Surface modification of W–Ta–V–Cr multi-component alloy after low-energy He plasma irradiation

This study explores a hypothetical scenario where low-activation refractory multi-component alloys (RMCAs) are considered for use as divertor target materials in fusion reactors. To investigate the surface modifications under divertor service conditions, a multi-phase W–Ta–V–Cr RMCA is irradiated with low-energy helium (He) plasma at varying temperatures to approximately 5.0 × 1025 He·m−2. The W-rich and Cr-rich phases in the multi-phase RMCA mimic the mono-phase W–Ta–V–Cr RMCA and segregation at grain boundaries, respectively. Following irradiation, fuzz layer formation is observed for all phases at temperatures lower than pure W requires. Additionally, nano He bubbles are identified in the fuzz tendrils at 920 °C. The modified layers exhibit reduced V and Cr content with increasing temperature, transitioning into W-Ta binary alloys at 920 °C. Notably, the fuzz layer on the W-rich phase is thinner than that on pure W at 920 °C. While a thinner fuzz layer suggests the alloy’s potential as a divertor target material, challenges include a lower fuzz formation temperature and potential high V and Cr sputtering yield, raising concerns for plasma contamination during fusion reactor operation. This dual perspective highlights both the promise and challenges of utilizing RMCAs as divertor target materials in severe fusion reactor environments.

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Introduction
Although tungsten (W) remains the primary candidate material for divertor targets owing to its high melting point, low fuel retention rate, low sputtering yield, and high thermal conductivity [1], it is not without its challenges.Issues such as a high ductile-to-brittle transition temperature, hightemperature recrystallization, and vulnerability to neutron irradiation effect pose significant drawbacks [1].These factors contribute to the degradation of mechanical properties [2,3], increased fuel retention rate [4], and reduced thermal conductivity [5,6] of W, subsequently impacting the component performance in the nuclear fusion reactor.Hence, developing novel plasma-facing materials (PFMs) for divertor targets emerges as one of the key challenges for future nuclear fusion reactors.
Low-activation refractory multi-component alloys (RMCAs) have higher resistance to irradiation effects, such as irradiation-induced defect growth and irradiation hardening, along with improved thermal stability and enhanced hightemperature mechanical properties compared to W [7][8][9][10].These RMCAs are composed of low-activation elements such as W, tantalum (Ta), titanium (Ti), vanadium (V), chromium (Cr), etc [11][12][13].El-Atwani et al [7] published experimental and simulated results on 1 MeV krypton (Kr) ion irradiation of nanocrystalline W-Ta-V-Cr RMCA.The irradiation temperature was 800 • C, and the damage reached up to 8 dpa.The indentation experiment results showed only a slight degree of irradiation hardening, and microscopy results showed no formation of dislocation loops after irradiation.However, the formation of second phases and the segregation of V and Cr to grain boundaries were observed.El-Atwani et al [8] also published another irradiation experiment on nanocrystalline W-Ta-V-Cr RMCA using 2 keV helium (He) ions irradiated to a dose of 1.25 × 10 21 m −2 at 950 • C. Microstructural analysis revealed the presence of numerous 2-3 nm He bubbles, which were uniformly distributed within the grains of RMCA.They attributed this phenomenon to the fact that He diffusion in RMCA is twice as difficult as in W, while the He bubble formation energy in RMCA is only half that of W, making it less prone to diffusion but more likely to form small helium bubbles.Besides the results for nanocrystalline W-Ta-V-Cr RMCA, Waseem et al [14] irradiated W 0.5 (TaTiVCr) 0.5 RMCA with 4.2 MeV fluorine (F) ions, with a dose of up to 3.2 × 10 16 m −2 .They noted an increase in hardness by 17% after irradiation, but the surface did not show significant morphological changes after irradiation, and the microstructure was not affected by irradiation.
Despite RMCAs exhibiting relatively low thermal conductivity [15], their improved high-temperature mechanical performance and microstructure stability serve as compensatory factors.Consequently, the estimated heat load capacity of the RMCA divertor target, specifically in the case of W-Ta-V-Cr-Ti alloy, is only 16% lower than that of a W divertor target [9].This may suggest that RMCAs are potentially suitable for application in plasma-facing components of nuclear fusion reactors.
In addition to high-energy particle irradiation, during the operation of a nuclear fusion reactor, the divertor targets will face exposure of high-flux plasma and high heat flux, leading to surface modification.In this study, our aim is to simulate a hypothetical scenario in which low-activation RMCA is utilized as a divertor target and to assess the effects of edge plasma on the surface of low-activation RMCAs during fusion reactor operation.A multi-phase W-Ta-V-Cr RMCA, comprising Wrich and Cr-rich phases, is subjected to low-energy He plasma irradiation at varying temperatures.The W-rich phase represents the W-rich mono-phase W-Ta-V-Cr RMCA, as previously reported [7,16], while the Cr-rich phases simulate Cr and V segregation at grain boundaries resulting from the highenergy particle irradiation [7].Following He plasma irradiation, a series of microstructural analyses are performed to investigate the morphology and chemical element variations among the different phases.

Material and specimen manufacture
The multi-phase W-Ta-V-Cr RMCA is manufactured using the arc-melting technique and supplied by Hebei Fengming New Material Technology Co., Ltd, China.The alloy undergoes over ten successive remelting cycles to ensure composition uniformity before being cast into semi-spherical ingots.While the specified elemental composition ratio in the design is 25 at% for all elements, the actual composition ratios of the ingot based on x-ray fluorescence (XRF) analysis are as follows: 27.05 at% for W, 25.46 at% for Ta, 20.66 at% for V, and 26.83 at% for Cr.Subsequently, the ingots are cut into square disks with dimensions of 10 mm × 10 mm × 1 mm using electric-discharge machining (EDM).After EDM, the specimens are polished to achieve a mirror-like surface in preparation for subsequent irradiation experiments and microstructural investigation.

Low-energy He plasma irradiation
The irradiation experiments are conducted using a linear plasma device known as the Compact LInear Plasma-Surface interaction device (CLIPS) at the University of Science and Technology of China.Detailed information regarding CLIPS can be found in our previous work [17].CLIPS is equipped with a water-cooled irradiation stage featuring a constant cooling rate.The irradiation temperature is monitored by a thermal couple attached behind the 1 mm thick irradiated specimen and controlled by manipulating magnetic field intensity and discharge power.Given the thinness of the specimen, the temperature difference between the irradiated surface and the back of the specimen should be negligible.Three irradiation temperatures are achieved in this study: 300 • C, 600 • C, and 920 • C, corresponding to He ion fluxes of 1.36 × 10 22 m −2 s −1 , 1.69 × 10 22 m −2 s −1 , and 1.39 × 10 22 m −2 s −1 , respectively.It takes approximately 5 min to reach the target temperature, with temperature fluctuations of around ±10 • C after reaching the target.The He ion energy is 90 eV, which is achieved by applying a bias voltage to the irradiated specimens via the thermal couple.The low-energy He plasma irradiation specimens are irradiated without heat treatment beforehand.Each specimen undergoes a 1 h irradiation to attain a fluence of approximately 5.0 × 10 25 m −2 .Furthermore, to facilitate a comparative analysis of surface modifications post-He plasma irradiation, a polished pure tungsten plate fabricated by AT&M China is also subjected to He plasma irradiation at 920 • C, with a flux of 1.39 × 10 22 m −2 s −1 , reaching a fluence of 5.0 × 10 25 m −2 .

Microstructural investigation
In order to measure the composition of the as-casted alloy, two quantitative analysis techniques, an XRF spectrometer and an electron probe microanalyzer (EPMA), are conducted.X-ray diffraction (XRD) analysis is implemented to determine phases and crystallographic structures of specimens before and after He plasma irradiation.Surface morphology is examined using a field-emission scanning electron microscope (SEM) after He plasma irradiation.In the cases of He plasma irradiated specimens, the surface chemical composition is semi-quantitatively analyzed using energydispersive x-ray spectroscopy (EDS), as analyzed surfaces are normal to the electron beam.To understand the thickness of the modified layer after He plasma irradiation, surface crosssection samples are prepared through focus ion beam milling.The thickness of the modified layer is determined by averaging measurements taken at nine points and employing geometric calculations that account for the specimen's tilt.The surface cross-section chemical composition is also analyzed by EDS to achieve a semi-quantitative measurement.Unlike the surface EDS analysis, irradiated specimen is tilted 45 • toward the EDS detector to maximize the signal intensity for cross-section EDS analysis.He bubbles in the fuzz tendrils are investigated through a transmission electron microscope (TEM).The fuzz tendril TEM specimens are prepared by scratching off the fuzz layer from the irradiated surface and drop-casting onto the copper grid.The He bubble size distribution analysis is achieved by analyzing the bright-field highresolution TEM image with ImageJ software [18].

First-principles calculation for lattice parameter
The density functional theory (DFT) method is implemented to calculate the lattice parameters (a 0 ) of different phases in the multi-phase W-Ta-V-Cr RMCA.These first-principles calculations are conducted using the VASP code [19].The exchange-correlation functional utilizes the generalized gradient approximation proposed by Perdew-Burke-Ernzarhof and projector-augmented wave potentials.A 5 × 5 × 5 k-point mesh is utilized for geometry optimization.All atomic positions are fully relaxed until the total free energy change is less than 10 −4 eV.The size of the supercell employed is 3 × 3 × 3 a 0 3 .

Multi-phase W-Ta-V-Cr RMCA
Figure 1(b) shows three distinct phases in the as-casted multi-phase W-Ta-V-Cr RMCA as identified through backscattered electron (BSE) imaging and EPMA.The brighter phase, designated as Phase 1, exhibits elevated content of W (figures 1(c) and (d).As presented in table 1, Phase 1 has a composition that aligns with the W-rich mono-phase W-Ta-V-Cr RMCA documented in the literature [7,16] but with a lower Ta content and higher V content.Conversely, the darker phases, denominated Phase 2-1 and Phase 2-2, display a higher content of Cr and V (figures 1(e)-(f ) and table 1).Furthermore, the branching structure indicates that these two phases are part of a eutectic alloy, as illustrated in figure 1(b).
The XRD analysis elucidates the presence of three distinct phases in the as-casted alloys, as shown in figure 2. The peaks indexed as (110), (200), and (211) signify a body-centered cubic (BCC) structure for all three phases.The broadening of the peaks can be attributed to lattice distortion arising from the disparate atomic sizes of the consisted elements.By comparing a 0 obtained from the XRD analysis with those calculated through DFT, as presented in table 2, individual groups of peaks can be discerned.Peaks corresponding to Phase 1 are annotated with red diamond markers, those for Phase 2-1 with green circle markers, and Phase 2-2 with blue triangle markers.The identification of Phase 1 peaks is further supported by their close proximity to values reported in the literature for the W-rich mono-phase W-Ta-V-Cr RMCAs [16,20,21].It is evident that a 0 decreases with decreasing W content and increasing Cr content, resulting in a noticeable shift of the peaks towards higher 2θ values.

Surface morphology after He plasma irradiation
The surfaces of as-received alloy after low-energy He plasma irradiation at various temperatures with a fluence around 5.0 × 10 25 m −2 are shown in figures 3(a)-(c).Since Phase 2-1 and Phase 2-2 form a eutectic phase, the following content will refer them as the Cr-rich phases.At an irradiation temperature of 300 • C, all phases exhibit a nano-wall surface morphology, a typical feature observed after low-temperature He plasma irradiation on W [22]. Upon increasing the irradiation temperature to 600 • C, a fuzz-like surface is observed in all phases.Notably, at 920 • C, the morphology of Cr-rich phases transforms into a pore-like surface, while Phase 1 retains a fuzz-like surface but with a thicker tendril diameter.
As presented in figures 3(d)-(f ), the cross-section analysis of the He plasma-irradiated surface reveals the absence of pores in all phases after irradiation at 300 • C.However, at an irradiation temperature of 600 • C, a pore-like structure emerges in Cr-rich phases.Moreover, Cr-rich phases have a thicker modified layer (∼2.5 µm) compared to the thin surface modification (∼0.7 µm) observed in Phase 1.Further    elevating the irradiation temperature to 920 • C results in larger pores in Cr-rich phases, forming an even thicker modified layer (∼2.9 µm).Simultaneously, at this irradiation temperature, the thickness of the modified layer in Phase 1 increases to ∼1.2 µm, accompanied by the observation of pores in Phase 1.
According to current theories, fuzz formation is commonly associated with the presence of He bubbles within the nanostructure of the surface or fuzz tendrils [23].Notably, Shi et al [20] have observed the presence of He bubbles on the irradiated surface of W 47 Ta 41 Cr 6 V 5 mono-phase RMCA after He plasma irradiation with a He ion energy of 100 eV, at a surface temperature of 400 • C, and to a fluence of 2.02 × 10 25 m −2 .Therefore, while our investigation primarily focuses on elucidating the surface morphology of multi-phase W-Ta-V-Cr RMCA after He plasma irradiation, we attempt to study deeper through TEM analysis to discern the presence of He bubbles within the fuzz tendrils resulting from He plasma  irradiation on multi-phase W-Ta-V-Cr RMCA.The initial findings, shown in figure 4, reveal a large number of He bubbles formed inside the fuzz tendrils of the specimen after He plasma irradiation at 920 • C to a fluence of 5.0 × 10 25 m −2 .Judging from the morphology of the specimen surface depicted in figure 3(c), it appears that fuzz tendrils may originate from the surface of Phase 1.The EDS analysis indicates that the fuzz tendrils are composed of 26.43 ± 8.24 at% of W, 67.04 ± 8.35 at% of Ta, and 6.53 ± 7.10 at% of Cr.The median diameter of the He bubbles, as derived from the analysis in figure 4(c), is 3.03 nm, smaller than the reported He bubble median diameter (∼4.2 nm) in pure W fuzz tendrils after being irradiated with 50 eV He plasma at 923 • C to a fluence of 4 × 10 26 m −2 [24].Nevertheless, these TEM findings show that the He bubble may participate in the fuzz tendril growth of W-rich mono-phase W-Ta-V-Cr RMCA during He plasma irradiation at high-temperature conditions.Here, we present the analysis solely focusing on Ta-rich fuzz tendrils under He plasma irradiation at the highest temperature of this study, as the TEM examination of other fuzz tendrils encounters technical challenges.Ongoing investigations are underway to explore additional fuzz tendrils or modified layers subjected to varying He plasma irradiation parameters, with forthcoming results to be presented in the following works.
The formation of a nano-structure surface on metal is known to occur at a He plasma irradiation temperature higher than 30% of its melting point [25].Given that a fuzz-like surface on W is observed only at an irradiation temperature exceeding 700 • C [26], it can be inferred that all phases possess a lower melting point than W.This inference aligns with their melting point estimations, calculated by averaging the melting points of the individual components-where the melting point of Phase 1 is ∼2800 • C and that of Cr-rich phases are ∼2300 • C. The lower fuzz formation temperature thresholds of Phase 1 and Cr-rich phases raise concerns regarding the application of W-rich mono-phase W-Ta-V-Cr RMACs as divertor target materials under high-energy particle irradiation, particularly considering the possibility of Cr and V segregation at grain boundaries.Given that the fuzz lacks structural strength, a lower formation temperature threshold implies a potential source of dust at lower temperatures.Furthermore, the likelihood of melt splashing and droplet ejection at lower temperatures from Cr-rich phases with lower melting points poses challenges for the confinement and steady-state operation of fusion plasma [27].
The variation in the thickness of the modified layer suggests a difference in the He diffusion coefficient in the alloys, with He diffusivity in Phase 1 being lower than in Cr-rich phases.El-Atwani et al [8] indicated that an RMCA, sharing a similar composition with Phase 1, exhibits a sluggish diffusion effect on He, resulting in smaller He bubbles.This sluggish diffusion effect could also account for Phase 1 having a thinner fuzz layer compared to pure W, which was reported to have a fuzz layer thickness of approximately 3 µm in our recent study [28], as shown in figure 5, under 90 eV He plasma irradiation at 920 • C to a fluence of 5.0 × 10 25 m −2 .According to the mechanism proposed by Kajita et al [29], fuzz formation is linked to the growth and rupture of He bubbles.Therefore, a material exhibiting higher resistance to He bubble growth may also indicate better fuzz formation resistance.

Surface composition after He plasma irradiation
In addition to the alterations in surface morphology, the nearsurface composition of Phase 1 and Cr-rich phases undergoes changes after He plasma irradiation.As illustrated in figure 6 and table 3, the content of lighter elements, V and Cr, on the surface gradually dissipates with increasing irradiation temperature, especially evident in Cr-rich phases.As a result, at an irradiation temperature of 920 • C, all phases predominantly consist of the heavier elements, W and Ta.EDS qualitative line analysis reveals that the composition-modified layer in all phases at temperatures below 600 • C exhibits a similar thickness to the morphologically modified layer discussed in section 3.2.However, the composition change in Phase 1 extends deeper than the morphological modification at 920 • C.
The reduction of lighter elements can be elucidated by the sputtering effect.Sputtering yield is a function of iontarget atom mass ratio and surface binding energy [30], with the surface binding energy remaining constant for the same alloy.Consequently, the sputtering yield for RMCA under He plasma irradiation is determined by the ion-target atom mass ratio.Clearly, the ion-target atom mass ratio of V and Cr is larger than that of W and Ta, resulting in a higher sputtering yield for V and Cr.This phenomenon leads to the depletion of V and Cr, leaving behind W-rich and Ta-rich alloys.Furthermore, upon comparing the EDS results of the Phase 1 fuzz tendril in section 3.2 with the Phase 1 surface EDS results from table 3, the variation in composition ratios suggests composition inhomogeneity within the fuzz layer on Phase 1 after He plasma irradiation at 920 • C. The formation of Tarich alloy on the surface of Cr-rich phases can also explain the pore-like surface observed at 920 • C in figure 3(c) since pure Ta under low-energy He plasma irradiation at this temperature tends to exhibit a similar surface morphology [31].Notably, the reduction of lighter elements on the surface may also be linked to temperature.However, further investigation is required to examine the thermal effect on surface chemical composition for verification.
Contamination from the sputtering of PFM, particularly involving high atomic number (Z) elements, can lead to radiative cooling of the plasma [32].In other words, maintaining contamination level is essential for stable plasma performance.Therefore, considering plasma stability, the higher sputtering yield of V and Cr in W-rich mono-phase W-Ta-V-Cr RMCA makes it less favorable as a divertor target material candidate.However, it should be noted that V and Cr possess lower Z compared to W and Ta, resulting in inherently lower cooling factors.The debate surrounding the radiative cooling effect from V and Cr contamination is complex and requires further investigation.
Significant differences in the modified layer thickness between Phase 1 and Cr-rich phases at various temperatures may result from the melting point differences between the  phases and the higher concentration gradient of Cr and V in the Cr-rich phases due to sputtering.At the same temperature, an alloy with a higher melting point exhibits a lower diffusion coefficient for its compositional elements, resulting in a shorter diffusion length according to Fick's law.Thus, Phase 1, with a higher melting point, has a thinner modified layer than Cr-rich phases.Moreover, since diffusion is a thermally activated and concentration gradient-driven process, the diffusion length increases with temperature and surface sputtering yield.This results in thickening the composition variation layer, particularly for Cr-rich phases.However, it is crucial to note that He bubbles, acting as barriers to He diffusion, can only form within a few hundred nm beneath the surface [33], thus limiting the depth of morphological modification.This reasoning may clarify why the composition change in Phase 1 extends deeper than the morphological modification at 920 • C. Given the modified layer is few µm thick, the distinct phases in the substrate remain identifiable by XRD even after He plasma irradiation, as shown in figure 7.No new phase is observed after He plasma irradiation at 300 • C.However, new peaks emerge with an increase in irradiation temperature beyond 600 • C.These additional peaks indicate a BCC structure with an a 0 larger than that of the original RMCA.By integrating results from EDS analysis, these new peaks can be attributed to the combined peaks of W-rich and Ta-rich binary alloys, considering that W-Ta binary alloys share the same structure and possess similar a 0 .The broadened full width at half maximum of these new peaks implies the fine-structure formation of W-Ta binary alloys, which aligns with the SEM results.As the fuzz-like surface is likely to be texture-free, the intensity of the new peak correlates with the fraction of W-Ta binary alloys in the diffraction volume.Consequently, the peak intensity of the W-Ta binary alloys increases proportionally with the thickness of the modified layer, which increases with elevating irradiation temperature.

Conclusions
This study subjects a multi-phase W-Ta-V-Cr RMCA consisting of a W-rich phase and two Cr-rich phases to lowenergy He plasma at varying temperatures, reaching a fluence of approximately 5.0 × 10 25 m −2 followed by comprehensive post-irradiation microstructural and chemical analyses.The key findings are summarized as follows: (I) A fuzz-like surface is observed on both W-rich and Crrich phases after irradiation at 600 • C to a fluence of 6.1 × 10 25 m −2 .As the irradiation temperature increases to 920 • C, the surface of the Cr-rich phases changes into a pore-like structure, while the surface of the W-rich phase retains a fuzz-like structure.These results imply that the temperature threshold for the appearance of a fuzz-like surface is lower for both phases compared to pure W.However, when evaluating fuzz thickness at an irradiation temperature of 920 • C, the W-rich phase exhibits a layer thinner than that observed in pure W irradiated to a similar fluence.(II) Large amount of nano He bubbles is observed in the fuzz tendrils of W-rich phase after He plasma irradiated at 920 • C and a fluence of 5.0 × 10 25 m −2 , which implies that the growth and rupture of He bubbles may also be the mechanism for fuzz growth on the W-rich W-Ta-V-Cr RMCA.(III) The analysis reveals a reduction in V and Cr content with increasing irradiation temperature, which may indicate a higher V and Cr sputtering yield during He plasma irradiation.As irradiation temperature increases to 920 • C, the surface composition of both phases mainly consists of W and Ta, transforming the W-rich and Cr-rich phases into W-rich and Ta-rich binary alloys, respectively, with trace amounts of V and Cr.
These insights into the structural and compositional alterations on the surface of the multi-phase W-Ta-V-Cr RMCA under low-energy He plasma irradiation highlight the challenges for developing an RMCA suitable for divertor target application in the perspective of plasma-materials interaction.Although the W-rich mono-phase W-Ta-V-Cr RMCA exhibits better resistance to fuzz layer thickening, concerns arise due to the reduction in V and Cr content and the thicker modification layer observed in segregation mimicked by Cr-rich phases, casting doubt on its potential as divertor target materials.Nevertheless, the compositional flexibility inherent in RMCA design offers a promising gateway for further exploration to address these challenges.

Figure 1 .
Figure 1.Multi-phase W-Ta-V-Cr RMCA in the as-casted state.Figures (a) and (b) are BSE images; (c)-(f ) are W, Ta, V, and Cr distribution based on EPMA mapping analysis, respectively.

Figure 2 .
Figure 2. XRD pattern for multi-phase W-Ta-V-Cr RMCA in the as-casted state.

Figure 4 .
Figure 4. Bright-field (a) underfocused and (b) overfocused TEM image of Ta-rich fuzz tendrils from specimen irradiated at 920 • C to 5.0 × 10 25 m −2 ; (c) bubble size distribution of Ta-rich fuzz tendrils based on projected area frequency.

Figure 5 .
Figure 5. Fuzz layer of pure W after He plasma irradiation with He ion energy of 90 eV at 920 • C to a fluence of 5.0 × 10 25 m −2 ; the specimen is 25 • tilted.

Table 1 .
Compositions of different phases in multi-phase W-Ta-V-Cr RMCA analyzed by EPMA.

Table 2 .
Lattice parameters of different phases obtained from XRD and DFT results.

Table 3 .
Surface compositions of multi-phase W-Ta-V-Cr RMCA after He plasma irradiation.