Microstructure evolutions of the W–TiC composite conducted by dual-effects from thermal shock and He-ion irradiation
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Considering that tungsten (W) materials served as the plasma-facing material in the fusion reactor would be exposed to edge-localized modes (ELMs)-like thermal shock loading accompanied with He-ion irradiation, the W–TiC composite produced with a wet-chemical method was conducted by the dual effects from the laser beam thermal shock first and He-ion irradiation later in this work. The microstructure changes of the W–TiC composite before and after two tests were characterized by scanning electron microscopy or transmission electron microscopy. After the laser beam thermal shock test, there was an obvious interface on the exposed surface of the W–TiC composite. Several main cracks and melting areas could be found nearby the interface and center, respectively. Furthermore, a mixture of tungsten oxide and TiC was easy to aggregate and form into circle areas surrounding the melting area. The thermal shock tested that W–TiC composite was then subjected to the He-ion irradiation. The typical features of fuzz structures could be detected on the surface of the W–TiC composite apart from the center of the melting area. Notably, several nano-sized He bubbles deeply distributed at grain boundaries in the melting area, owing to the grain boundary functioning as the free path for He diffusion.
KeywordsPlasma-facing materials Wet-chemical method W–TiC composite Thermal shock He-ion irradiation
Tungsten (W) is considered as one of the most promising candidates for plasma-facing materials (PFMs) because of its several decisive advantages of high thermal conductivity, high melting point, low tritium retention, and low coefficient of thermal expansion [1, 2, 3, 4]. However, there exist severe disadvantages of recrystallization embrittlement, irradiation embrittlement, and high ductile–brittle transition temperature (DBTT) for W severed as PFMs [5, 6, 7]. Currently, many researchers consider that the addition of the second phase such as TiC , ZrC , Y2O3 , and La2O3  could be an efficient way to improve the brittle behavior of W. The TiC with a high melting point and low thermal expansion coefficient could be used to improve the toughness of W and reduce the DBTT.
When the plasma operates normally, PFMs are subjected to the steady-state heat load, which can reach up to 20 MW m−2 (~ 10 s). However, PFMs are still subjected to the high-energy transient heat load because of the instability of plasma, such as vertical displacement events (VDEs, ~ 60 MJ m−2, ~ 300 ms) and edge-localized modes (ELMs, ~ 1 MJ m−2, < 0.5 ms) [12, 13]. Such a high-energy density can lead to recrystallization (for deformed W materials), surface melting, ablation, cracking, and other serious irreversible damages for PFMs , resulting in reducing its basic physical properties and shortening its service life [15, 16]. The particle irradiation from the plasma also causes some damages for PFMs. For example, He-ion irradiation can induce some surface damages in W materials, such as helium bubbles, filaments, and fuzz structures [17, 18].
W materials served as PFMs would be subjected to the dual actions from the transient thermal loading and He-ion radiation simultaneously. Actually, it is hard to find a facility that could conduct these two tests at the same time. Sinclair et al.  conducted the He-ion irradiation first and thermal shock test later on pure W to study the surface structure changes during the process of these two tests, and found that the fuzz density on the damaged surface decreased gradually and disappeared finally with the increasing energy density of thermal shock. In addition, melting and cracking could also be detected. In this work, we tried to figure out the surface change behaviors of the produced W–TiC composite exposed to the thermal shock first and then conducted by He-ion irradiation. The laser beam thermal shock test was designed to simulate the ELMs-like transient thermal shock loading. The He-ion irradiation experiment was to simulate the behavior of He ion on W materials during plasma operation. To further study the changes of microstructures of the irradiated W–TiC composite, the composite was cut by a focused ion beam (FIB).
The W–TiC composite powder was prepared by a wet-chemical method . An appropriate amount of ammonium paratungstate ([NH4]10H2W12O42·xH2O) was dissolved into deionized water, and nano-sized TiC particles were added into the solution. Then, oxalic acid (C2H2O4·2H2O) severed as a precipitator was dissolved into the mixture solution. Heating up to 165 °C, the W–TiC precursor could be obtained by continuously stirring and evaporating the mixture solution until water was completely evaporated. The precursor was ground and then heated in a tubular furnace in a hydrogen atmosphere to obtain the W–TiC composite powder. The heating temperature was raised to 200, 500 and 800 °C at a heating rate of 5 °C min−1 and dwelled for 30, 60, and 60 min, respectively. Finally, the W–TiC composite bulk with a relative density of 18.71 g cm−3 was consolidated by the spark plasma sintering technique. The shape of the W–TiC bulk was a disk with the diameter of 20 mm and the thickness of 2 mm.
The surface morphologies or the phase structures of the W–TiC specimens before and after the thermal shock test or He-ion irradiation were characterized by field-emission scanning electron microscope (FESEM, SU8020, Japan) and X-ray diffraction (XRD, D/MAX2500 V, Japan). To find out the influences during the thermal shock test or He-ion irradiation, the FESEM equipped with energy-dispersive X-ray spectrometry (EDS) was applied to confirm the composition changes of the W–TiC composite. To further investigate the microstructure changes of the W–TiC composite, the specimen was cut from the center of the melting area using the FIB (FEI Helios NanoLab 650 DualBeamTM field-emission electron microscope) technique, and the specimen was analyzed by transmission electron microscope (TEM, JEM-2100F, Japan) in detail.
3 Results and discussion
3.1 ELMs-like laser beam thermal shock test
To study the detailed information of the W–TiC composite after the thermal shock test, the magnified images of selected areas were performed and are shown in Fig. 2b, c. From Fig. 2b, a nearly circular grey block with a diameter of 25 μm is located at the center of the melting area, and according to the EDS mapping, as shown in the insert of Fig. 2b, the grey block is composed of W and O elements. The presence of O would be ascribed to the oxidation during the thermal shock test. The melting point of formed tungsten oxides is lower than that of W, indicating that the molten state is more likely to occur. Notably, the width of cracks becomes more and more narrow from the outside to inside, as shown in Fig. 2c, indicating that the cracks are originated from the outside of the melting area. This means that the formation of cracks is due to the induced tensile stress during the beam-off stage. In general, the addition of the TiC particles acted as the strengthening phase to strengthen grain boundaries, thereby hindering the formation and propagation of cracks . From Fig. 2c, the paths of crack are deflected at the location of the TiC particle, which would be an effective way to release the induced thermal stress. In addition, some grey or dark areas can be found on the surface of the W–TiC specimen after the thermal shock test, as shown in Fig. 2c. Based on EDS spectra of selected areas, which composed of W and O, or W, Ti, and O, indicating that these areas may exist tungsten oxide or a mixture of tungsten oxide and TiC. Notably, the grey or dark areas are presented with a dotted distribution, which might be resulted by the jet stream of argon sputtering in the local melting area.
3.2 He-ion irradiation
In this work, the W–TiC composite was prepared with the wet-chemical method, and the microstructure changes of the W–TiC composite conducted by the laser beam thermal shock first and He-ion irradiation later were studied. After the laser beam thermal shock, a significant circular interface appeared on the surface of the composite. A melting area was detected in the center of the circular interface. Cracks were detected nearby the interface, which was owing to the thermal stress generated during the beam-off stage of the thermal shock test. In addition, a mixture of tungsten oxide and TiC was aggregated around the melting area, which might be related to gas sputtering. After He-ion irradiation, the fuzz structure was formed on the surface of the composite, except for the center of the melting area. Due to the grain boundary which might act as the diffusion path for He ions, He bubbles were more likely to aggregate at the deeper grain boundary rather than on the surface of the center of the melting area. This would be the reason why the fuzz structure was not formed on the surface of the center of the melting area.
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51574101), the Fundamental Research Funds for the Central Universities (Grant Nos. PA2018GDQT0010, PA2019GDZC0096, JZ2019HGTA0040), the Foundation of Laboratory of Nonferrous Metal Material and Processing Engineering of Anhui Province (15CZS08031), the Natural Science Foundation of Anhui Province (Grant Nos. 201904b11020034, 1908085ME115), the Foundation of Laboratory of Nonferrous Metal Material and Processing Engineering of Anhui Province, the Open Foundation of Key Laboratory of Advanced Functional Materials, Devices of Anhui Province and Double First Class enhancing independent innovation and social service capabilities of Hefei University of Technology (Grant No. 45000-411104/011).
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