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 /Nb 5 Si 3 and Nb ss /Nb 5 Si 3 /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 /Nb 5 Si 3 , but no marked change is observed in the Nb ss /Nb 5 Si 3 /NbC. A crack propagates straightly in Nb-16Si-5Mo-15W, while crack deflection and branching take place in the Nb ss Mb 5 Si 3 /NbC. Fractography reveals that Nb ss /Nb 5 Si 3 fractures completely in a transgranular mode, while Nb ss Mb 5 Si 3 /NbC fractured in a mixed mode of transgranular fracture and interface decohesion between NbC and Nb ss or Nb 5 Si 3 and Nb ss . It is suggested that the presence of NbC dispersoids in Nb ss /Nb 5 Si 3 is effective to increase high temperature strength without a significant decrease of fracture toughness.