Microstructure and fracture toughness of Nb-Si based alloys with Ta and W additions

The alloying effects of Ta and W on the phase composition, microstructure and room-temperature fracture toughness of Nb-Si based alloys were investigated. The nominal compositions of Nb-Si based alloys were Nb-15Si-24Ti-4Cr-2Al-2Hf (base alloy, at.%), Nb-15Si-24Ti-4Cr-2Al-2Hf-1Ta (1Ta alloy, at.%) and Nb-15Si-24Ti-4Cr-2Al-2Hf-1W (1W alloy, at.%). Results showed that the base alloy, 1Ta alloy and 1W alloy were composed of Nb solid solution (Nbss), αNb₅Si₃ and small volumes of hexagonal γNb₅Si₃ phases. The elements of Ta and W were primarily partitioned in the Nbss phases. The addition of Ta improved the fracture toughness of the base alloy from 10.2 to 12.2 MPa m^1/2, but the addition of W reduced the fracture toughness to 8.2 MPa m^1/2. The fracture of Nbss phases in the three alloys exhibited a cleavage fracture mode, and river patterns were created on individual facets, while the Nb₅Si₃ phases showed a brittle fracture mode with flat and featureless fracture surfaces. Different with base alloy and 1W alloy, 1Ta alloy involved the occurrence of secondary cracking as well as interface decohesion. The toughening mechanisms of Nb-Si based alloys with Ta and W additions were emphasized.