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

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 Nbss by adding W, low diffusivity of Win Nbss and microstructural change caused by W addition. With increasing W content, fracture toughness decreases in the Nbss/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/NbsSi3 is effective to increase high temperature strength without a significant decrease of fracture toughness.