Damian Sobieraj

To design a proper material for fusion energy, improved properties can be achieved by varying the composition of alloys. Computational methodologies and tools are readily available to efficiently explore the vast compositional space. In this work, we combined Density Functional Theory (DFT) with the Cluster Expansion (CE) method to develop a predictive model, explored possible equilibrium phases in different systems with a small addition of Hf (1-5 at.%), and calculated the order–disorder transition temperature (ODTT) for each composition. A key finding is that the effect of Hf addition on ODTT is composition dependent. In some subsystems, the ODTT remains nearly unchanged with Hf addition, whereas in others, an increase in ODTT is observed even at low Hf concentrations. These results provide valuable guidance for future experimental studies aimed at optimizing thermodynamic stability and thermo-mechanical properties in irradiation-resistant materials.

Due to the extreme operating temperatures in nuclear fusion reactors, current divertor monoblock materials, tungsten (plasma-facing material) and CuCrZr (heat sink), exhibit a substantial mismatch in mechanical and thermal properties. Therefore, a thermal barrier interlayer is required to guarantee continuous operation. In this work, (CrVW)(1-x)(TaTi)xalloys (x = 10 and 20 at.%) were studied for application as interlayers in nuclear fusion reactors. Atomistic modelling using Monte Carlo simulations combined with a first-principles-based Cluster Expansion Hamiltonian has been performed as a function of temperature to predict the order-disorder transitions in the investigated high entropy alloys.Moreover, the experimental alloys were prepared by mechanical alloying followed by spark plasma sintering. Both alloy compositions display strong negative average chemical short-range ordering below 1500 K for the W-Ta and W-Ti chemical pairs, with an estimated transition temperature around 1300 K. Atomic structures obtained from Monte Carlo simulations for (CrWV)80(TaTi)20 indicate that the ordering observed in this composition is stronger than that for (CrWV)90(TaTi)10. Moreover, both systems exhibit a bcc structure, and heat-treated samples exhibit phase growth and a microstructure with four phases. The thermal diffusivity increases with temperature, reaching lower values than those of pure W and CuCrZr.

The realisation of fusion energy depends on the development of advanced materials for challenging environments. Rapid screening of prototype alloys using magnetron sputtering and high throughput characterisation is currently being applied to candidate W alloys with improved mechanical performance, reduced activation and tolerance to damage from neutron irradiation. Delamination of these films is a barrier to the application of this approach. In this study, delamination of W films was determined to be a result of high compressive residual stresses, and inadequate adhesion to the Si substrate. It was found that using a Ti or Cr interfacial layer increases adhesion, something that was strongly supported by interface W/Cr studies from first-principles calculations based on the Density Functional Theory (DFT). It is shown for all configurations that the interface binding energies between W and Cr are attractive with the W(110)/Cr(110) is predicted to be the most stable with a higher adhesive energy.

Self-passivating Metal Alloys with Reduced Thermo-oxidation (SMART) are under development for the primary application as plasma-facing materials for the first wall in a fusion DEMOnstration power plant (DEMO). SMART materials must combine suppressed oxidation in case of an accident and an acceptable plasma performance during the regular operation of the future power plant. Modern SMART materials contain chromium as a passivating element, yttrium as an active element and a tungsten base matrix. An overview of the research and development program on SMART materials is presented and all major areas of the structured R&D are explained. Attaining desired performance under accident and regular plasma conditions are vital elements of an R&D program addressing the viability of the entire concept. An impressive more than 10⁴-fold suppression of oxidation, accompanied with more than 40-fold suppression of sublimation of tungsten oxide, was attained during an experimentally reproduced accident event with a duration of 10 days. The sputtering resistance under DEMO-relevant plasma conditions of SMART materials and pure tungsten was identical for conditions corresponding to nearly 20 days of continuous DEMO operation. Fundamental understanding of physics processes undergone in the SMART material is gained via fundamental studies comprising dedicated modeling and experiments. The important role of yttrium, stabilizing the SMART alloy microstructure and improving self-passivating behavior, is under investigation. Activities toward industrial up-scale have begun, comprising the first mechanical alloying with an industrial partner and the sintering of a bulk SMART alloy sample with dimensions of 100 mm × 100 mm × 7 mm using an industrial facility. These achievements open the way to further expansion of the SMART technology toward its application in fusion and potentially in other renewable energy sources such as concentrated solar power stations.