Characterisation by phase mappings of microstructural-thermal-mechanical properties in equimolar refractory high-entropy alloys with reduced neutron cross-section

Abstract

High-entropy alloys (HEA) hold promising potential as advanced technology fuel cladding materials for nuclear fission reactors. The HEAs typically exhibit low thermal conductivity, influencing substantially thermal spikes caused by nuclear collisions. In this framework, we screened over fifteen million combinations of quaternary and quinary equimolar HEAs to select the best alloy candidates for lower thermal neutron absorption cross-section combined with propensity to form a single-phase solid solution at high temperatures. Three of these HEAs NbZrTiMo, NbZrTiVMo, and NbZrTiV were arc-melted and characterised after thermal annealing at 1200 °C for 100 h. While a single-phase field was not achieved, each alloy exhibited a predominant bcc phase. We employed a unique combination of co-located advanced mapping techniques, including scanning electron microscopy, time-domain thermoreflectance (TDTR), and nanoindentation. High-resolution TDTR mapping was integrated with conventional mapping techniques (SEM, EDS, EBSD, and nanoindentation) to produce a micrometre-scale profile of the material properties. This multi-technique approach enabled a detailed characterisation of each phase, covering aspects such as phase size, morphology, distribution, crystalline orientation, chemical composition, thermal conductivity, nanohardness, and elastic modulus. The insights gained from this comprehensive characterisation provide a strong foundation for further HEAs optimisation, including efforts to enhance beneficial phases and suppress undesired ones.