中文摘要：

在熔体温度为2000℃的条件下,分别以2.5, 5, 10, 20, 50和100μm/s的抽拉速率对Nb-Ti-Si基超高温合金进行了有坩埚整体定向凝固.采用XRD, SEM和EDS等分析方法,研究了抽拉速率对定向凝固共晶组织及固/液界面形貌的影响,并分析了该合金的凝固过程. 结果表明：合金定向凝固组织主要由沿着试棒轴向排列的横截面呈花瓣状的共晶胞Eutectic I（Nbss/α（Nb,X）5Si3）以及分布于共晶胞周围的沿试棒轴向耦合生长的共晶组织EutecticII（Nbss/γ（Nb, X）5Si3组成.随着凝固速率的增大, 组织细化,花瓣状共晶胞由以硅化物或细小共晶为中心的近似圆形形貌逐渐演变为以十字形Nbss为中心、α （Nb,X）5Si3呈片状向外辐射生长的四边形形貌; EutecticII则呈沿纵向耦合生长的层片状形貌. 固/液界面形貌经历了由胞枝状→树枝状→胞枝状的演变过程.

英文摘要：

Nb-Ti-Si base ultrahigh temperature alloys that possess higher melting points, relatively lower densities and attractive high temperature strength have received worldwide attention for their potential applications as next-generation turbine blade materials. In this work, integrally directional solidification of an Nb-Ti-Si base ultrahigh temperature alloy was conducted at different withdrawing rates （2.5, 5, 10, 20, 50 and 100 μm/s） with a constant melt temperature of 2000℃. Effect of solidifying rate on the integrally directionally solidified eutectic microstructure and solid/liquid interface morphology of this alloy has been investigated by XRD, SEM and EDS, and its directional solidification behavior has been discussed. The results show that the directionally solidified microstructure is mainly composed of petal-like Nbss/α（Nb,X）5Si3 eutectic colonies （Eutectic I） and coupled grown lamellar Nbss/γ（Nb,X）5Si3 eutectic （Eutectic II） which distributed in the intercellular area. The Eutectic I and Eutectic II are both aligned straight and uprightly along the growth direction. When the solidifying rate increases from 2.5 μm/s to 100 μm/s, the microstructure becomes finer and finer, and the petal-like eutectic colonies evolve from round morphology to tetragonal morphology. Either silicides or fine eutectics locate in the centers of round eutectic cells, while cross-like Nbss locates in the centers of tetragonal eutectic cells. Eutectic II exhibits a well-aligned lamellar structure on longitudinal-section. The solid/liquid interface of the alloy undergoes an evolution from cellular dendrite, dendrite and finally to cellular dendrite morphologies.