High temperature strength and room temperature fracture toughness of Nb–Mo–W refractory alloys with and without carbide dispersoids

Room temperature fracture toughness and high temperature strength at 1773 K of Nb–Mo–W based alloys with and without carbides were investigated by three-point bending and compression tests. W addition to Nb–5Mo solid solution gives rise to an increase in yield stress at 1773 K but to a decrease in fracture toughness at room temperature. On the basis of the experimental results obtained, it is suggested that solid solution hardening and cleavage fracture propensity are responsible for the increase of yield stress at 1773 K and for the decrease of fracture toughness at room temperature, respectively. The microstructure of Nb–5Mo–15W–x(Hf+C) alloys consists of Nb-rich bcc solid solution and (Nb,Hf,Mo)C carbide with B1-structure. Yield stress at 1773 K and fracture toughness at room temperature increase concurrently with increasing content of (Hf+C) in the Nb–5Mo–15W–x(Hf+C) alloys. The increase of yield stress at 1773 K due to the presence of (Nb,Hf,Mo)C phase is explained by the mechanism based on dispersion hardening, and the increase of fracture toughness at room temperature is attributable to the transition of fracture mode from cleavage to quasi-cleavage accompanied by decohesion between carbide and matrix phase, crack branching and deflection. Details will be discussed in relation to microstructural characteristics.