Thermal Transport Properties of Ni-Containing High-Entropy Alloys

High-entropy alloys (HEAs) refer to alloys comprising five or more principal elements, each in relatively high concentrations (5-35 at.%), exhibiting unique properties arising from lattice distortion, such as phase stability, solid-solution hardening, and irradiation resistance. Especially, nickel-based HEAs are of great industrial importance in extreme environments involving intense radiation, which can be attributed to effective defect recombination due to their low thermal conductivity and enhanced electron-phonon coupling. Although understanding thermal transport behavior is crucial for managing heat flow in materials, reports on thermal conductivity of nickel-based HEAs are still limited compared to those on their mechanical properties. Furthermore, previous studies have reported a notable contribution of phonons to the thermal conductivity in solid solutions containing nickel, whereas the phonon thermal conductivity of most metals is significantly suppressed due to the electron-phonon scattering. In this study, we prepare pure nickel and four nickel-based alloys: NiCo, NiCoFe, NiCoFeCr, and NiCoFeCrMn. The electrical resistivity is determined by using the four-point technique, while the thermal conductivity is measured using time-domain thermoreflectance (TDTR) in the temperature range of 300 – 773 K, from which the electron and phonon contributions to thermal conductivity are evaluated through the Wiedemann-Franz law. Moreover, we quantitatively investigate the electron-phonon coupling parameters of the alloys by using magnetic transducers and time-resolved magneto-optical Kerr effect (TR–MOKE). These optical methodologies can provide insight into electron-phonon interactions on non-equilibrium, picosecond time scales, as well as microscopic transport behaviors of individual heat carriers. We expect this study to contribute to a better understanding of transport mechanisms of heat carriers governing the thermal transport properties in nickel-based HEAs.