Synthesis of nanosized composite powders via a wet chemical process for sintering high performance W-Y2O3 alloy

Accepted Manuscript Synthesis of nanosized composite powders via a wet chemical process for sintering high performance W-Y2O3 alloy

Zhi Dong, Nan Liu, Zongqing Ma, Chenxi Liu, Qianying Guo, Yusuke Yamauchi, Hatem R. Alamri, Zeid A. Alothman, Md. Shahriar A. Hossain, Yongchang Liu PII: DOI: Reference:

S0263-4368(17)30478-X doi: 10.1016/j.ijrmhm.2017.09.001 RMHM 4510

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date: Revised date: Accepted date:

19 July 2017 3 September 2017 3 September 2017

Please cite this article as: Zhi Dong, Nan Liu, Zongqing Ma, Chenxi Liu, Qianying Guo, Yusuke Yamauchi, Hatem R. Alamri, Zeid A. Alothman, Md. Shahriar A. Hossain, Yongchang Liu , Synthesis of nanosized composite powders via a wet chemical process for sintering high performance W-Y2O3 alloy, International Journal of Refractory Metals and Hard Materials (2017), doi: 10.1016/j.ijrmhm.2017.09.001

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ACCEPTED MANUSCRIPT Synthesis of nanosized composite powders via a wet chemical process for sintering high performance W-Y2O3 alloy

Zhi Dong,1 Nan Liu,1 Zongqing Ma1*, Chenxi Liu,1 Qianying Guo,1 Yusuke Yamauchi,2,3 Hatem R. Alamri,4 Zeid A. Alothman,5 Md. Shahriar A. Hossain,2,3 and Yongchang Liu1

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Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Australian Institute for Innovative Materials (AIIM), University of Wollongong, North Wollongong, NSW 2500, Australia Physics Department, Jamoum University College, Umm Al-Qura University, Makkah, 21955, Saudi Arabia Advanced Materials Research Chair, Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

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ACCEPTED MANUSCRIPT Abstract: With the aim of preparing high performance oxide dispersion strengthened tungsten-based alloys by powder metallurgy, the W-Y2O3 composite nanopowders were prepared by an improved bottom-up wet chemical method. Ultrasonic treatment and anionic surfactant sodium dodecyl sulfate (SDS) addition were innovatively introduced into this wet chemical method in order to fabricate homogeneous, ultrafine W-Y2O3 composite nanopowders. As a result, the average tungsten grain size of 40~50 nm was obtained for this

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composite nanopowders. For comparison, W-Y2O3 composite powders were also prepared by traditional

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mechanical milling. After that, spark plasma sintering (SPS) was employed to consolidate the powders

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prepared by either mechanical milling or wet chemical method to yield high density as well as suppress grain growth. It is found that the W-Y2O3 alloy prepared by wet chemical method and subsequent SPS

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possesses smaller grain size (0.76±0.17μm) and higher relative density (99.0%) than that prepared by

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mechanical milling and subsequent SPS. Moreover, the oxide nanoparticles (about 2-10 nm) are dispersed within tungsten grains and at grain boundaries more uniformly in W-Y2O3 alloy prepared by wet chemical

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method and subsequent SPS. Due to the ultrafine grains, high sintering density and homogeneously

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distributed oxide nanoparticles, the Vickers microhardness of yttria dispersion strengthened tungsten-based alloy prepared in our work reaches up to 598.7±7.3 HV0.2, higher than that reported in the previous studies.

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These results indicate that the improved bottom-up wet chemical method combined with ultrasonic

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treatment and anionic surfactant addition developed in our work is a promising way to fabricate high performance oxide dispersion strengthened tungsten-based alloys with ultrafine grain and high density.

Keywords: W-Y2O3 alloy; wet chemical method; SPS; ultrafine grain

ACCEPTED MANUSCRIPT 1. Introduction With high melting point, low sputtering rate, high thermal conductivity, low tritium inventory, high strength at elevated temperatures and low thermal expansion, tungsten-based materials have been widely applied in many fields, such as aerospace industry [1], engineering [2] and nuclear industry [3]. However, a lot of improvements need to be done for tungsten-based materials in order to achieve higher performance to

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meet the application requirement. Among them, how to get high-density sintered products is always the

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main obstacle [4, 5]. Therefore, to enhance the density, many measures have been taken to improve the

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sintering activity of these materials. At first, trace amounts of activators such as Cu [6, 7], Fe and Ni [8] were added in tungsten matrix to increase the density during sintering at low temperature. Besides,

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mechanical alloying that introduce many defects such as vacancies and dislocations and enhance special

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surface area is also an effective way to get high-density sintered alloys [9]. Also, ultrafine grains with great surface energy can achieve rapid densification and high sintering density at lower temperature [10]. But it is

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worth noting that all the techniques mentioned above for increasing density easily lead to the abnormal

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growth of tungsten grains during sintering. It is well accepted that tungsten has a recrystallization temperature ranging from 1150 ˚C to 1350 ˚C that depends on deformation history and chemical purity.

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Because the operation temperature in plasma-facing materials during operation of future nuclear fusion

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reactors usually exceeds 1350 ˚C, tungsten material can recrystallize during the duty operation, which results in grain growth and the formation of coarse equiaxed grains [11]. Coarse grains resulting from recrystallization not only weaken the high-temperature strength but also increase the low-temperature embrittlement of tungsten-based materials. At the same time, recrystallization also can cause degradation of thermal shock resistance [12]. Besides, it has been confirmed that coarse-grained tungsten will produce more defects under the same dose of neutron irradiation and weaken the irradiation resistance [13]. Accordingly, to obtain tungsten-based alloys with excellent performance, ultrafine grains and high density should be achieved simultaneously.

ACCEPTED MANUSCRIPT In recent years, earth rare oxides or carbides dispersion strengthened tungsten-based alloys have been confirmed to obtain excellent performance, including W-HfC [14], W-TiC [15, 16] and especially W-Y2O3 alloys [17-19]. Some previous works have indicated that Y2O3 is the best oxide dispersion phase to obtain high-density tungsten-based alloys with ultrafine grains [18]. Y2O3 dispersively distributed in tungsten matrix can effectively inhibit the recovery and recrystallization at high temperature. Moreover, yttria

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dispersion strengthened tungsten-based alloys possess lower ductile-brittle transition temperature (DBTT),

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improving the low-temperature brittleness [11, 12]. Till now, the sintering precursors, W-Y2O3 composite

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powders, are generally prepared by traditional mechanical alloying, but the introduction of impurities and the inhomogeneous distribution of oxide dispersion particles, especially the segregation of oxide particles at

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the grain boundaries are great challenges for this technique. In addition, spark plasma sintering (SPS) is

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frequently used to prepare tungsten-based alloy owing to its remarkable advantages such as low sintering temperature, fast heating rate, short sintering time. Nevertheless, some technical difficulties to scale up the

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production of tungsten-based materials always exist. Above all, the inhomogeneous temperature distribution

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restricts the application of SPS, especially for the production of large-sized materials. Secondly, the capacity of impulse current should be expanded to meet the need of large-sized products preparation. Besides, it is

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urgent to develop dies with higher strength and repetition usage rate with the aim of reducing costs [20].

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In our previous work, polyvinylpyrrolidone (PVP, K17) was utilized to synthesize the Y2O3 doped W composite powders. As a result, the average W grain size was 10.7 nm. Especially, the high performance ultrafine grain W-Y2O3 alloy can be prepared by SPS with this kind of composite powders, which makes this work meaningful [21]. Based on these backgrounds, in the present work, an improved wet chemical method where ultrasonic treatment and anionic surfactant SDS addition were employed was developed with the aim of preparing homogeneous and ultrafine W-Y2O3 composite nanopowders. For comparison, W-Y2O3 composite powders were also prepared by mechanical milling. After that, SPS was employed to prepare Y2O3 dispersion strengthened tungsten-based alloys. The sintered W-Y2O3 alloy made of nanopowders

ACCEPTED MANUSCRIPT prepared by wet chemical method possesses smaller grain size and higher relative density compared to the sintered W-Y2O3 alloy made of powders prepared by mechanical milling. Moreover, the oxide particles are dispersed within tungsten grains (about 2-10 nm) and at tungsten grain boundaries more uniformly in this

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alloy and its Vickers microhardness is up to 598.7±7.3 HV0.2.

ACCEPTED MANUSCRIPT 2. Experimental procedure Preparation of powders. Y2O3-doped W nanopowders with Y2O3 content corresponding to 5 wt% (denoted by 5%-C) were synthesized through a wet chemical process from ammonium paratungstate hydrate (H42N10O42W12·4H2O) and yttrium nitrate hydrate (YN3O9·6H2O). At first, 2 g of anionic surfactant SDS and 2.57 g of yttrium nitrate hydrate were dissolved in 120 ml of water. Then 20 g of ammonium

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paratungstate hydrate was added into the transparent solution. After mechanical stirring and ultrasonic

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treatment for 10 min, 20 ml of nitric acid (65%-68%) was added into the solution and left to react for 30 min.

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After that, 140 ml of ethanol was added into the formed suspension. After further reaction for 3 h, the suspension was filtered. Mechanical stirring and ultrasonic treatment were carried out throughout the whole

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process. The collected precipitate was washed with ethanol for four times and left to dry in vacuum drying

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oven at 60 ˚C for 24 h.

Using a temperature profile including three isothermal steps (450 ˚C for 1 h, 600 ˚C for 3 h, and 800 ˚C

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for 6 h), thermal processing (calcination and reduction) of the composite powders was carried out in a tube

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furnace. The ramping rate was 5 ˚C·min-1. Argon gas was passed through the furnace from room temperature to the end of isothermal stage at 450 ˚C, followed by reduction with hydrogen gas. Then the composite

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powders were left in furnace to cool down to room temperature under hydrogen atmosphere.

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For comparison, commercial pure W (99.9% purity, 0.6-1 μm in size) and Y2O3 (99.99% purity, 20-50 nm in size) powders were used as starting materials for mechanical milling. The mechanical milling experiment was conducted in a QM-3SP2 planetary ball mill in an argon atmosphere at a speed of 400 rpm. With a ball to powder ratio of 16:1, the stainless steel jar and tungsten carbide grinding balls were used. The mechanical milling lasted for 20 h, during which ethanol was added to inhibit agglomeration of powders. The Y2O3 content was 5 wt% (denoted by 5%-B). Sintering the powders. A SPS machine “Dr. Sinter 1050” (Sumitomo coal mining company) was used to sinter the prepared powders under vacuum condition. 10 g of powders were firstly pressed in a graphite die

ACCEPTED MANUSCRIPT (12 mm in inner diameter) by manual. Then the die was placed into the SPS chamber. The temperature was firstly ramped to 600˚C and held for 2 min. Then the temperature was soared to 1600 ˚C and held for 2 min under a pressure of 50 MPa. When the temperature was lower than 1500 ˚C, the heating rate was 100 ˚C·min-1. When the temperature was higher than 1500 ˚C, the rate was changed to 50 ˚C·min-1. Characterizations. The microstructure of powders and sintered alloys were examined by X-ray diffraction

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(XRD, D/MAX-2500) with Cu Kα radiation, scanning electron microscopy (SEM, Hitachi Model No. S

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4800) and transmission electron microscopy (TEM, JEM-2100) equipped with EDX detector, respectively.

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The density of the sintered alloys were measured by Archimedes method (in water at room temperature). The Vickers microhardnesses of the sintered alloys were tested on the polished surfaces under 200 gf load

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and a dwell time of 20 s at room temperature. For each sintered alloy, 20 individual microhardness tests

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ACCEPTED MANUSCRIPT 3. Results and discussion Fig. 1 shows the wide-angle XRD patterns of as-prepared powders from the chemical solution and the corresponding thermally treated W-Y2O3 composite powders. As for the as-prepared powders, an amorphous phase is observed. On the other hand, with tungsten as the dominant phase, a small peak corresponding to Y2O3 is observed in the XRD pattern after the thermal treatment. The average crystallite size is calculated to

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be 39.6 nm using the Scherrer’s equation for the thermally treated W-Y2O3 composite powders. Fig. 2a

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presents the SEM image of the reduced W-Y2O3 composite powders. The grain size distribution is quite

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narrow, and the grain size is almost consistent with the result calculated from XRD data using Scherrer’s equation. Fig. 2b shows the TEM image to further characterize the detailed microstructure. It can be seen

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that the composite powders with uniform grain size is obtained, which is in agreement with the result

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observed in SEM image (Fig. 2a). Surface modification induced by SDS inhibits the grain growth significantly. Moreover, combined with ultrasonic treatment, the SDS adsorbed on the surface of grains can

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prevent them from aggregation. Therefore, ultrafine tungsten grains with uniform size distribution can be

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obtained using this novel route.

Fig. 3 shows the XRD patterns of W-Y2O3 composite powders prepared by mechanical milling. It is

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observed that the full width of half maximum (FWHM) of the peaks corresponding to W become broader

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and intensity of these peaks decrease after mechanical milling for 20 h, which results from the refinement of W grains. In addition, the lattice strain arising from lattice defects and distortion after mechanical milling is clearly confirmed. On the other hand, the peak of Y2O3 disappears after mechanical milling, which suggests that Y2O3 may exist in small crystallites or amorphous state. Besides, Y2O3 is possibly introduced into tungsten crystal lattice by solid-solution during mechanical milling because it is found that the lattice parameter of W expands from 0.31589 nm to 0.31634 nm. Fig. 4a shows the SEM image of W-Y2O3 composite powders prepared by mechanical milling. It is found that the particles are composed of nanosized lamellar crumbs and microsized lamellar agglomerates

ACCEPTED MANUSCRIPT which are formed due to high surface energy of fine tungsten grains after mechanical milling (Fig. 4b-c). As seen from the TEM image, cracks, nanosized grains, and a plenty of grain boundaries and dislocations are also observed in the particles (Fig. 4c). Fig. 4d shows the HRTEM image of milled W-Y2O3 composite powders. It can be seen from the image that most of Y2O3 particles (circled by white solid lines) with irregular shape have a size of about 5 nm, which is attributed to plastic deformation during mechanical

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milling. However, no amorphous Y2O3 particles are found. The same size evolution of Y2O3 particles after

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mechanical milling is also observed [22].

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During mechanical milling, the plastic deformation and fracture effect caused by friction, shock and impact from balls and jar will give rise to the lamellar morphology, refinement of grains and formation of

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defects [23]. In addition, the increase of stress and defect density will also bring about microcracks and grain

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refinement [9]. Using the Scherrer’s equation, it is found that the average crystallite size of milled W-Y2O3 composite powders is 59.3 nm, which is much larger than that of the powders prepared by wet chemical

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method. Besides, grain size inhomogeneity of the former is also more significant than the latter, as seen from

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the SEM images (see Fig. 2a and Fig. 4a). Based on all the results discussed above, we can conclude that the W-Y2O3 composite powders prepared by wet chemical method is much more homogeneous and refined

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than those prepared by mechanical milling.

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The SEM images of polished surfaces of sintered alloys are present in Fig. 5. It can be clearly seen that the grain size of W-Y2O3 alloy prepared by mechanical milling and subsequent SPS (Fig. 5a) is approximate ten times larger than that of the alloy prepared by wet chemical method and subsequent SPS (Fig. 5b), and their average grain size are 7.08±1.63 μm and 0.76±0.17 μm, respectively. The driving forces for W grain boundary migration during sintering of the W-Y2O3 composite powders stem from two souces; (i) internal structure, i.e., grain size of W and Y2O3, uniformity of W grain size, distribution of Y2O3 grains, internal stress and dislocation density; and (ii) externally applied stress [24]. Compared with the powders prepared by wet chemical method, the larger W grain size in powders prepared by mechanical alloying is one of the

ACCEPTED MANUSCRIPT factors that lead to discrepant grain size in the two alloys after SPS. Besides, the uniformity of W grain size in the powders prepared by wet chemical method is greater, which can alleviate the abnormal grain growth during SPS according to Ostwald ripening mechanism. In addition, for the same Y2O3 volume fraction, the smaller Y2O3 particle size in the powders prepared by wet chemical method is more effective at pinning W grain boundary migration, comparing with the powders prepared mechanical alloying [25]. Moreover, the

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dissipating degree of Y2O3 particles in the former is greater, which causes a higher contacting probability of

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Y2O3 particles with grain boundary, leading to the moving grain boundary is frequently impeded by the

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smaller Y2O3 particles [26]. Finally, high internal stresses induced by mechanical alloying, which can be confirmed by microstrain, microcracks and dislocations (as shown in Fig. 4b-c) to some degree, also can

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greatly promote grain growth for the corresponding powders.

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Because the same sintering pressure was applied for both W-Y2O3 powders, externally stress cannot cause the difference in grain size of sintered alloys. The smaller grain size (700 nm) was obtained by Liu et

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al. [27] for the W-1wt%Y2O3 alloy sintered at 1500 ˚C. Besides, the average grain size of the W-1wt%Y2O3

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alloy prepared by Yar et al. [17] was 650 nm. However, the relative density of the two samples was only 93.8 % and 86 %, respectively, much lower than that of our sample (99%, Table 1). When the similar

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relative density (99 %) was obtained by Kim et al. [18], the grains have grown up severely to 3.7 μm. Thus,

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with nearly full densification, the grain size of W-Y2O3 alloy prepared by wet chemical method and subsequent SPS in our work is much smaller than that reported in these previous studies. Observing more carefully, one can see that many white particles (marked by white arrows) distribute at the W grain boundaries as well as within the W grains in the SEM image of the alloy prepared by mechanical milling and subsequent SPS (Fig. 5a). According to EDX result of the selected particle region (Fig. 5c), small amounts of W element is detected, which indicates W and Y2O3 are soluble to some degree during sintering. However, there is no evidence to support the formation of new W-Y-O phase, which is in accordance with the result observed by Kim et al.

ACCEPTED MANUSCRIPT [18]. However, Kuribayashi et al. observed that W preferentially diffuses to Y2O3 side in the reaction of the pellets of both Y2O3 and WO3 at 1000 ˚C for 72 h, resulting in the formation of new phase including Y6WO12 and Y2WO6. Therefore, in our opinions, W can be incorporated in Y2O3 lattice during sintering, but new W-Y-O phase formation depends on whether the amount of W atoms diffusing into Y2O3 exceed the minimum reaction requirements. For example, Y6WO12 phase formation needs the minimum W content

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comparing with other W-Y-O phases [29]. In terms of SPS, sintering time of only a few minutes is too short

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to diffuse sufficient W into submicron Y2O3, therefore no new phase is generated. Noteworthily, with a size

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ranging from nano to submicron, the homogeneity of size distribution for these oxide particles needs to be improved in this alloy prepared by mechanical milling and subsequent SPS. On the contrary, the oxide

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particles are difficult to be distinguished in the SEM image of W-Y2O3 alloy prepared by wet chemical

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method and subsequent SPS (Fig. 5b), probably because the size of dispersed oxide particles is too small to be detected under low resolution. To further characterize the distribution of oxide particles in the alloy

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prepared by wet chemical method and subsequent SPS, the high-resolution TEM was employed to observe

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the detailed microstructure and the result is shown in Fig. 6. It is obvious from Fig. 6a that many black dots (marked by white arrows) are uniformly dispersed in the tungsten grains while bright and white particle with

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size of approximate 300 nm exists at grain boundary. As seen from the high magnification TEM image (Fig.

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6b), near spherical Y6WO12 particles with a size of 2-10 nm are homogeneously embedded in tungsten grains, which can be further confirmed by the HRTEM image of these particles within W grain (Fig. 6d). Moreover, the HRTEM image of the black square region in Fig. 6a is shown in Fig. 6c and the bright and white particle distributed at W grain boundaries is found to be Y2O3 particle. It is well known that SDS is anionic surfactant and it can disperse Y ions during the process of chemical reaction [30]. However, the oxide particles prepared by mechanical milling distribute inhomogeneously and tend to agglomerate at grain boundaries to some extent. Therefore, under the combined action of ultrasonic treatment and SDS addition, Y2O3 particles in powders prepared by wet chemical method can become much more refined and also more

ACCEPTED MANUSCRIPT homogeneous dispersion than that prepared by mechanical milling. During subsequent SPS, W grain boundary can pass across these nanosized Y2O3 particles and then form intragranular structure [31]. Then W atoms in W matrix diffuse into these nanosized oxides and its amount can reach the minimum reaction requirements easily for these nanosized Y2O3 particles, triggering reaction of W and Y2O3. However, the amount of W atoms diffusing into the submicron oxide particles distributed at grain boundary is limited in

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the larger space, as mentioned above. Therefore, the formation of W-Y-O phase depends on Y2O3 particle

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size and sintering process. Isolated by tungsten matrix, these intragranular oxide particles can hardly

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aggregate each other, thus keeping original size. These oxide dispersoids within the tungsten grains can effectively generate, pin down and accumulate dislocations and thus could lead to the enhanced mechanical

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properties. Meanwhile, partial Y2O3 particles distributed at grain boundary will coarsen such that they have a

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relatively large size because of the aggregation and coarsening effects during SPS [32]. According to previous study, Y2O3 located at W grain boundary coarsen through Ostwald ripening mechanism, i.e.,

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long-range diffusion [33]. Due to the more significant interface energy increase when grain boundary move

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away from these large Y2O3 particles, W grain boundary can hardly pass across these large Y2O3 particles, making them still stay at W grain boundary. During loading, these Y2O3 particles at W grain boundaries are

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preferential sites for crack initiation. Fortunately, compared with previous preparation methods, the

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improved wet chemical method developed in our work can make the more oxide homogeneously distribute within W grains after sintering (see Fig. 6b), mitigating the tendency to intergranular fracture.

The XRD patterns of the sintered alloys are shown in Fig. 7. As for the W-Y2O3 alloys prepared by wet chemical method and subsequent SPS, tungsten is confirmed as main phase and relatively weak peaks corresponding to Y2O3 are also observed. However, peaks of Y6WO12 is not obvious, probably due to its small amount and nanometer size. Meanwhile, some tungsten oxides such as W3O and WO1.09 are found besides Y2O3 in the sample prepared by mechanical milling and subsequent SPS. It is explained that more

ACCEPTED MANUSCRIPT oxygen can be introduced into the powders during mechanical milling process and finally results in the formation of additional tungsten oxides impurities in the subsequent SPS. As listed in Table 1, the relative density of W-Y2O3 alloy prepared by wet chemical method and subsequent SPS is 99.00%, which is higher than the one prepared by mechanical milling and subsequent SPS (89.07%). One possible reason is that the composite powders prepared by wet chemical method with

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smaller grain size and narrower size distribution have higher initial activity than that produced by

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mechanical milling. In the subsequent SPS process, such nanopowders can be easily consolidated under the

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same sintering condition. In addition, the mechanical milling process could introduce a lot of oxygen that promotes the melting and volatilization of Y-W-O phases at high temperature [34], leading to higher porosity

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in the sintered alloy. The Vickers microhardness of the W-Y2O3 alloy prepared by wet chemical method and

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subsequent SPS is measured to be 598.7±7.3 HV0.2, much higher than the one prepared by mechanical milling and subsequent SPS, as listed in Table 1. The higher microhardness originates from finer grains and

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higher relative density, as shown in Table 1. Besides, the oxide particles uniformly dispersed in tungsten

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grains also contribute to the increase of microhardness. It was reported that the microhardness was only 393±5 HV0.2 for the W-1.15wt%Y2O3 alloy produced by wet chemical method and sintering at 1600 ˚C for 3

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min [35]. For the W-1wt%Y2O3 alloy prepared by wet chemical method and sintering at 1100 ˚C, the

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microhardness reached up to 570±30 HV0.2, however, the relative density was only 86% [17]. Compared with these previous results, our sample prepared by wet chemical method and subsequent SPS possesses the best microhardness, which can be attributed to the ultrafine grains, high sintering density as well as homogeneously dispersed oxide nanoparticles, as discussed above. 4. Conclusions With the aim of obtaining high performance oxide dispersion strengthened tungsten-based alloys by powder metallurgy, the W-Y2O3 composite nanopowders were prepared by an improved bottom-up wet chemical method. Ultrasonic treatment and anionic surfactant SDS were innovatively introduced into this wet chemical method in order to fabricate homogeneous, ultrafine W-Y2O3 composite nanopowders. For

ACCEPTED MANUSCRIPT comparison, W-Y2O3 composite powders were also prepared by mechanical milling. It is found that the oxide particles are dispersed both within tungsten grains and at the tungsten grain boundaries homogeneously in the W-Y2O3 alloy prepared by wet chemical method and subsequent SPS. Meanwhile, with a size range from nano to submicron, the uniformity of distribution of the oxide particles and particle size need to be improved in the W-Y2O3 alloy prepared by mechanical milling and subsequent SPS. The W-Y2O3 alloy prepared by wet chemical method and subsequent SPS possesses the smaller grain size

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(0.76±0.17 μm) and the higher relative density (99.0%) than that prepared by mechanical milling and subsequent SPS. Moreover, its Vickers microhardness is up to 598.7±7.3 HV0.2. All these results suggest that

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the improved wet chemical method developed in our work is a promising way to fabricate high performance

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oxide dispersion strengthened tungsten-based alloys.

Acknowledgement The authors are grateful to the National Natural Science Foundation of China (Grant

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No.51574178) and China National Magnetic Confinement Fusion Energy Research Program (Grant No. 2015GB119001). Z.A.A is grateful to the Deanship of Scientific Research, King Saud University for funding

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through Vice Deanship of Scientific Research Chairs.

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[13] H. Kurishita, Y. Amano, S. Kobayashi, K. Nakai, H. Arakawa, Y. Hiraoka, T. Takida, K. Takebe, H. Matsui, Development of ultra-fine grained W-TiC and their mechanical properties for fusion applications, J. Nucl. Mater. 367 (2007) 1453-1457. [14] Y.K. Wang, S. Miao, Z.M. Xie, R. Liu, T. Zhang, Q.F. Fang, T. Hao, X.P. Wang, C.S. Liu, X. Liu, L.H. Cai, Thermal stability and mechanical properties of HfC dispersion strengthened W alloys as plasma-facing components in fusion devices, J. Nucl. Mater. 492 (2017) 260. [15] L.-M. Luo, X.-Y. Tan, H.-Y. Chen, G.-N. Luo, X.-Y. Zhu, J.-G. Cheng, Y.-C. Wu, Preparation and characteristics of W-1wt%TiC alloy via a novel chemical method and spark plasma sintering, Powder Technol. 273 (2015) 8-12. [16] M. Xia, Q. Yan, L. Xu, L. Zhu, H. Guo, C. Ge, Synthesis of TiC/W core-shell nanoparticles by precipitate-coating process, J. Nucl. Mater. 430 (2012) 216-220.

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ACCEPTED MANUSCRIPT Table and Figure Captions Table 1 Relative density, grain size and Vickers micro-hardness of the sintered W-Y2O3 alloys. Fig. 1

Wide-angle XRD patterns of powders prepared from wet chemical method and the corresponding the thermally treated W-Y2O3 composite powders.

Fig. 2

(a) SEM image of the W-Y2O3 composite powders prepared by wet chemical method, (b) corresponding TEM image of the W-Y2O3 composite powders. Wide-angle XRD patterns of W-Y2O3 composite powders prepared by mechanical milling for 0 h

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(a) SEM image of the W-Y2O3 composite powders prepared by mechanical milling for 20 h, (b, c)

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TEM images of the W-Y2O3 composite powders prepared by mechanical milling for 20 h with (b)

of the milled W-Y2O3 composite powders.

SEM images for the polished and etched surfaces of (a) sintered W-Y2O3 alloy prepared by

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Fig. 5

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microcracks and nano-grains and (c) grain boundaries and dislocations, (d) typical HRTEM image

mechanical milling and subsequent SPS, (b) sintered W-Y2O3 alloy prepared by wet chemical

(a) Low magnification TEM image of the W-Y2O3 alloy prepared by wet chemical method and

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subsequent SPS, (b) high magnification TEM image showing the oxide particles within W grain, (c)

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HRTEM image of black square region in Fig. 6a, (d) HRTEM image of oxide particles within W grain.

Wide-angle XRD patterns of the sintered W-Y2O3 alloys (wet chemical method and mechanical

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Fig. 7

milling method).

ACCEPTED MANUSCRIPT Table 1 Relative density (%)

Grain size (μm)

Microhardness (HV0.2)

5%-B+SPS

89.07

7.08±1.63

505.5±6.2

5%-C+SPS

99.00

0.76±0.17

598.7±7.3

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Samples

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Fig.1 (Single column use)

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Fig.2 (Single column use)

Fig. 3 (Single column use)

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Fig. 5 (Single column use)

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Fig. 6 (Double column use)

Fig. 7 (Single column use)

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights Average tungsten grain size of 40~50 nm was obtained for the composie powders;

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Small grain size of about 700 nm and high density was achieved for sintered alloys;

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Nano oxide particles were uniformly dispersed within tungsten matrix;

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The Vickers microhardness of sintered W-Y2O3 alloy is up to 600 HV0.2.

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