In the present work, a series of high-entropy alloys based on refractory metals Hf, Nb, Ta, Ti, V, and Zr with various compositions have been systematically investigated. Our study revealed that a bcc single-phase solid solution of a Hf-Nb-Ta-Ti-V-Zr system is thermodynamically stable only at high temperatures above 1000 °C. At lower temperatures, the phase separation into disordered bcc phases with slightly different chemical compositions occurs. Despite the phase separation, a single-phase random solid solution can be saved at room temperature as a metastable phase by rapid cooling of the sample from high temperature. The microstructure of a single-phase metastable random solid solution was characterized and compared with the microstructure of the as-cast state. Furthermore, the mechanical properties of annealed and as-cast alloys were compared. Interestingly, both states exhibit comparable mechanical properties. It indicates that from the point of view of practical applications, a mechanical mixture of disordered bcc solutions is as good as single-phase random solid solution.

Aluminium—boron carbide metal matrix composites (Al-B₄C MMCs) belong to the class of materials extensively used in the nuclear industry as a thermal neutron absorber in spent fuel casks. This article investigates a novel production method of Al-B₄C MMCs—Friction Stir Additive Processing (FSAP)—as an alternative production method to casting or sintering. FSAP is derived from friction stir welding, which can be used to local modifications of microstructure, or it can be used to incorporate the second phase into the processed material. During this study, a variant of FSAP for MMC production was proposed, and its mechanical and thermal neutron absorbing properties have been investigated. Further, the influence of neutron irradiation on mechanical properties has been studied. Results show that FSAP can successfully produce Al-B₄C MMCs with 7 mm thickness. Neutron irradiation causes only a slight increase in hardness, while its effect on tensile properties remains inconclusive.

Low-alloyed zirconium alloys are widely used in nuclear applications due to their low neutron absorption cross-section. These alloys, however, suffer from limited strength. Well-established guidelines for the development of Ti alloys were applied to design new two-phase ternary Zr alloys with improved mechanical properties. Zr-4Sn-4Nb and Zr-8Sn-4Nb alloys have been manufactured by vacuum arc melting, thermo-mechanically processed by annealing, forging, and aging to various microstructural conditions and thoroughly characterized. Detailed Scanning electron microscopy (SEM) analysis showed that the microstructural response of the alloys is rather similar to alpha + beta Ti alloys. Duplex microstructure containing primary alpha phase particles surrounded by lamellar alpha + beta microstructure can be achieved by thermal processing. Mechanical properties strongly depend on the previous treatment. Ultimate tensile strength exceeding 700 MPa was achieved exceeding the strength of commercial Zr alloys for nuclear applications by more than 50%. Such an improvement in strength more than compensates for the increased neutron absorption cross-section. This study aims to exploit the potential of alpha + beta Zr alloys for nuclear applications.