First-principles investigation of W
V and WMo alloys as potential plasma facing materials (PFMs) for nuclear application
Highlights
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First-principles calculations of the BCC W
V and WMo systems were investigated. - •
Both systems are thermodynamically favourable and mechanically stable at 0 K.
- •
The Hv of BCC W1-xMox and W1-xVx was predicted using an ab initio technique.
- •
Mo reduced the hardness while up to 15 at.% V composition sustained high hardness of solid solutions.
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The VCA tool is reliable for design and product development of advanced W-based alloys.
Introduction
Despite its key concerns such as environmental impact, safety and raw materials supply limitations, nuclear fusion still has a potential to become a source of safe and clean power generation, thus rendering nuclear power as a solution for future energy generation. However, with better selection of materials closer to the fusion reaction prone to becoming radioactive, such that they have short half-life, then the issue of disposing radioactive waste can be minimized or eliminated [1]. Plasma-facing materials (PFM) and plasma facing components (PFC) are exposed to the harsh nuclear fusion power plants environments as they act as the interface between the plasma and the material part [1,2]. Hence the operational conditions in such extreme environment require materials that can withstand high heat flux and surface neutron irradiation. Such a combination of demand poses a stiff challenge in terms of material performance. Consequently, great effort has been dedicated to search (theoretical and experimental) for suitable materials with high thermal conductivity, low sputtering yield and sufficient mechanical properties even under neutron irradiation. Due to their supreme high temperature properties such as high melting temperature, good thermal conductivity, high creep resistance, high temperature strength and good erosion resistance, tungsten (W) and tungsten based materials are found to be amongst the frontrunners as potential candidates for various fusion applications [[2], [3], [4]]. However, prior to application of such materials certain properties still need to be enhanced to improve their performance. The key properties that need to be improved are the low temperature ductility and fracture toughness as well as the ductile-to-brittle transition temperature (DBTT) while maintaining high temperature properties. One of the methods to improve the ductility and DBTT is through solid solution alloying. In many studies, the first-principles calculations have demonstrated the capability to investigate solid solution alloying in various systems [[5], [6], [7], [8]] and was successful in predicting data that supported the existing experimental data.
Over the past decades, density functional theory (DFT) based methods have become a useful and viable tool to investigate behaviour of crystalline materials. These methods aid in facilitating the development of advanced materials by providing guidelines for design of new materials and give physical mechanistic insight into the origins of experimentally-derived trends. This is attributed to massive development efforts complemented by exponential increase in computational power. Consequently, the DFT-based first-principles calculations have demonstrated the capability to generate results with high level of accuracy and comparable to experimental data [[5], [6], [7], [8]]. The obtained materials data do not only provide insights about materials behaviour at electronic or atomic scale but can also be fed into a range of length and time scales modelling and simulation techniques capable of predicting materials performance at engineering scale. At this scale, the predictions of the macroscopic mechanical properties, such as flow stress, work hardening, and fatigue behaviour, in metals or other materials undergoing dislocation mediated plastic flow, is key [9].
In this study, a first-principles based virtual-crystal approximation (VCA) solid-solution unit-cell approach is used investigate the effect of alloying on various properties of W. It is well-known from metallurgy that solid-solution may have softening or hardening (strengthening) effect while in other systems induces phase transformation [5]. The WV and WMo binary solid solution systems were chosen since they present 100% solubility [10,11], although in the current work the focus is limited to 50 at.% solute. A random solid solution (SS) with BCC crystal structure is represented by a model shown in Fig. 1. Ideally using this model, W-X (X = V, Mo) binary solid solution may be formed when an atom of W or X occupy randomly any position in the crystal, thus only changing the composition. In this manner, correct symmetry and crystal structure are maintained as they are both critical to predicting structural and elastic properties, with the latter linked to mechanical properties reported in experimental data. The composition can be varied by representing atoms as a fractional composition, A1-xBx. The details of the calculation for the elastic constants are presented in previous studies [5]. Unit cell approach allows for small composition variation WV and WMo BCC SS. For further clarification, the full details of this method are described elsewhere [5]. Current approach differs slightly from earlier ab initio calculations studies conducted on similar materials in that supercell and ordered crystal structures were used [12].
Section snippets
Methodology
The ab initio modelling work was carried out using the CASTEP module within the Materials Studio software package [13]. The CASTEP code, a first principles quantum mechanical based programme for performing electronic structure calculations within the Hohenberg-Kohn-Sham density functional theory (DFT) [14,15], was used within the generalized gradient approximation (GGA) formalism [16] to describe the electronic exchange-correlation interactions. We used the recent Perdew-Burke-Ernzerhof (PBE) [
Phase stability
As shown in Fig. 2(a), the lattice parameter decreases with increase in V composition as expected since the metallic atomic radius of V is smaller than that of W. However, this decrease in lattice parameter is linear but deviates negatively from the Vegard's rule shown by the dotted line. Nonetheless, the predicted trend as well as the negative deviation from Vegard's law are both in agreement with experimental observations [10]. In addition, the predicted lattice parameter of 3.181 Å for
Conclusion
Both systems considered in this study are thermodynamically favourable and mechanical stable at 0 K as characterized by negative mixing enthalpy and C′ > 0. The Vickers hardness of BCC W1-xMox and W1-xVx solid solutions predicted for the first time using an ab initio technique are presented. Introduction of Mo significantly reduces the hardness of W while the introduction of up to 15 at.% V composition seems to sustain high hardness complimented by slight increase in ductility but A is reduced.
Declaration of Competing Interest
No conflict of interest exists.
Acknowledgements
The authors are grateful to the support of Department of Science and Innovation (DSI) through Center for High Performance Computing for their computational resources.
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