Development of Novel Low Activation Refractory High Entropy Alloys for Nuclear Fusion Applications

Nuclear fusion power has the potential to meet the growing global energy demand. However, for commercial fusion energy to be realised, serious reactor design concerns need to be addressed. Suitable materials for the first wall of the reactor need to be able to withstand harsh conditions including: extreme heat loads (0.25-0.5 MWm-2), significant transient heat fluxes between 10-20 MWm-2, and high energy neutron bombardments (14 MeV). A novel class of materials, high entropy alloys, have recently been identified to have interesting properties including high temperature capabilities and the self-healing effect upon irradiation.
Preliminary findings, reported in this thesis, demonstrated the excellent radiation damage resistance of the refractory high entropy alloy V2.5Cr1.2MoWCo0.04 in the as-cast condition. This metastable BCC phase showed no microstructural difference upon 5 MeV Au 2+ ion implantation at room temperature. Despite this, after a prolonged heat treatment at intermediate temperatures the phase transformation to a mixture of phases including a BCC, tetragonal, and an orthorhombic phase was observed.
Computational alloy design tools were implemented for the development of novel low activation refractory high entropy alloys for consideration as plasma-facing materials. High entropy alloy empirical parameters for the prediction of a BCC solid solution, and Thermo-Calc simulations for the reduction of intermetallic phases were used for the design of five novel refractory low activation high entropy alloys: V35Cr33Mn2Fe12W18, V46Cr3Mn1Ta10W40, V21Cr18Mn9Fe17Ta20W15, V36Cr18Fe8Ta17W21, and V26Cr17Fe32Ta25. The thermal stability of these alloys was assessed, and the elemental segregation of the alloys could not be reduced due to insufficient homogenisation time. Additionally, only VCrMnTaW and VCrMnFeW displayed BCC phases which were retained upon thermal treatment. From these results, the deficiencies of the design tools were highlighted. Vickers microhardness studies also showed a strong correlation between atomic size difference and as-cast hardness values.
The most promising alloy, V35Cr33Mn2Fe12W18 displayed strong microstructural similarities to V2.5Cr1.2MoWCo0.04, so potential for radiation resistance in the as-cast condition may result. Further high temperature X-ray diffraction experiments on V35Cr33Mn2Fe12W18 showed the formation of a σ phase at 750 °C which was irreversible upon cooling to room temperature.
In this work, the investigation of novel low activation refractory alloys and their potential as plasma-facing materials is assessed and insight is given into the design process of high entropy alloy systems with BCC solid solutions.