*In-Situ* **Helium Implantation and TEM Investigation of Radiation Tolerance to Helium Bubble Damage in Equiaxed Nanocrystalline Tungsten and Ultrafine Tungsten-TiC Alloy**

**Osman El Atwani 1,\*, Kaan Unal 1, William Streit Cunningham 2, Saryu Fensin 1, Jonathan Hinks 3, Graeme Greaves <sup>3</sup> and Stuart Maloy <sup>1</sup>**


Received: 3 December 2019; Accepted: 3 February 2020; Published: 10 February 2020

**Abstract:** The use of ultrafine and nanocrystalline materials is a proposed pathway to mitigate irradiation damage in nuclear fusion components. Here, we examine the radiation tolerance of helium bubble formation in 85 nm (average grain size) nanocrystalline-equiaxed-grained tungsten and an ultrafine tungsten-TiC alloy under extreme low energy helium implantation at 1223 K via in-situ transmission electron microscope (TEM). Helium bubble damage evolution in terms of number density, size, and total volume contribution to grain matrices has been determined as a function of He<sup>+</sup> implantation fluence. The outputs were compared to previously published results on severe plastically deformed (SPD) tungsten implanted under the same conditions. Large helium bubbles were formed on the grain boundaries and helium bubble damage evolution profiles are shown to differ among the different materials with less overall damage in the nanocrystalline tungsten. Compared to previous works, the results in this work indicate that the nanocrystalline tungsten should possess a fuzz formation threshold more than one order of magnitude higher than coarse-grained tungsten.

**Keywords:** nanocrystalline tungsten; alloy; in-situ electron microscopy; helium bubbles; radiation tolerance

### **1. Introduction**

Fusion energy applications require materials to possess superior properties able to withstand extreme environments of high thermal loads, transient heat fluxes, high fluxes of helium (He) plasma particles, and fast neutrons [1]. Furthermore, neutron irradiation can also lead to solid transmutation products and He gas formation [2]. These challenging conditions can exacerbate damage in materials facing the plasma. Tungsten is currently one of the best candidates for the divertor armor and is to be used in ITER (originally the International Thermonuclear Experimental Reactor), but has demonstrated several drawbacks (in terms of microstructural and mechanical property changes) when exposed to extreme irradiation conditions [3–5]. One of the most studied microstructural changes is the formation of fuzz: a high density of dendritic structures that can develop on the surface of tungsten at high temperatures and He plasma fluxes [3]. While the precise mechanisms of fuzz formation are still under investigation, it is understood to occur due to a high density of He bubbles forming near the tungsten surface [6,7]. Several routes have been suggested in the quest to design irradiation resistant materials capable of mitigating microstructural changes and fuzz formation in a plasma facing material. Refining the grain size to the nanocrystalline regime, and thereby increasing the density of interfaces that act as defect sinks, has been proposed as a method to mitigate these changes [8,9]. Particle reinforcement, where the particles act as defect annihilation sites, is also a suggested route to increase the irradiation resistance of the material and enhance its mechanical properties and thermal stability [10–12]. The hypothesis is that materials that are radiation tolerant to bubble formation should lead to an increased He<sup>+</sup> fluence threshold for fuzz formation (at fuzz formation conditions) if fuzz formation depends on the same conditions as proposed in the literature [6]. El Atwani et al. studied nanocrystalline tungsten with a 35 nm average grain size [13] and an ultrafine tungsten alloy with TiC dispersoids (W-TiC 1.1%) [14] under heavy ion irradiation (to mimic fast neutron damage). Both types of materials have shown enhanced irradiation tolerance when compared to commercially-available tungsten. Damage due to loop formation was shown to be minimal in the nanocrystalline tungsten and decreased with time (due to loop annihilation and dissolution under heavy ion cascades) in the W-TiC (1.1%). The response of these materials, however, due to low energy He<sup>+</sup> implantation is still unknown. The testing of severe plastically deformed (SPD) nanocrystalline and ultrafine tungsten, where both elongated ultrafine (<500 nm) and nanocrystalline (<100 nm) grains coexist under low energy He<sup>+</sup> implantation and high temperatures (1223 K), showed a trend in both damage and He bubble formation as a function of grain size [15]. Grains of less than 60 nm (in elongated grains, size was defined to be the distance across the middle in the narrower direction) demonstrated a high irradiation tolerance to bubble formation and grains of ~35 nm were shown to possess minimum damage.

Here, we present a detailed study investigating the radiation tolerance to bubble formation under low energy (2 keV). Implantation of He<sup>+</sup> at a high temperature (1223 K) (similar conditions to previous SPD work [15]) of the equiaxial nanocrystalline tungsten with an average grain-size 85 nm (referred to as NCW herein) was formed by magnetron deposition, and the tungsten-TiC (1.1%) (labelled as W-TiC (1.1%)) formed by hot iso-static pressing of tungsten powders with TiC dispersoids. The experiments were performed in-situ within the transmission electron microscope (TEM) in the MIAMI-2 system (Microscope and Ion Accelerators for Materials Investigations) at the University of Huddersfield [16]. Bubble formation, distribution, and evolution in the materials were studied by quantifying bubble density, average size, and total change in volume (in the grain matrices of the material due to bubble formation) as a function of implantation He<sup>+</sup> fluence. Together with the previous work in SPD tungsten, the results constitute a comparable set of bubble/damage evolution profiles and distribution for different ultrafine and nanocrystalline grades (pure and alloyed) that should assist in the understanding of nanocrystalline and ultrafine material behavior under He<sup>+</sup> implantation and the design of materials with higher irradiation resistance for fusion applications.

#### **2. Materials and Methods**

The TEM sample (~100 nm thickness) preparation methodology and detailed morphology of the samples prior to implantation have been described in detail previously [13,14]. The implantations were incident at 18.7◦ from the surface normal with fluxes of 8.8 <sup>×</sup> <sup>10</sup><sup>13</sup> and 6.8 <sup>×</sup> <sup>10</sup><sup>13</sup> ion.cm<sup>−</sup>2.s−<sup>1</sup> for the NCW and W-TiC (1.1%), respectively, to a total He<sup>+</sup> fluence of 3.6 <sup>×</sup> 1016 ion.cm<sup>−</sup>2. Displacement damage and He distributions were found by the Kinchin-Pease model in the Stopping Range of Ions in Matter (SRIM) Monte Carlo computer code (version 2013) [17], using 70 eV as a displacement threshold [18]. He bubbles were characterized using bright-field TEM imaging at under-focused (bubbles appear bright) conditions [19]. For every He<sup>+</sup> fluence reported, different grains (about 7 grains in the W-TiC (1.1%) sample and 15 grains in the NCW sample) in every sample were quantified at different He<sup>+</sup> fluences. In every grain, several small circles (3–8 depending on the grain size) of the same area were drawn randomly and the number of He bubbles and their corresponding sizes were found. Averages were calculated from the results in all grains. A detailed illustration of the quantification process was published in the supplemental of reference [20]. Figure 1 shows a schematic diagram of the sample, implantation conditions and overlapping ion and displacement damage distributions. The projected He peak is ~12–15 nm, which is much less than the nominal thickness of the film (~100 nm). This is an important consideration when comparing and illustrating phenomena (such as bubble formation and defect annihilation/recombination) that can be affected by the proximity to free surfaces. However, surface proximity effects becomes negligible in nanocrystalline and ultrafine grains when the grain boundary to surface ratio approaches a value of 1 [21]. The grain boundary to surface ratios on the equiaxed 85 nm tungsten and the W-TiC (1.1%) were measured taking into consideration the upper and the bottom surface of the foils and assuming edge-on grain boundaries, and were found to be 3.8 and 0.3, respectively. Interfaces of TiC particles and the tungsten matrix inside the grains were not considered in the case of the W-TiC (1.1%) grade. These values are relatively high compared to fine- or coarse-grained grades. For example, a corresponding grain boundary to surface ratio for a fine-grained tungsten grade with an average grain size of ~2 μm would be ~0.04 [13]. Due to the shallow depth of implanted He in this work, no considerable variations in proximity effects are expected between the two grades investigated.

**Figure 1.** A schematic showing the sample shape, implanted He (red curve), and displacement per atom (dpa) damage (blue curve) distributions of 2 keV He<sup>+</sup> (as determined by SRIM). The thickness of the sample is magnified for the purpose of overlapping the implanted He and displacement damage distributions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

#### **3. Results and Discussion**

#### *3.1. He Bubble Formation and Growth*

At the temperature studied in this work (1223 K), vacancies, interstitials, and small He–vacancy complexes are mobile and hence He bubbles are expected to form [22]. The He bubble density, average size, and the consequential change in volume in the grain matrices due to He bubble formation, as a function of He<sup>+</sup> fluence, not only reveal the total damage (as defined here) in the sample but also demonstrate the damage evolution profiles and elucidate the variations in the response of the two materials. Figures 2 and 3 show bright-field TEM images taken under Fresnel conditions (under-focused conditions where bubbles appear bright) of the NCW and W-TiC (1.1%) as a function of He<sup>+</sup> implantation fluence and indicate He bubble formation. In both cases (Figures 2b and 3b), He bubble formation was more evident at the grain boundaries than in the bulk grain matrices. As the fluence was increased, grain boundaries demonstrated larger He bubble formation than that in the grain matrices. While this demonstrates efficient He trapping by the grain boundaries, the mechanisms

involved in this process can be complicated [23]. At the He<sup>+</sup> energy in this work (2 keV), the number of vacancies generated per ion is 0.5 for a 70 eV displacement threshold [18] according to SRIM calculations. Therefore, HenVm complexes of n/m larger than 1 are expected to dominate especially taking the inevitable dynamic annealing of vacancies into account. Such complexes have very large migration energies and are not expected to migrate [24]. Only complexes with migration energies close to that of vacancies (1.7 eV) [25] can contribute to He bubble formation at the grain boundaries under these conditions and these are mainly complexes with a higher vacancy content [24,26]. Interstitial-He trapping by the grain boundaries (2D trapping) can also occur and then He bubbles can form through trap mutation processes as well as via vacancy migration to the grain boundary [27]. The probability of trap mutation and He bubble formation on the grain boundaries can be high at this temperature due to higher gas pressure and the decrease in self-interstitial formation energy [28]. Such mechanisms are therefore expected to contribute to the observed formation of larger He bubbles at the grain boundaries; nevertheless, understanding large He bubble growth on grain boundaries requires further coordinated and complimentary experimental and modelling work.
