Recently, new progress has been made by the researchers from Institute of solid state physics, collaborating with the researchers from Insitute of Plasma Physics and Institute of Modern Physics. The new progress has been publised in Acta Mater. 66, 172-183 (2014).
Tungsten has received particular attention as the most promising candidate for various plasma-facing components, such as the divertor plate in the International Thermonuclear Experimental Reactor. It possesses many good properties, such as high melting point, good resistance to neutron irradiation, and low sputtering corrosion. The main drawback associated with technological applications of W is its high ductile-to-brittle transition temperature. To improve the material ductility, the alloying of W with some ductile components, e.g. Re, Ta, Ti and V, has been proposed. However, during its lifetime as the divertor, tungsten will be exposed to high-energy, high-flux neutrons. The high-energy neutron irradiation gives rise to a large number of point defects, such as vacancies and self-interstitial atoms, in the lattice. These point defects interact strongly with solute atoms, changing their migration and aggregation properties and ultimately leading to macroscopic changes in the material properties, such as hardening and embrittlement. Thus, understanding the fundamental mechanisms of solute-point defect interactions has an important role in developing predictive models of changes in the microstructure and mechanical properties of metals for engineering applications in fission and fusion reactors. We performed a series of first-principles calculations to quantify the intrinsic properties of transition metal (TM) solutes and their interactions with point defects in W, including vacancies and <111>-crowdions. This work provides good explanations for recent experimental results on the influence of solute on radiation response and might aid future material design regarding the choice of alloy composition. We find that the early TM elements do not segregate together while the late elements tend to accumulate to form small clusters in dilute W alloys. The solute-point defect interactions are mostly attractive with a few exceptions, and can be well understood in terms of the combination of, and competition between, electronic effects and strain-relief effects, which are characterized by the solute electronegativity and atomic size, respectively (see Fig. 1). Solute atoms with larger electronegativity more favorably bond to the vacancy and the smaller ones prefer to bind to the <111>-crowdion, and vice versa (See Fig. 2). The present results, together with previous experimental results, suggest that Re might be a relatively suitable alloying element compared to other possible candidates, and Ta seems suitable for adding in W-Re alloy to adjust the concentration of Re and Os.
The above investigations are supported by the National Magnetic Confinement Fusion Program, National Natural Science Foundation of China, and Research Program of Chinese Academy of Sciences.
Fig. 1. Solute-vacancy (a) and solute-<111>-crowdion (b) binding energies of TM elements in W.
Fig. 2. The correlation between the solute-point defect binding energy and the Pauli electronegativity ((a) and (c)) and the atomic volume ((b) and (d)).