Surface modification of ZrC dispersion-strengthened W under low energy He plasma irradiation

ZrC dispersion-strengthened W exhibits high strength/ductility, low ductile-to-brittle transition temperature, and excellent thermal shock resistance, making it a promising candidate plasma-facing material for future fusion devices. In this study, surface modification of 0.5 wt.% ZrC dispersion-strengthened W (WZrC) under low energy and high fluence He plasma irradiation at high temperature was presented. Under the energy of 90 eV and fluence ranging from 6 × 1024 He·m−2–2 × 1026 He·m−2 He irradiation at 920 °C, a typical fuzz nanostructure appeared on the W matrix of WZrC. The thickness of fuzz layer is proportional to the square root of He irradiation fluence. The fuzz showed comparable thickness and structural features to pure W, indicating limited effects of the particle’s addition on resistance to high fluence He irradiation at high temperatures. Under continuous He injection, the fuzz would grow extending onto the particle area, making the particle obscured. Besides, the erosion behavior of particles under He plasma irradiation has been investigated, which is thought to be dominated by a sputtering process. Under the He influence of 6 × 1024 He·m−2, only nanopores were observed in the surface region. With fluence increasing to 5 × 1025 He·m−2, the surface became relatively uneven with larger holes. W aggregated in spots and distributed on the surface of the particle, which might be the result of subthreshold sputtering and deposition. When fluence further increased to 2 × 1026 He·m−2, the particles were eroded completely and covered by the extended fuzz, forming cavities. In addition, distinctive layered nanotendrils were observed above the cavities, which were characterized to consist of inner W-riched skeletons and outer Zr-riched layers. It indicates that the layered nanotendrils should be the result of fuzz extension combined with particle sputtering/deposition.

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Introduction
Tungsten (W) is the primary candidate for plasma-facing materials (PFMs) in fusion devices due to its high melting point, high thermal conductivity, low sputtering yield, low swelling, low tritium retention, etc [1][2][3].However, some drawbacks, like high ductile-brittle transition temperature (DBTT), embrittlement under irradiation, and recrystallization, limit its performance [4,5].To mitigate these problems, various materials have been developed in recent years.Dispersion-strengthened W are tungsten-based materials strengthened with a small amount of dispersed secondphase particles.Among these ZrC dispersion-strengthened W has recently aroused much interest for its excellent performance in strength/ductility, DBTT, and thermal shock resistance [6][7][8].
There has been some preliminary work on the response of ZrC dispersion-strengthened W to He irradiation.Liu et al [19] studied the evolution of dislocation loops around the ZrC particles under 30 keV He + irradiation.Lang et al [20] studied the behavior of ZrC dispersion-strengthened W under 250 eV He irradiation.The work focused on the early-stage He-induced modification (only slight blisters on the particles) under low fluence (10 24 He•m −2 ).Liu et al [21] performed 620 eV He irradiation at 900 • C-1000 • C.However, its characterization after irradiation focused on the modification of the W matrix, while lacking the behavior of particles.Therefore, the response of ZrC dispersion-strengthened W to low energy He (<100 eV) irradiation, especially the behaviors of both W matrix and particles under both reactor-relevant high temperature and high fluence (∼10 26 m −2 ) He irradiation, needs further study.In this paper, both the surface morphology changes of the W matrix and particle behaviors under low energy and high fluence He plasma irradiation at high temperatures were presented.

Experimental
The materials studied in experiments were 0.5 wt.% ZrC dispersion strengthened W (referred to as WZrC) and polycrystalline W (referred to as ATW).WZrC samples were cut from a hot-rolled plate with a thickness of 8 mm, developed by the Institute of Solid State Physics, Chinese Academy of Sciences (ISSP, CAS) [8].ATW samples were cut from a commercial hot-rolled plate with purity > 99.97%, manufactured by Advanced Technology & Materials Co. Ltd Inc., China.The WZrC and ATW samples were cut to the size of 10 mm × 8 mm × 0.6 mm and 10 mm × 10 mm × 1 mm, respectively.Before irradiation, all samples were mechanically polished to mirror-like surfaces and then electropolished in baths of 1.5 wt.% NaOH aqueous solution.
The characterization of microstructural features of aspolished samples were shown in figure 1.Based on the results of electron back scatter diffraction (EBSD), the average grain size of ATW (figure 1(b)) was ∼37.9 µm, while for WZrC (figure 1(b)), the average grain size was ∼16.4 µm.The particle features were investigated by both EBSD (figure 1(b)) and SEM BSE (figures 1(c) and (d)).There were particles of two different sizes in WZrC.The smaller ones were nano-sized ZrC particles (as indicated by yellow arrows in figure 1(d)) with an average size of ∼60 nm, which were mainly distributed in the grain and only a few amounts distributed at the grain boundary.More detailed information on this microstructure can be found in [8].The larger ones were sparsely distributed micro-sized particles (as indicated by red circles in figure 1(c)) with an average size of ∼4.2 µm.The crystal structures and elemental maps were investigated with transmission electron microscopy (TEM).Selected area electron diffraction (SAED, figures 1(f )-(h)) and energy-dispersive xray spectroscopy (EDX, figure 1(i)) analysis indicated that the particle was tetragonal structure and the atomic fractions of C, Zr, and O were 45.2%, 33.7%, and 21.1%, respectively.Furthermore, these micro-sized particles were composed of ZrO 2 (figure 1(g)) and ZrC (figure 1(h)).The formation of large micro-sized particles might be the result of the aggregation and coarsening of the nano-sized ZrC particles at high temperatures in the fabrication process [8], combined with the reaction of O impurities.Above all, in WZrC the nanosized particles were ZrC, while the micro-sized particles were ZrC x O y .After He plasma irradiation, the surface modifications of samples were characterized with a cold field emission scanning electron microscope HITACHI UHR FE-SERM SU8220.The cross-section samples were prepared and characterized with a focus ion beam (FIB) (15 keV Ga + ) in Helios NanoLab 650 FIB-SEM.The samples for TEM and HAADF-STEM were cut from the irradiated samples using a FIB, with a thickness of about 100 nm.Then, the TEM, HAADF-STEM, and EDX analysis were conducted with a high-resolution transmission electron microscopy Talos F200X.

The fuzz nanostructure on the W matrix of WZrC
The surface modifications of WZrC after exposure to 90 eV He ions with various fluences of 6 × 10 24 , 5 × 10 25 , and 2 × 10 26 He•m −2 at 920 • C are shown in figure 2. Interweaved nanotendrils (also called fuzz) appeared on the W matrix under all three fluences (figures 2(a)-(c)).Under 6 × 10 24 He•m −2 (figure 2(d)), nanotendrils with a diameter of tens of nanometers (averaging ∼50-60 nm) appeared.As fluence increased to 5 × 10 25 He•m −2 (figure 2(e)), the nanotendrils developed further and grew finer with an average diameter of ∼20-30 nm.With higher fluence up to 2 × 10 26 He•m −2 (figure 2(f )), the diameter of nanotendrils did not change.In addition, the surface modifications of ATW (figures 2(g)-(i)) under the same He fluence were investigated for comparison.They showed similar morphological characteristics under the same He fluence.Furthermore, the thickness of the fuzz layer in the specific sample was the average of five measurements through FIB cross-section analysis.And the thicknesses of the fuzz layer on both ATW and WZrC were presented in table 1.It implied that the thickness of the fuzz layer increased with increasing He fluence for both materials, with no significant difference under the same He fluence.

Modified structures of the particle and its surrounding area
Except for the fuzz layer on the W matrix, the gradual disappearance of micro-sized particles was observed on WZrC.Under 6 × 10 24 He•m −2 (figure 2(a)), the particles could be distinctly observed.As fluence increased to 5 × 10 25 He•m −2 (figure 2(b)), the interface of the particle and W matrix became blurred, but the particles could still be identified.Under 2 × 10 26 He•m −2 (figure 2(c)), the particles completely disappeared, and only fuzz layer existed.
Cross-section images (figure 3) of WZrC under different He irradiation fluences revealed further details of the disappearance process.Before He irradiation (figure 3(a)), the tightly bonded interface between particle and W matrix can be identified, with few pores at the interface.Under 6 × 10 24 He•m −2 (figure 3(b)), many nanopores appeared on the surface of particle, which maintained a relatively flat surface.
As fluence increased to 5 × 10 25 He•m −2 , as shown in figure 3(c), the surface of particle became relatively uneven and further eroded.SEM characterization in figure 4(a) showed that there were larger holes and small spots on the particle.Moreover, as indicated by the arrows in figure 3(c), fuzz growing on the W matrix crossed the W-particle interface (the white dashed line), and extended onto the particle area.This extension of fuzz would cause the particles to be obscured.Moreover, SEM and EDX analysis (figure 4) show that the obscure effect is particle size dependent.As shown in figure 4(a), the particle with a size of ∼15 µm could still be distinctly observed, despite heavily eroded.While the smaller particle (as marked by the white circle in figure 4(a)) was almost covered by fuzz.More observations on other particles show that particles smaller than about 3-5 µm are almost completely obscured, while larger particles are only partially obscured.In addition, as marked by white circles in figures 4(d) and (f ), some W aggregated in spots and distributed on the surface of the particle.
With higher fluence up to 2 × 10 26 He•m −2 , only uniform fuzz existed, and no particles can be identified (as shown in figure 2(c)).But the cross-section indicated two different structures: 1. Uniform fuzz grew on the W matrix, as shown in figure 5(a).2. Extended fuzz above a cavity, as shown in figures 3(d) and 5(b).The uniform fuzz on the W matrix, as mentioned in figure 2(f ), are interweaved nanotendrils with a diameter of about 20-30 nm.EDX analysis indicated that the uniform fuzz had extremely small amounts of Zr (as shown in figure 5(c), the emission peak of Zr was detected, but the content was less than the detection limit).The extended fuzz had significantly thicker tendrils (as shown in figure 3(d)).As shown in figure 5(b), the extended fuzz did not originate from the bottom but, rather, grew from the adjacent W matrix. Notably, unlike the uniform fuzz, considerable Zr existed in the extended fuzz above the cavity (figure 5(d)).
To unravel the intricate details of two different fuzz, additional characterization was conducted.As shown in figure 6(a), the uniform fuzz tendrils on the W matrix are ∼20-30 nm in diameter, with many He bubbles inside.As shown in the Zr mapping (figure 6(c)), a small amount of Zr was doped inside the tendrils (Zr/W atomic ratio = 0.03 ± 0.02).For the extended fuzz, HAADF-STEM (figure 6(d)) revealed a layered structure consisting of an inner skeleton and an outer coating layer.EDX analysis (figures 6(e) and (f )) revealed that the inner skeleton and outer layer were W-riched and Zrriched, respectively.The Zr content in the layered fuzz is much higher (Zr/W atomic ratio = 2.28 ± 0.15) than the uniform fuzz (Zr/W atomic ratio = 0.03 ± 0.02).The characterization of an individual layered tendril (figures 6(d)-(f )) further revealed its characteristics.As shown in figures 6(g) and (h), the inner W-riched skeleton had a diameter (∼20-30 nm) similar to the uniform fuzz on the W matrix, also with nanoscale He bubbles inside.As shown in figures 6(f ) and (i), the outer Zr-riched layer uniformly covered the surface of the inner skeleton.

The growth of the fuzz layer
The typical fuzz structure in this paper has been widely observed in related experimental researches [18,22].These experimental results [23] suggest that fuzz nanotendrils would reach a steadily-growing condition under high flux/fluence He irradiation.The structural characteristics of steady-state growth are mainly related to incident energy and temperature [12,24].Moreover, after reaching the steadily-growing condition, the thickness is well characterized by a fluence 1/2 dependence [11,25], suggesting that the fuzz growth process may be dominated by diffusion.And as shown in figure 7, fuzz in this work also matched well with this relationship.And if the simple one-dimensional growth law, d = (2Dt) 1/2 (d: thickness of fuzz, D: effective diffusion coefficient, t: time), arising from Fick ' s law is assumed, the fitting lines correspond to a diffusional growth process characterized by effective diffusion coefficients of D ATW = 8.88 × 10 −14 m −2 s −1 and It is also worth mentioning that similar fuzz structures with comparable thickness were observed on WZrC and ATW, despite the difference in the grain orientation, size, and grain boundary density of the two materials.However, some experimental works [20,[26][27][28] showed significantly different responses to He irradiation between dispersion-strengthened W and pure W.These works hold that the W-particle interface could affect the formation of He-induced defects, and further change the response to He irradiation.This discrepancy could be explained by that reference works were conducted under relatively lower flux/fluence (<10 25 He•m −2 ) and temperature, which means the modified structures just evolved in their early stages.However as indicated by Parish et al [23], the effect of defects like grain boundaries and dislocation loops on response to He irradiation would be overwhelmed In addition to increasing thickness, the fuzz layer was also observed to extend onto the particle region.Angled He irradiation conducted by Meyer et al [24] has shown that fuzz tendrils would be parallel to the irradiation direction when the He energy is higher than a threshold (∼418 eV).While under low energy He irradiation, Meyer et al [24] and some experimental results [11,12,29] indicated that fuzz tendrils would interweave with each other, without preferential growth direction.The macroscopic fuzz layer composed of multiple tendrils would grow extending onto the particles, making them partially or completely obscured.
The W sputtering and subsequent deposition onto the particle surface may be the reason for the aggregated W spots on the particle surface in figure 4(d).It is generally believed that 90 eV is not high enough for He ions to induce the sputtering of W because the physical sputtering threshold is about 105-110 eV [30,31].However, the subthreshold sputtering of W has been observed in some experiments [32][33][34].The work [32] of Guseva et al indicated that the subthreshold sputtering is attributed to the possible sputtering of W adsorbed atoms (adatoms).The knock-out energy of adatoms from the surface induced by He ions bombardments is about one-third of the physical sputtering threshold [35].Considering the experimental temperature of 920 • C and the He ion energy of 90 eV in this paper, some W atoms could be sputtered from the extended fuzz below the physical sputtering threshold and deposited on the particle.The concentration of adatoms on the surface is usually very low [36], resulting in a low subthreshold sputtering yield.The general sputtering/deposition process [37] is that atoms gather into clusters, and then gradually extend into layers.The mentioned low subthreshold sputtering yield would only form sparsely distributed W cluster spots (the white circle in figure 4(d)), rather than W deposition layer on the particle surface.
With a sufficient He injection (2 × 10 26 He•m −2 in this work), the extended fuzz will block all the particles (no particle could be observed in figure 2(c)).The details of the extended fuzz will be discussed in the next discussion 4.2 section in conjunction with the particle erosion behavior.

The particle behavior under He irradiation
The particle erosion behavior under continuous He irradiation deserves more attention.The possible mechanisms that dominate particle erosion are hereby discussed.SAED (figures 1(d)-(f )) showed that the micro-sized particles were composed of ZrO 2 (figure 1(e)) and ZrC (figure 1(f )).Given that the melting point of ZrC (∼3700 K [38]) and ZrO 2 (∼2988 K [39]) is much higher than the experimental temperature (∼1200 K), the likelihood of evaporation or melting as the dominant erosion mechanisms is low.Related experimental studies [40] have indicated that the aggregation of He at the grain boundary could decrease the cohesive strength, resulting in W grain ejection.Considering the He plasma and thermal load in this work, the thermal/internal stress or He 12 aggregation may cause some particles to eject or flake off, analogous to W grain ejection.However, clear particle ejection or spalling phenomena were not observed.Instead, complex structures like nanopores and holes were identified on the particle surfaces postirradiation, which might be related to a sputtering process.When surface binding energy is used to estimate the collisions of He [41][42][43], the sputtering threshold for Zr and C in ZrC is about 50 eV and 12 eV, respectively.The sputtering threshold for Zr and O in ZrO 2 is 55 eV and 14 eV, respectively.Hence, it is reasonable to expect that the sputtering process would occur under 90 eV He irradiation.To confirm the sputtering effect of He ions on ZrC x O y , additional He irradiation was conducted on a ZrC x O y target under the same He irradiation  conditions, yielding similar erosion behavior (related results could be seen in supporting figure 1) as ZrC x O y particles in WZrC.This observation reinforces the conclusion that particle erosion in this study might be related to the sputtering process.
Notably, no nano-sized ZrC particles were found in the uniform fuzz tendrils grown from the W matrix of WZrC, and the composition analysis showed that a small amount of Zr is evenly distributed on the fuzz tendrils.Considering the sputtering threshold mentioned above, it is reasonable to infer that ZrC particles would also be eroded by sputtering.The sputtered Zr would be doped in the fuzz tendrils, resulting in a uniform fuzz containing W and Zr.Considering that the proportion of nano-sized particles is less than 0.5 wt.%, they could only produce little sputtering, resulting in a low Zr content.
Moreover, the micro-sized particle erosion was observed to occur simultaneously with the fuzz extension.The diagram in figure 8 illustrates the possible relationship between these two processes.As shown in figure 8(a), the fuzz would grow extending onto the particle region.As marked by the black solid arrow in figure 8(a), some W would be sputtered (subthreshold sputtering) and deposit on the particle surface, which resulted in the W cluster spots in figure 4(d).At the same time, as marked by the black dashed arrow in figure 8(a), sputtering of particles also occurred.Zr would be sputtered away from the particle, captured by the extended fuzz above it, leaving holes caused by sputtering.In this way, as shown in figure 8(b), under continuous He injection, the extended fuzz will finally cover the particles, and the eroded particle below will leave a cavity.The extended fuzz would contain W and Zr, which is consistent with the EDX results in figure 5(d).The sputtered Zr from the particles might deposited onto the extended fuzz tendrils.This deposition may explain the coating of Zr on the extended fuzz tendrils.In the process of sputtering/deposition, the deposited atoms will first nucleate and gather into clusters [37], and then grow up (diffusionmediated, adsorption-driven, etc) and extend into layers [44,45].Therefore, under continuous He injection in this work, sputtered Zr atoms would deposit on the extended fuzz, and evolve into a film layer coating the W fuzz tendrils.In summary, the outer Zr-riched layer might be dominated by a combination process of both particle sputtering/deposition and W fuzz extension.
It should be noted that simultaneous subthreshold sputtering of W and sputtering of Zr have different yields.The He incident energy of 90 eV is insufficient to induce the W sputtering because the physical sputtering threshold is about 105-110 eV [30,31].The W subthreshold sputtering only came from weakly bonded, absorbed on the surface, loose adatoms [36].Its concentration on the surface is usually very low [36], resulting in a low subthreshold sputtering yield.While 90 eV is much higher than the sputtering threshold energy for Zr (both in ZrC and ZrO 2 ), resulting in a relatively high sputtering yield.Therefore, the mentioned low W subthreshold sputtering yield would only form sparsely distributed W cluster spots.While, the sputtered Zr atoms from micro-sized particles would deposit, and evolve into a film layer coating the W fuzz tendrils.And the formed layered structure will add Zr-related interfaces, which have shown moderate trapping ability of hydrogen atom [46].The effect of these interfaces on hydrogen isotope retention needs further study.

Conclusion
The surface modifications of WZrC and ATW have been investigated under the energy of 90 eV and fluence ranging from 6 × 10 24 He•m −2 -2 × 10 26 He•m −2 He irradiation at 920 • C. The typical fuzz structure appeared on the WZrC with a thickness comparable to ATW, which might mean that the He-W interactions would dominate the surface modification of WZrC under low energy and high fluence He irradiation.The erosion of both ZrC x O y and ZrC particles was also observed.As a result of micro-sized ZrC x O y particle sputtering/deposition combined with fuzz extension, a distinctive layered fuzz structure formed.It consisted of an inner W-riched skeleton and an outer Zr-riched coating layer, which has never been noticed by reference works.When used as PFM, the erosion of Zr-containing particles could increase Zr, C, and O impurities.And Zr-related interfaces in the layered fuzz could increase hydrogen retention.These effects should be taken into consideration for the application of particle dispersion-strengthened W in future fusion reactors.

Figure 1 .
Figure 1. Materials before He irradiation.(a) EBSD of ATW.(b) EBSD of WZrC.(c) BSE SEM image of WZrC.As marked by green circles, micro-sized particles with an average size of ∼4.2 µm were distributed in the W matrix.(d) BSE SEM image (high magnification) of WZrC.Nano-sized ZrC particles with an average size of ∼60 nm are indicated by yellow arrows [8].(e) TEM image of a slice cut from a micro-sized particle.(f ) SAED patterns of a micro-sized particle.(g) Diffraction pattern of ZrO 2 .(h) Diffraction pattern of ZrC.(i) EDX result of particle.The micro-sized particle was composed of ZrO 2 and ZrC (ZrCxOy).

Table 1 .
Thicknesses of the modified fuzz layer of WZrC and ATW.The thickness in the specific sample was the average of five measurements through FIB cross-section analysis.The thickness of the fuzz layer increased with increasing He fluence.The fuzz thickness on WZrC was comparable to ATW under the same He fluence.He fluence (He•m −2 ) 6 × 10 24 5 × 10 25

Figure 4 .
Figure 4. WZrC under middle He fluence (5 × 10 25 He•m −2 ).(a) The particles circled by white dashed line were further eroded, with holes and small spots.The partially obscured particle was marked by a white circle.(b) and (c) EDX mappings of W and Zr in (a).As shown in white circles, overlapped W and Zr signals indicated that the extended fuzz obscured the particles.(d) As marked by the white circles, W aggregated in spots and distributed on the surface of the particle.(e) and (f ) EDX mappings of W and Zr in (d).

Figure 5 .
Figure 5. Two structures on WZrC under high He fluence.(a) Uniform fuzz on W matrix.(b) Fuzz above a cavity.Its roots were connected to the fuzz on the W matrix. (c) EDX results of uniform fuzz (the selected area is marked with a yellow square in (a)).(d) EDX analysis of fuzz above a cavity (the selected area is marked with a yellow square in (b)).

Figure 6 .
Figure 6.(a) HAADF-STEM of uniform fuzz on W matrix.(b) and (c) W and Zr mappings of the uniform fuzz.(d) HAADF-STEM of layered nanotendrils above cavities.(e) and (f ) W and Zr mappings of layered nanotendrils.It revealed a W-riched skeleton and a Zr-riched layer.(g) HAADF-STEM of an individual layered nanotendril.(h) and (i) W and Zr mappings of a layered nanotendril.The dashed red lines indicated an internal W-riched skeleton with a diameter of about 20-30 nm, with nanoscale He bubbles inside.

Figure 7 .
Figure 7.The thickness of the fuzz layer of WZrC and ATW as a function of the square root of He fluence.

Figure 8 .
Figure 8. Diagram of simultaneous fuzz extension and particle erosion processes.(a) Fuzz extended onto the particle.The solid arrows indicated that W would be sputtered away from extended fuzz, and then deposited on the particle.The dashed arrows indicated that Zr would be sputtered away from the particle, and captured by the extended fuzz.(b) Under continuous He injection, the extended fuzz will finally cover the particle, and the eroded particle below will leave a cavity.The extended fuzz would contain both W and Zr.Note: The green stripes in (b) only indicate that extended fuzz contains Zr, and do not represent the distribution form of Zr.