在2050℃的熔体温度下, 在自行研制的定向凝固炉内实现了Nb-Ti-Si-Cr-Hf-Al-B-Y超高温合金的有坩埚整体定向凝固. 采用XRD, SEM, EDS等方法分析了凝固速率分别为2.5, 5, 10, 20, 50和100 μm/s时的整体定向凝固组织、组成相的择优取向及固/液界面形貌, 并讨论了其共晶生长机制. 结果表明: 合金的定向凝固组织主要由沿着试棒轴向排列的横截面为多边形的柱状初生(Nb, X)₅Si₃(X=Ti, Hf, Cr)相与耦合生长的层片状Nbss/(Nb, X)₅Si₃共晶团(Nbss表示铌基固溶体)组成. 横截面上共晶胞界明显.当凝固速率由2.5 μm/s变化到100 μm/s时, 定向凝固组织细化, 固/液界面经历粗胞状→细胞状→胞枝的演化过程. Nbss/(Nb, X)₅Si₃共晶两相较低的熔化熵及其前沿较大的动力学过冷度是形成规则共晶的主要原因.

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 ℃as structural materials. Alloying and directional solidification are generally used to obtain a better combination of room temperaure 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. 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/liqid interface of the alloy undergoes an evolution from coarse cellular, fine cellulaand finally to cellular dendrite morphologies. Both the average diameter of eutectic cells and lamellar spacing in hem decrease with the increase in solidifying rate. The formatin of a regular bss/(Nb, X)₅Si₃ euecic morphology is attributable to a large kinetic undercoling and a low fusion entropies of alloy phases.