中文摘要：

采用电磁冷坩埚定向凝固技术研究了加热功率、抽拉速率和保温时间对Nb-22Ti-16Si-3Cr-3Al-2Hf（原子分数,%）合金固液界面的影响.采用正交实验制备合金试样.结果表明,延长保温时间、减小抽拉速率和提高加热功率有利于保持固液界面的宏观形态为平界面.随着抽拉速率的增加,初生Nb固溶体（Nbss）一次枝晶臂间距和二次枝晶臂间距逐渐减小;随着加热功率的增加,初生Nbss一次枝晶臂间距和二次枝晶臂间距逐渐增加;随着保温时间的延长,初生Nbss一次枝晶臂间距和二次枝晶臂间距先增大后减小.增大抽拉速率、减小加热功率和缩短保温时间有利于一次枝晶臂间距和二次枝晶臂间距的细化.

英文摘要：

Nb-Si base alloys have attracted considerable attentions as the potential high temperature structural materials working in the service temperature range of 1200-1400℃ because of their high melting points （〉 1750 ℃）, moderate densities （6.6～7.2 g/cm^3） and excellent high temperature strength. However, the mismatching between room temperature fracture toughness and high temperature strength has limited their practical applications. Directional solidification （DS） and alloying have been proved to be the effective methods to overcome this critical issue. The DS processes used to prepare Nb-Si base alloys included Czochralski directional solidification in a copper crucible, electron beam directional solidification, optical floating zone melting, integrally directional solid- ification and electromagnetic cold crucible directional solidification （ECCDS）. The previous studies focused on the effect of process parameters on microstructure and mechanical properties in the steady-state growth region （SS-GR）. However, the microstmcture in the SSGR was controlled by the solid-liquid interface, and the solid-liquid in- terface was controlled by process parameters. Therefore, the study about the effect of process parameters on solid- liquid interface was very important. In this work, the master alloy with the nominal composition of Nb-22Ti-16Si- 3Cr-3A1-2Hf （atomic fraction, %） was prepared by vaccum non-consumable arc-melting first, and then induction skull melting. The DS experiments were performed in the ECCDS device equipped with a square water cooled copper crucible （internal dimension： 26 mm×26 mm× 120 mm） and a Ga-In alloy pool. There were three processing parameters in ECCDS including heating power of power supply, withdrawal rate and holding time. The DS ingots were prepared according to the orthogonal test （L_9 （3^3））. Instability degree was defined as the ratio of the height of solid-liquid interface to the width of the DS ingot. The results showed that there were three