中文摘要：

Nb基超高温合金的制备技术主要包括粉末冶金、真空电弧熔炼和定向凝固等，其中定向凝固可以显著提高该合金的高低温力学性能。Nb基超高温合金的定向凝固组织主要由沿着定向凝固方向生长的初生Nbss枝晶、铌硅化物块或板条以及（Nbss＋铌硅化物）共晶等组成，定向凝固速率和固液界面前沿液相中的温度梯度会显著影响其组织形貌及尺寸。在其定向生长过程中，Nbss、铌硅化物及（Nbss＋铌硅化物）共晶各相之间竞争生长，形成不同的组织形貌及尺寸，但在合适的条件下，共晶两相可以实现平行耦合生长。

英文摘要：

Nb-based ultrahigh temperature alloys can be prepared by powder metallurgy, vacuum arc melting and directionally solidification techniques. Directionally solidification can improve the alloys＇ mechanical properties evidently. Generally speaking, the directionally solidified （DS） microstructure is mainly composed of primary Nbss dendrites, Nb silicide blocks or plates and Nbss＋Nb silicide eutectic colonies aligned along the growth direction. The motphology and size of the DS microstructure are affected by the withdrawing rate and thermal gradient ahead of the solid/ liquid interface significantly. During the directional growth process, Nbss dendrites, Nb silicide blocks and Nbss＋ Nb silicide eutectic structures solidify in a competitive growth manner, forming microstructures with different morphologies and sizes. However, they can form colonies in parallelly coupled growth manner with suitable growth parameters.