Microstructure and Mechanical Properties of an Advanced Niobium Based Ultrahigh Temperature Alloy

摘要

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 is primarily composed of Nb solid solution (Nbss), α -(Nb)_5Si_3 and (Nb)_3Si 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)_5Si_3/(Nb)_3Si eutectic colonies (lamellar or rod-like) and divorced Nb silicide plates align along the longitudinal axes of the specimens. When R = 2.4 mm/min, the best directional microstructure is obtained. Directional solidification has significantly improved the σ_b at 1250℃ and K_q. The maximum σ_b occurs for the specimens with R = 2.4 mm/min and is about 85.0 Mpa, meanwhile, the K_q is about 19.4 Mpam~(1/2).