Effect of solidifying rate on integrally directionally solidified microstructure and solid/liquid interface morphology of an Nb-Ti-Si base alloy

Since temperatures of airfoil surfaces in advanced turbine engines are approaching the limit of nickel base superalloys, Nb-Ti-Si base alloys as their potential materials have attracted much attention recently. Nb-Ti-Si base alloys have high melting temperature, suitable densities, good elevated temperature creep strength and acceptable room temperature fracture toughness, therefore, they are expected to be employed in the temperature range of 1200-1450°C as structural materials. Alloying and directional solidification are generally used to obtain a better combination of room temperature fracture toughness with high temperature creep strength and oxidation resistance for an elevated-temperature alloy. In this paper, the master alloy ingot with a nominal composition of Nb-20Ti-16Si-6Cr-5Hf-4Al-2B-0.06Y (atomic fraction, %) was prepared by using vacuum consumable arc-melting. The integrally directional solidification of this alloy was conducted in a high vacuum and ultrahigh temperature directional solidification furnace with the use of a ceramic crucible at melt temperature of 2050°C. The integrally directionally solidified microstructure, preferred orientation of constituent phases and solid/liquid (S/L) interface morphology at different solidifying rate (2.5, 5, 10, 20, 50 and 100 /μm/s) for this alloy have been investigated by XRD, SEM and EDS, and the growth mechanism of Nbss/(Nb, X)₅Si₃ (where Nbss denotes Nb solid solution, X represents Ti, Hf and Cr elements) eutectic in it has been discussed. The results show that the directionally solidified microstructure of the alloy is mainly composed of hexagonally cross-sectioned primary (Nb, X)₅Si₃ columns and coupled grown lamellar Nbss/(Nb, X)₅Si₃ eutectic colonies both aligned straight and uprightly along the growth direction. When the solidifying rate varies from 2.5 /μm/s to 100 /μm/s, the solid/liquid interface of the alloy undergoes an evolution from coarse cellular, fine cellular and finally to cellular dendrite morphologies. Both the average diameter of eutectic cells and lamellar spacing in them decrease with the increase in solidifying rate. The formation of a regular Nbss/(Nb, X)₅Si₃ eutectic morphology is attributable to a large kinetic undercooking and a low fusion entropies of alloy phases.