Microstructure and mechanical properties of Nb based ultrahigh temperature alloy directionally solidified by EBFZM

The microstructure and mechanical properties including room temperature fracture toughness K_q, tensile strength σ_b and elongation δ at 1250° of the Nb based alloy directionally solidified in an electron beam floating zone melting (EBFZM) furnace have been evaluated. The microstructure was primarily composed of Nb solid solution (Nbss), α-(Nb)₅Si₃ and (Nb), Si phases. After directional solidification with the moving rate of electron beam gun R being respectively 2.4, 4.8 and 7.2 mm/min, the primary Nbss dendrites, Nbss + (Nb)₅Si₃/(Nb)₃Si eutectic colonies (lamellar or rod-like) and separated Nb silicide plates aligned along the longitudinal axes of the specimens. When R = 2.4 mm/min, the best directional microstructure was obtained. Both of the tensile strength σ_b at 1250°C and room temperature fracture toughness K_q were improved significantly by the directional solidification. The maximum σ_b incurred for the specimens with R = 2.4 mm/min and was about 85.0 MPa, meanwhile, the K_q was about 19.4 MPa·m^1/2.