Enhancement of comprehensive properties of Nb–Si based in-situ composites by Ho rare earth doping

This study examines the microstructure, mechanical properties (with a focus on room-temperature toughness), and oxidation resistance of Ho-doped Nb–Si based in-situ composites. The base alloy consists of the coarse primary Nb₅Si₃ phase and the Nb₅Si₃ + Nbss (Nb solid solution) eutectic cells. Ho doping influences the solidification path. When the Ho doping is higher than 0.2 at%, the alloys transform into eutectic alloys. Ho can be solid-solved in trace amounts in the Nbss phase. However, most of Ho forms a stable Ho oxide phase, which alleviates oxygen contamination problem to some extent. Moreover, the interface separation between Ho oxide and other phases reduces the plastic deformation constraint. Thus, with 0.4 at% Ho doping, the K_Q value is 18.03 MPa·m^1/2, which is 31.1% higher than that of the base alloy. The strength of the Ho-doped alloys does not deteriorate with an increase in toughness. However, the large network-like Ho₂O₃ in the 0.8Ho alloy causes a decrease in toughness and strength. In addition, the Ho oxide phase effectively blocks the inward oxygen intrusion. With 0.8 at% Ho doping, the oxidation mass gain per unit area is 10.16 mg·cm², which is 39.7% lower than that of the base alloy.