The mechanical properties of Nbss/Nb5Si3 in-situ composites alloyed with Mo and W have been studied in relation to the microstructure. The samples were prepared by arc-melting, or floating zone melting method to produce a directional structure. Microstructures were characterized by XRD, EPMA and BEI. Room temperature fracture toughness and high temperature strength were examined by three point bending test and compression test, respectively. In Nb-5Mo-xW-16Si and Nb-5Mo-xW-20Si(x=0∼15 mol%) alloy series, the strength increased and fracture toughness decreased monotonously with increasing content of W. Furthermore, the strength of Nb-15Mo-18Si produced by directional solidification was slightly higher than arc-melted samples, but the fracture toughness was extremely low. It is suggested that intrinsic toughness of Nbss and thickness of Nbss phase in the microstructures are very important to room temperature fracture toughness.
