Abstract
W-Cr-Y smart alloys are potential material candidates for plasma facing components due to their protective behaviour during the loss-of-coolant accident (LOCA), while maintaining beneficial properties of W during the normal operation of the fusion power plant. During plasma exposure, the lighter alloying elements are preferentially sputtered at the surface, but in case of a LOCA, the plasma quenches and sputtering stops and diffusion of the alloying elements to the surface becomes intensive. The diffusion of Cr to the surface due to alloying elements (Y, Ti) yields a protective oxide layer stopping the sublimation of WO3. The phase stability and short-range ordering of ternary alloys in W-Cr-Y(Ti) systems has been investigated, using combination of Density Functional Theory (DFT) and Cluster Expansion (CE) methods with Monte-Carlo (MC) simulations. It has been found out from the DFT calculations, that all pairs in the W-Cr-Y system have positive values of the enthalpy of mixing, while most of the Cr-Ti and Ti-W binary structures have negative enthalpies of mixing. The shift in the short-range order as a function of temperature between Cr and W has been predicted as a result of Y addition in W-Cr-Y alloys compared to W70Cr30, by around 400 K towards lower temperatures. A strong tendency towards clustering of Y has been observed even at elevated temperatures (1800 K). The decrease of the order–disorder transition temperature (ODTT) as a result of the Y addition has been observed, while the addition of Ti has not shown any significant changes in the ordering of W-Cr-Ti alloys compared to W-Cr alloy. Our MC simulations showed that for the W70Cr29Y1 alloy the enthalpy of mixing (Hmix) value is positive in the whole analysed temperature range. Free energy of mixing above 1000 K has been calculated from the first nearest neighbours approximation for W70Cr29Y1 and W70Cr29Ti1 alloys. The results of the present investigations provide an insight enabling for optimizing chemical composition of materials for future plasma facing components.
Highlights
Plasma-Facing-Materials (PFM) for the first wall of future fusion power plants are required to possess extraordinary properties while being able to withstand very high temperatures and radiation damage
The summation is performed over all clusters ω that are not equivalent to each other via symmetry operations applied to a bcc lattice, m(|ωs)|,n is the number of clusters equivalent by symmetry to the considered cluster, J|(ωs)|,n are the concentration-independent effective cluster interactions (ECIs), derived from a set of Density Functional Theory (DFT) calculations using the structure inversion method, and Γ(|ωs)|,n(σ) are the average correlation functions defined as a product of point functions of occupation variables on a specific cluster ω averaged over all the clusters ω that are equivalent by symmetry to cluster ω [40]
One can analyse the influence of the specific elements and their pair concentrations on the order–disorder transition temperature (ODTT), which allows us to find the optimal composition of an alloy with the lowest temperature of the disordered solid solution presence
Summary
Plasma-Facing-Materials (PFM) for the first wall of future fusion power plants are required to possess extraordinary properties while being able to withstand very high temperatures and radiation damage. Smart alloys are potential candidates for plasma facing material components due to their protective behaviour during the LOCA, while maintaining beneficial properties of W during the normal operation of the fusion power plant. Ti and Cr decay to activities below the LLW limit at least as quickly as W in this simulation (representing one of the most severe exposure conditions in DEMO), and so their use in a W-alloy is unlikely to cause any long-term waste issues in comparison to pure W. The force components were relaxed to 10−3 eV/Å, and the total energy convergence criterion was set to 10−6 eV/cell
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