Harnessing Secondary Phases in Titanium-containing Refractory High-Entropy Alloys: Towards Microstructure Evolution and Mechanical Performance

To meet high-temperature service demands in energy and aerospace, refractory high-entropy alloys (RHEAs) have garnered attention for retaining strength at high temperatures, yet room-temperature brittleness and impurity sensitivity continue to impede deployment—necessitating holistic alloy-and-process design to balance strength, ductility, creep resistance, density, and oxidation resistance. In Ti-containing RHEAs, second-phase strengthening offers an effective lever to remedy strength deficits and tailor deformation mechanisms; representative phases include nano-ordered B2, micron-scale Zr–Al intermetallics, and micron-scale ceramics, whose size, spatial statistics, and interfacial character govern strength–ductility matching and embrittlement risk. This review synthesizes formation mechanisms, microstructural evolution, and mechanical roles of these three classes and consolidates corresponding control strategies. Key insights are that nano-ordered B2 enables order-shearing–driven precipitation strengthening that can coordinate with matrix dislocation activity to achieve strength–ductility synergy; Zr–Al intermetallics, while high-modulus and potent obstacles, promote intergranular cracking and room-temperature embrittlement when boundary-percolated; and fine, discrete, non-percolating morphologies with controlled volume fractions are essential to suppress embrittlement while sustaining work hardening and ductility. Building on these analyses, we propose a multiscale second-phase design framework for Ti-containing RHEAs and emphasize the need for coupling with interstitial behavior within a holistic design space, providing an actionable microstructure–property optimization route toward lightweight, heat-resistant structural materials with high strength and ductility.