Influence of grain boundaries on the radiation-induced defects and hydrogen in nanostructured and coarse-grained tungsten

Abstract

We have studied the influence of grain boundaries (GBs) on the radiation-induced defect evolution and on H retention at 300 K, both experimentally and by computer simulations. For this purpose, coarse-grained tungsten (CGW) and nanostructured tungsten (NW) samples were implanted with H and C ions at energies of 170 keV and 665 keV respectively. Three different sets of experiments were carried out: (i) H single implantation, (ii) C and H co-implantation and (iii) C and H sequential implantation. Computer simulations were performed by using the Object Kinetic Monte Carlo (OKMC) methodology, which was parameterized by new and pre-existing Density Functional Theory (DFT) data. The three sets of experiments were simulated in monocrystalline tungsten (MW) and NW, resulting that (i) GBs have a clear influence on the amount and distribution of vacancies, being the vacancy concentration larger in NW than in MW samples, (ii) H retention is highly influenced by both the GBs themselves and the vacancy concentration, (iii) the size of H_nV_m clusters is slightly influenced by the presence of GBs and (iv) it can be inferred, from the comparison between experimental and computational results, that GBs act as preferential paths for H diffusion.

Introduction

Due to its outstanding properties, tungsten is considered one of the most promising candidates as plasma-facing material (PFM) in future fusion reactors, both in magnetic (MC) and in inertial confinement (IC) approaches [1], [2], [3], [4], [5]. In principle, W is supposed to fulfill the majority of the highly demanding requirements, such as transient temperatures and stresses which will take place in a laser fusion chamber [6], [7]. However, light species such as hydrogen [8], [9] and helium [10], [11] tend to nucleate in defects resulting in detrimental effects such as cracking, exfoliation or blistering, which is unacceptable for a PFM. Therefore, there is a need to develop materials capable to withstand the expected harsh conditions (extreme irradiation and large thermal loads) taking place in this kind of reactors.

In the case of direct drive targets in IC, H ions are expected to reach the PFM at high enough energies to produce Frenkel pairs (FPs), i.e., vacancies (Vs) and self-interstitial atoms (SIAs) [3]. According to the literature, H diffusivity in W strongly depends on the presence of vacancies, since H atoms get easily trapped at them with a high binding energy [12], [13], [14], [15], [16]. Nanostructured materials have a large density of GBs which, depending on the irradiation conditions, have a large influence on the distribution of both FPs and light species such as H. FPs have been described to annihilate at GBs under certain conditions [17], [18], [19]. However, the H behaviour at GBs is still unclear. On the one hand, some authors showed that H atoms experience enhanced diffusion along GBs with a migration energy lower than in the bulk [20], [21]. Thus, GBs might act as preferential diffusion paths for H atoms [22]. On the other hand, other works report that the migration energy along the GB is higher than in the bulk [23]. Therefore, the physical mechanism for H behaviour at GBs is not well understood. Another important point about H trapping at GBs is whether H gets trapped at vacancies formed in the GBs or in their vicinity, or at GBs themselves. The fact that the formation energy of vacancies in the GB region [20], [21] is lower than that in the bulk [24] may facilitate the trapping of H atoms at vacancies formed at the GBs. Thus, the role of vacancies in the case of the observed H concentration enhancement at the GB region is not clear [23], [25].

In this study, we investigate the H behaviour in three varieties of tungsten with different GB densities: nanostructured tungsten (NW) with a large GB density, coarse-grained tungsten (CGW) with a lower GB density and monocrystalline tungsten (MW), with no GBs at all. The goal is to contribute to a better understanding of the role of GBs on the H behaviour in W at a temperature of 300 K. For this purpose, we compare experimental measurements to theoretical results calculated with the MMonCa [26], [27] code, an Object Kinetic Monte Carlo (OKMC) simulator. The code is parameterized by a Density Functional Theory (DFT) database, which provides the required input for the OKMC simulations applied to NW and MW subject to different irradiation scenarios: (i) H single implantation (NW-H and MW-H), (ii) C and H co-implantation (NW-Co-CH and MW-Co-CH) and (iii) sequential C and H implantation (NW-Seq-CH and MW-Seq-CH), see Table 1 . The simulation conditions were selected to mimic the experiments. The H and V distribution as well as the type and size of H_nV_m clusters are analyzed in detail in order to assess the influence of the GB density on them. The simulation results show that the density of radiation-induced vacancies is larger in NW than in MW samples. The reason stems from the fact that FPs exhibit a much higher recombination rate in MW than in NW samples. The resulting higher vacancy concentration in NW compared to MW leads to a higher H retention. Finally, by comparing the experimental and computational data, we conclude that GBs act as efficient migration paths for H.