Er-induced microstructural refinement and internal oxidation enhancing fracture toughness in Nb-Si alloys

Owing to their elevated melting temperature and reduced density, Nb-Si superalloys are prime candidates for replacing Ni-based superalloys. This research comprehensively examines the microstructure, fracture toughness at ambient temperature, and compressive behavior under elevated temperatures within the Nb-30Ti-13Si-12Zr alloy system modified by varying Er additions. The results demonstrate that all alloys consist of Nbss and γ-Nb₅Si₃ phases. The incorporation of Er effectively refined the alloy microstructure and modified the eutectic morphology. Owing to its high chemical activity, Er preferentially reacted with oxygen during solidification, forming discrete oxide particles that purified the alloy melt through an internal oxidation mechanism. The Er addition significantly enhanced the frequency of occurrence of toughening mechanisms including crack bridging, branching, and deflection, thereby improving the room-temperature fracture toughness. Meanwhile, the resulting oxides also contributed to enhanced mechanical performance at elevated temperatures. Among all investigated alloys, the 0.3Er alloys exhibited the most pronounced microstructure refinement and optimal fracture toughness of 16.89 MPa·m^1/2, representing a 15.1 % improvement compared to the base alloy.