Non-uniform He bubble formation in W/W2C composite: Experimental and ab-initio study
Graphical abstract
Introduction
Materials considered for nuclear reactor environments must perform under extreme conditions, including simultaneous radiation damage, elevated temperature, and mechanical stress. Helium (He) is the by-product generated in fission [1] and fusion [2] reactions. Due to the extremely low solubility in most materials, He tends to coalesce to form gas stabilised voids or cavities (bubbles), significantly degrading the physical properties of the materials [3]; therefore, characterisation of the He behaviour in solids is one of the steps to predict the material performance under irradiation. Helium-ion implantation is the prevailing technique to mimic nuclear-reaction induced radiation damage of materials by creating radiation-damage-like lattice defects, allowing examination of the interaction of the injected He with structure imperfections [[4], [5], [6]]. Post-implantation microstructural analysis by transmission electron microscopy (TEM) of cavity density, size, and spatial distributions is used for determining the physical origins of mechanical property degradation. Up to now, helium-implanted tungsten (W) has been experimentally and theoretically probed for varying conditions of temperature and He fluence [7,8]. On the other hand, He implantation effects on W2C and multi-phase, polycrystalline W-based composites are generally unknown.
In a deuterium-tritium fusion reaction, plasma-facing materials (PFMs) must withstand high energy neutron bombardment and hydrogen isotope ion (plasma) exposure, combined by high thermal flux and continuous production of He through (n, p) and (n, α) nuclear reactions in PFMs. The contribution of He produced from neutron-induced reaction depends very much on the choice of the PFMs [9]. Metallic tungsten (W) and its carbides are some of the candidates proposed as fusion-relevant materials because of their high melting point, high thermal conductivity, low physical sputtering yield and low hydrogen isotope retention [10,11]. After five full-power years of neutron irradiation of W, we can expect to get 30 atomic parts per million (appm) He through neutron capture and α particle emission (n,α) [9], with an additional 600 appm due to tritium decay with the assumption of 1 at.% tritium retention [12]. As also transmutation of carbon under neutron irradiation results in He production [9], the carbon present in bulk might even further increase the predicted He concentration in the material. Helium will influence the macroscopic properties of the material, such as tensile strength, creep and fatigue behaviour or swelling [13,14], while emitted α-particles, having energies of the order of MeV, will cause displacement damage in the crystal lattice [9,15]. At elevated irradiation temperature, rapid He accumulation along the grain boundaries and dislocations will promote inter-granular fracture known as helium embrittlement, leading to pronounced surface blistering and exfoliation [15,16]. Additionally, sintered W is characterised by high porosity and low recrystallization temperature. To address this issue, the incorporation of transition-metal ceramic particles (TiC, TaC and W2C) into the W matrix was suggested to suppress recrystallization and consequent grain growth at high temperatures [17], [18], [19], [20], [21].
The primary objective of the present research was to provide experimental insight into the behaviour of W/W2C composite under helium implantation and its effect on the microstructure of the multi-phase material, micrometres below the surface. The sample consolidated by field-assisted sintering technique (FAST) was He-implanted at room temperature (RT) and subsequently annealed at 1873K for 20 min to mimic the short temperature excursions above 1500K that can be expected in high heat loads and particle flux areas in the fusion reactor [22]. The microstructure features of the He-irradiated samples were examined by scanning electron microscopy (SEM), while transmission electron microscopy (TEM) was used to assert the He-implantation-induced defects. The experimental observations of He bubble formation in the W/W2C composite were complemented by first-principles based density functional theory (DFT) calculations to establish a fundamental understanding of He clustering, migration and dissolution in W and W2C.
Section snippets
Starting materials
The W/W2C composite was densified by FAST under moderate vacuum (30 – 50 Pa), as described in detail in [21]. The starting materials were commercially available pure tungsten powder (99.9 % purity with particle size < 1.5 µm, Global Tungsten & Powder) and submicron WC particulates (99 % purity with particle size in the range 0.15-0.2 µm, Sigma-Aldrich); the impurity contents of both powders, as provided by the supplier, are listed in the Supplementary information file (Table S I). In the final
Initial composite microstructure
The detailed microstructure analysis and phase composition of the starting composite material, the as-sintered W/W2C composite, is reported in [21]. The results of EBSD analysis of the native, non-irradiated and non-annealed W/W2C composite microstructure are summarised on Fig. 2. The composite has a bimodal grain size distribution: the W grains have a size range of 1.0 - 26.7 µm with a mean size of 7.5 µm in diameter (standard deviation (SD) 4.6), while the W2C grain size ranges from 0.4 µm to
Conclusions
The implantation of 4He into W/W2C at room temperature, with subsequent annealing, was performed to study bubble evolution at a given He content. After annealing at 1873 K for 20 min, visible bubbles, which can be observed in cross-section SEM, appear in the sub-surface region at a depth of ∼1.2 µm. The detailed TEM analysis of as-implanted and post-annealed sample shows bubbles throughout the as-prepared lamella, but surprisingly not in W2C grain. These experimental observations were the
Authors and their contribution list
The authors confirm contribution to the paper as follows: A. Š.: experiment design, manuscript draft, SEM and EBSD, and data curation; L. S.: DFT calculation and interpretation, manuscript drafting; S. M.: He-implantation experiment, design, simulations, and interpretation; M. K.: He-implantation experiment, design, and interpretation; J. Z.: TEM analyses and interpretation of the data; C. H. L.: electron microscopy and manuscript editing; G. D.: experiment design and manuscript editing; T. H.:
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Katharina Hengge is greatly acknowledged for her help with the FIB preparation of the TEM samples. Barbara Šetina Batič is acknowledged for her help with the EBSD analyses. L.S. and T.H. acknowledge fruitful discussions with Prof. Dr. J. Neugebauer. A.Š. acknowledges financial support within the EUROfusion education & training scheme. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme
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