 5.49 5.67 3.82 2.95
± ± ± ± ±
1.41 1.24 1.73 0.74 0.55
Average Vickers hardness (HV)
401.32 413.88 408.87 475.97 505.24
± ± ± ± ±
12.78 15.62 14.53 11.14 13.62
Fig. 4. Surface morphologies of (a-e) AKS-W with different yttrium doping percentage of 0, 0.05 wt%, 0.1 wt%, 0.5 wt%, and 1 wt%.
judging from optical micrographs, i.e., sintering void, dispersion phase particles and potassium bubbles. SEM micrographs and EDX spectra characterization are applied to differentiate these black spots in this work. The morphology of sintering void, yttrium particles, potassium bubbles, and relative EDX spectra were shown in Fig. 6. Comparing Fig. 5a with Fig. 5b, the size of sintering void is slightly bigger than that of the yttrium particle. The biggest observed sintering void is up to 1 mm large, while the yttrium particles is less than 1 mm. The second phase particle is yttrium or yttrium oxide dispersion phase. The potassium bubbles are much smaller than sintering void and
dispersion phase particles which are regular hexagon with a diameter 50e100 nm and mainly located in the grain, which is close to the previous report [8]. The distributions of yttrium and potassium are clearly shown on the fraction section of the AKS-W bulk samples with 1 wt% yttrium doping. Potassium bubbles disperse irregularly in grains and on grain boundaries. The bubbles located on grain boundaries are much bigger than those in grains. The EDX spectra in Fig. 6a suggested that these little bubbles are actually potassium bubbles. EDX spectra in Fig. 6b implied that the second phase particle is yttrium or its oxide. Our other work demonstrated that the second phase
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Fig. 5. SEM micrographs of (a) sintering void (b) yttrium particle (c) potassium bubbles on AKS-W bulk samples with 0.05 wt% yttrium doping.
may have different performance with yttrium oxide in improving the mechanical properties of tungsten. The pure yttrium core has higher toughness than yttrium oxide and thus brings less brittleness to the material. And the hard yttrium oxide shell plays the same role in pinning dislocation with yttrium oxide. Thermal shock resistance is an important factor to assess the property of PFMs. Pintsuk et al. investigated fully recrystallized WK in transient heat load tests at 0.32 GW/m2 (the value of absorption coefficient in their work is 0.46) for 100 shots, 1000 shots and 10,000 shots at room temperature [21]. These samples did not display obvious damage until 1000 shots. The value of cracking threshold was estimated around 0.19e0.38 GW/m2 from their work (the value was calculated by an absorption coefficient of 0.55 instead of 0.46), and it is similar to the one of pure tungsten [21]. Actually, previous work has proved that traditional vertical sintering W-K does not improve the cracking threshold of tungsten [8]. The transient heat load tests under the energy density of 0.37 GW/ m2 for 5 ms single shot were carried out to study the plasma disruption resistance of W-K-Y. In Fig. 7a the 82 ppm W-K sample showed great thermal shock resistance. The positive improvement of nano-size potassium bubbles on SPS-sintering tungsten had been study in our previous work [8]. The surface morphologies of W-K-Y bulk samples after thermal shock was shown in Fig. 7bee and the inserted SEM images are the full view of the test area. Comparing the surface morphology of these W-K-Y samples, an obvious major crack formed on the surface of yttrium 1% doped sample while on others no crack was found. At the high magnification morphology, grain boundaries can be observed on each surface of these samples, probably induced by the annealing effect of the transient heat load. Local ruptured grain boundaries could be observed on the surface of 0.1 wt% Y-W-K sample. The rupture of the grain boundaries attributed to thermal stresses induced by different thermal expansion which caused by high temperature gradient. When the thermal stresses exceed the yield strength of the material, the grain boundaries first began to rupture as they were the weakest part of the heated surface [6]. The 0.1% Y-W-K samples have lower relative density than other samples and their grain size is not refined comparing with 0.05% Y-W-K samples. It is probably due to the poor reproducibility of SPS sintering process and thus induces poor yield strength. That can explain why grain boundary rupture appeared on the surface of 0.1 wt% Y-W-K sample while others did not. For the sample with 1 wt% yttrium, the formation of major crack can mainly be attributed to induced brit-
Fig. 6. Fraction section of AKS-W bulk samples with 1 wt% yttrium doping and (a) EDX spectra of potassium bubble, (b) EDX spectra of yttrium particle of the selected area.
particles are yttrium particles covered in thin oxide layer, which will be reported later. The core-shell structure of yttrium particles
tleness due to an excess amount of the doping yttrium [16]. During one pulse, the surface temperature normally rise above DBTT and
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Fig. 7. SEM micrographs of (a-e) the surface morphology of yttrium doping AKS-W bulk samples with 0, 0.05 wt%, 0.1 wt%, 0.5 wt% and 1 wt% yttrium doping percentage after single shot transient heat load tests.
relieve thermal stresses by plastic deformation [14]. But for brittle material like 1 wt% Y-W-K, the stress relief was not possible due to poor plasticity and major crack formed. In addition, multiple (100) shots transient heat load tests under the energy density of 0.37 GW/m2 and 1 ms for each pulse were performed. The surface morphology of yttrium doping AKS-W samples are shown in Fig. 8(a-d). The inserted SEM images are the full view of the test area. A major crack formed at the loading area of the 0.5 wt% W-K-Y sample but the 1.wt% yttrium doping one did not. An anomalous particle (almost 60 mm in size) can be observed at the middle of the major crack for the sample 0.5 wt% W-K-Y (Fig. 8e). The EDX spectrum (Fig. 8g) suggests the huge particle is yttrium doping particle. The anomalous size of this particle was attributed to the unobservable ball milling process. Some starting yttrium powder stuck in the corner of the vessel and has not been crushed into small size during the whole process. Considering the different thermal expansion coefficient between the huge yttrium doping particle and tungsten, as well as the brittleness of yttrium doping particles, the reason why a major crack formed become clear. In addition, the annealing effect of 0.5 wt% W-K-Y sample could be easily observed in Fig. 8f, suggests that plastic deformation did happened at the heated area, which is further confirmed by 3D topography in Fig. 9 below. The surface roughness of the W-K-Y samples after 100 pulses transient heat load tests are shown in Fig. 9. The surface roughness
deceased with the increase of yttrium doping percentage, which indicates that the plastic deformation became weaker with the rising yttrium content. It is a predictable result of increasing yield strength by refining the grain size. The results of transient heat load under 0.37 GW/m2 for a single shot suggested that SPS sintered 82 ppm potassium doped and moderate (<0.5 wt%) yttrium doped AKS tungsten performs better thermal shock resistance than traditional commercial W-K [8], owing to the improved mechanical properties and ductility. It is implied that high yttrium doping (1 wt%) brings negative effect on the thermal shock resistance behavior. It was reported that potassium doping lowers the DBTT and increase the yield strength [8,22], and yttrium (mainly its oxide) doping brings high temperature strength and brittleness [16,23]. In this work, the core-shell-like yttrium doping plays the similar role with yttrium oxide. For transient heat load under 0.37 GW/m2 for 100 shots, the 0.5 wt% WK-Y sample cracked due to an abnormal huge yttrium particle. Comparing the surface roughness of these W-K-Y samples in multiple shots test with previous work, the plastic deformation effect are much better, indicates that SPS sintering W-K-Y alloy performs better than traditional commercial W-K [21]. Both potassium doping and yttrium doping could effectively improve the mechanical properties, but has opposite effect on the ductility of tungsten. One of the efforts in this work is to find the best yttrium doping percentage to balance the strength and
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Fig. 8. SEM micrographs of (a-d) the surface morphology of yttrium doping AKS-W bulk samples with 0.05 wt%, 0.1 wt%, 0.5 wt% and 1 wt% yttrium doping percentage after 100 pulses transient heat load tests, (e) morphology of anomalous particle for 0.5 wt% Y doping, (f) non-cracked area of 0.5 wt% W-K-Y and (g) EDX spectra of selected area.
ductility of tungsten. The results of transient heat load tests, which mainly reflect on the strength and ductility of material, is one of the most direct ways to determine the best yttrium doping percentage. Present transient heat load tests at the energy density of 0.37 GW/ m2 for a single shot 100 shots suggests that appropriate yttrium doping rate (less than 0.5 wt%) perform better thermal shock resistant than traditional commercial W-K and pure tungsten. In order to find the best binary doping content, higher heat flux transient heat load experiments will be studied to obtain the optimal yttrium content for W-K, and the micro structure optimization also need to be modulated to further improve thermal shock resistance in the future.
4. Conclusion Novel tungsten-based W-K-Y ingots were sintered by sparkplasma-sintering (SPS) method. High relative density (up to 98.60%) AKS-W bulk samples with 0.05 wt%, 0.1 wt%, 0.5 wt%, 1 wt% yttrium doping were sintered obtained. Nano size (50e100 nm) potassium bubbles are formed during this process and the size of the yttrium particles are about 500 nm. The Vickers-hardness enhances with the increasing of doping percentage as the result of refinement of grain size and the highest one reaches 505.24 HV. The samples with 0.05 wt%, 0.1 wt% and 0.5 wt% yttrium doping improved thermal shock resistance compared with traditional commercial W-K and pure tungsten. This provides a promising way to improve the properties of tungsten-based PFMs and more
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Fig. 9. (a-d) 3D topography and surface roughness of yttrium doping AKS-W bulk samples with 0.05 wt%, 0.1 wt%, 0.5 wt% and 1 wt% yttrium doping percentage after 100 pulses transient heat load tests.
detailed investigations are needed to this doping system. Acknowledgements This work was supported by the International Thermonuclear Experimental Reactor (ITER) Program Special (No. 2011GB108005), the National Natural Science Foundation of China (No. 11475118), and the National Fund of China for Fostering Talents in Basic Science (J1210004). References [1] H. Bolt, V. Barabash, W. Krauss, J. Linke, R. Neu, S. Suzuki, N. Yoshida, A.U. Team, Materials for the plasma-facing components of fusion reactors, J. Nucl. Mater 329e333 (Part A) (2004) 66e73. [2] J. Linke, Plasma facing materials and components for future fusion devicesddevelopment, characterization and performance under fusion specific loading conditions, Phys. Scr. T123 (2006) 45e53. [3] R. Liu, Y. Zhou, T. Hao, T. Zhang, X.P. Wang, C.S. Liu, Q.F. Fang, Microwave synthesis and properties of fine-grained oxides dispersion strengthened tungsten, J. Nucl. Mater 424 (2012) 171e175. [4] M. Rieth, S.L. Dudarev, S.M. Gonzalez de Vicente, J. Aktaa, et al., Recent progress in research on tungsten materials for nuclear fusion applications in Europe, J. Nucl. Mater 432 (2013) 482e500. € dig, L. Singheiser, Performance of plasma-facing ma[5] J. Linke, T. Hirai, M. Ro terials under intense thermal loads in tokamaks and stellarators, Fusion Sci. Technol. 46 (2004) 142e151. [6] A. Suslova, O. El-Atwani, S.S. Harilal, A. Hassanein, Material ejection and surface morphology changes during transient heat loading of tungsten as plasma-facing component in fusion devices, Nucl. Fusion 55 (2015) 033007. [7] P. Schade, 100years of doped tungsten wire, Int. J. Refract. Metals Hard Mater. 28 (2010) 648e660. [8] B. Huang, Y. Xiao, B. He, J. Yang, J. Liao, Y. Yang, N. Liu, Y. Lian, X. Liu, J. Tang, Effect of potassium doping on the thermal shock behavior of tungsten, Int. J. Refract. Metals Hard Mater. 51 (2015) 19e24. [9] M. Faleschini, H. Kreuzer, D. Kiener, R. Pippan, Fracture toughness investigations of tungsten alloys and SPD tungsten alloys, J. Nucl. Mater 367e370 (Part A) (2007) 800e805.
[10] O. Horacsek, L. Bartha, Development of the bubble structure from selectively deforming potassium-pores in doped tungsten wires, Int. J. Refract. Metals Hard Mater. 22 (2004) 9e15. [11] F.C. Sze, R.P. Doerner, S. Luckhardt, Investigation of plasma exposed We1% La2O3 tungsten in a high ion flux, low ion energy, low carbon impurity plasma environment for the International Thermonuclear Experimental Reactor, J. Nucl. Mater 264 (1999) 89e98. [12] X. Zhang, Q. Yan, Morphology evolution of La2O3 and crack characteristic in WeLa2O3 alloy under transient heat loading, J. Nucl. Mater 451 (2014) 283e291. €ublin, P. Sp€ [13] M. Battabyal, R. Scha atig, N. Baluc, We2wt%Y2O3 composite: microstructure and mechanical properties, Mater. Sci. Eng. A 538 (2012) 53e57. [14] N. Lemahieu, J. Linke, G. Pintsuk, G. Van Oost, M. Wirtz, Z. Zhou, Performance of yttrium doped tungsten under ‘edge localized mode’-like loading conditions, Phys. Scr. T159 (2014) 014035. [15] X.-S. Kong, Y.-W. You, Q.F. Fang, C.S. Liu, J.-L. Chen, G.N. Luo, B.C. Pan, Z. Wang, The role of impurity oxygen in hydrogen bubble nucleation in tungsten, J. Nucl. Mater 433 (2013) 357e363. €ublin, T. Plocinski, M. Walter, N. Baluc, Processing and [16] L. Veleva, R. Scha characterization of a We2Y material for fusion power reactors, Fusion Eng. Des. 86 (2011) 2450e2453. [17] O. Horacsek, L. Bartha, Influence of surface particles of AKS-doped TBO on the NS-structure of tungsten wires, Int. J. Refract. Metals Hard Mater. 20 (2002) 271e276. [18] D. Zhang, Processing of advanced materials using high-energy mechanical milling, Prog. Mater. Sci. 49 (2004) 537e560. [19] X. Shu, H. Qiu, B. Huang, Z. Gu, J. Yang, J. Liao, Y. Yang, N. Liu, J. Tang, Preparation and characterization of potassium doped tungsten, J. Nucl. Mater 440 (2013) 414e419. [20] B. Huang, B. He, Y. Xiao, R. Ang, J. Yang, J. Liao, Y. Yang, N. Liu, D. Pan, J. Tang, Microstructure and bubble formation of AleKeSi doped tungsten prepared by spark plasma sintering, Int. J. Refract. Metals Hard Mater. 54 (2016) 335e341. [21] G. Pintsuk, I. Uytdenhouwen, Thermo-mechanical and thermal shock characterization of potassium doped tungsten, Int. J. Refract. Metals Hard Mater. 28 (2010) 661e668. [22] K. Sasaki, K. Yabuuchi, S. Nogami, A. Hasegawa, Effects of temperature and strain rate on the tensile properties of potassium-doped tungsten, J. Nucl. Mater 461 (2015) 357e364. [23] L. Veleva, Z. Oksiuta, U. Vogt, N. Baluc, Sintering and characterization of WeY and WeY2O3 materials, Fusion Eng. Des. 84 (2009) 1920e1924.