Microstructure and solid/liquid interface morphology evolution of integrally directionally solidified Nb-silicide-based ultrahigh temperature alloy

Nb-silicide-based ultrahigh temperature alloys have attracted considerable attentions as potential high temperature structural materials because of their high melting point, suitable density, good elevated temperature creep strength and acceptable room temperature fracture toughness. However, the shortcoming in both high temperature strength and high temperature oxidation resistance retarded their practical applications. Directional solidification and alloying can be used in overcoming these deficiencies at certain degree. In this paper, the alloy with the composition of Nb-22Ti-16Si-6Cr-4Hf-3Al-3Mo-2B-0.06Y (atomic fraction, %) was designed and the master alloy ingot was prepared by firstly vacuum non-consumable arc melting and then vacuum consumable arc melting. The integrally directional solidification of this alloy was conducted with the use of special ceramic crucibles in a self-made resistance heating directional solidification furnace with ultrahigh temperatures and high thermal gradients. The microstructure and solid/liquid (S/L) interface morphology evolution of directionally solidified alloy were investigated under the condition of different melt superheat temperatures θ_s (1950, 2000, 2050, 2100 and 2150°C) but with a constant withdrawing rate of 15 μm/s. The results revealed that when the melt superheat temperature θ_s=1950°C, the directionally solidified microstructure is composed of straight primary Nb_ss dendrites and couple grown lamellar (Nb_ss+γ-(Nb, X)₅Si₃) eutectic colonies (petal-like) along the longitudinal axes of the specimens. When θ_s=2000 and 2050°C respectively, the directionally solidified microstructure is completely composed of straight petal-like eutectic colonies. As θ_s increased to 2100 and 2150°C respectively, the directionally solidified microstructure evolves into straight coarse primary Nb_ss dendrites and fine lamellar eutectic colonies along the longitudinal axes of the specimens. The S/L interface morphology changes from coarse dendrite to cellular, then to coarse dendrite with the increase of melt superheat temperature.