Refractory High-Entropy Alloys for Liquid Rocket Engine Combustion Chambers and Throat Liners

Refractory High-Entropy Alloys (RHEAs) have rapidly emerged as one of the most promising classes of ultra-high-temperature materials for liquid rocket engine (LRE) combustion chambers and throat liners, where structural components are subjected to some of the most aggressive thermochemical environments in aerospace engineering. LRE combustion zones routinely exceed 3000 K, expose walls to heat fluxes of 15–25 MW/m², and generate extreme thermal gradients often surpassing several hundred kelvin per millimetre. These conditions accelerate creep deformation, promote oxidation induced embrittlement, and destabilise strengthening phases in conventional Ni-based superalloys, ultimately limiting their service life and reusability (Miracle & Senkov,2017). As propulsion systems move toward higher chamber pressures and more demanding duty cycles, material systems capable of maintaining structural stability beyond the limits of current superalloys are required. RHEAs—particularly multi-principal-element systems such as Ti–Zr–Nb–Ta–Mo—offer a fundamentally different pathway for achieving the thermomechanical performance required for next-generation reusable engines. Their exceptionally high melting points (>2000°C), combined with severe lattice distortion and sluggish atomic diffusion, suppress dislocation mobility, delay diffusion-controlled degradation, and enhance creep resistance under sustained thermal loading. The entropy-stabilised BCC solid-solution phases commonly exhibited in these alloys provide outstanding thermal stability, resisting phase separation and coarsening under repeated thermal cycling. Furthermore, their intrinsic ability to form multi-layer protective oxides—including TiO₂, ZrO₂, Nb₂O₅, and Ta₂O₅—significantly improves resistance to oxidation, volatilisation, and hot-gascorrosion (Yeh, 2004; Senkov et al., 2010). These oxide layers develop in situ during exposure to high-enthalpy exhaust flows, providing a passive, self-healing barrier that slows oxygen ingress and minimises mass loss. This study presents a systematic evaluation of RHEA performance within LRE combustion environments. Thermodynamic analyses explore Gibbs free-energy minimisation, mixing enthalpy effects, and valence electron concentration criteria governing BCC stability. Microstructural evolution is assessed through dendritic solidification modelling, diffusion-controlled homogenisation behaviour, and grain-boundary stability at elevated temperatures. High-temperature deformation mechanisms—including dislocation glide resistance, lattice-distortion strengthening, and creep–fatigue interactions—are investigated to quantify long-duration structural reliability. Oxidation behaviour is examined through oxygen-diffusion kinetics, multi layer oxide growth modelling, and thermochemical compatibility with oxygen-rich hydrogen or methane combustion products. Heat-transfer and thermal-shock simulations further highlight RHEAs’ ability to withstand rapid thermal transients common during engine start-up and shutdown. Taken together, these findings demonstrate that RHEAs possess the integrated thermodynamic, mechanical, and environmental stability required for future high- performance and reusable rocket engine architectures. Their superior resilience under extreme thermomechanical and oxidative conditions positions them as transformativecandidates for next-generation combustion chamber and throat-liner materials.