Cr-Triggered FCC Ti-Rich Precipitation and Its Effects on the Mechanical and Oxidation Performance of TiVNbTa Refractory High-Entropy Alloys

This study investigates the effects of Cr content on the microstructure, room-temperature and 600 °C mechanical properties and high-temperature oxidation behavior of TiVNbTaCrx (x = 0, 0.25, and 0.5). Cr addition significantly intensified Ti segregation in the interdendritic regions during solidification, inducing the formation of an Face-Centered Cubic (FCC) Ti-rich precipitate phase. As the Cr content increases from 0 to 0.5, the Ti-rich phases evolve from a straw-like morphology to a blocky morphology, and the mechanical properties at room temperature continue to decrease. Fracture analysis reveals that cracks initiate at grain boundaries associated with the FCC Ti-rich phase, leading to a transition from ductile to brittle fracture in the Cr-containing alloys. At 600 °C, all Cr-containing alloys exhibit elongations below 0.20%, indicating that the precipitation of the FCC Ti-rich phase severely degrades the deformation capability of the alloys at 600 °C. Oxidation tests demonstrate that Cr addition provides only a marginal beneficial effect on the mass gain rate during the initial stage (0–10 h at 600 °C), while the Cr-containing alloys continue to gain mass upon prolonged exposure due to the reduced integrity of the oxide film. The oxide scale consists primarily of (Ta, Nb)9VO25, and no protective Cr2O3 scale was observed under the present experimental conditions. In summary, within the composition range studied, the FCC Ti-rich phase precipitates at Cr contents of 5.88 and 11.11 at.%, resulting in the fragmentation of the body-centered cubic lattice (BCC) matrix continuity and a comprehensive deterioration of mechanical properties at both room temperature and 600 °C, while failing to improve the oxidation resistance at 600–700 °C.