Effect of W Alloying and NbC Dispersion on High Temperature Strength at 1773 K and Room Temperature Fracture Toughness in Nb₅Si₃/Nb In-situ Composites

Abstract

The effects of W alloying and NbC dispersion on high temperature strength at 1773 K and room temperature fracture toughness are investigated using Nb_ss/Nb₅Si₃ and Nb_ss/Nb₅Si₃/NbC in-situ composites with the hypoeutectic composition, where Nb_ss denotes Nb solid solution. With increasing W content, 0.2% offset yield stress for both composites increases at 1773 K. The increase in the yield strength is attributable to the solid solution strengthening in Nb_ss by adding W, low diffusivity of W in Nb_ss and microstructural change caused by W addition. With increasing W content, fracture toughness decreases in the Nb_ss/Nb₅Si₃, but no marked change is observed in the Nb_ss/Nb₅Si₃/NbC. A crack propagates straightly in Nb–16Si–5Mo–15W, while crack deflection and branching take place in the Nb_ss/Nb₅Si₃/NbC. Fractography reveals that Nb_ss/Nb₅Si₃ fractures completely in a transgranular mode, while Nb_ss/Nb₅Si₃/NbC fractured in a mixed mode of transgranular fracture and interface decohesion between NbC and Nb_ss or Nb₅Si₃ and Nb_ss. It is suggested that the presence of NbC dispersoids in Nb_ss/Nb₅Si₃ is effective to increase high temperature strength without a significant decrease of fracture toughness.