Development of NbSi_2-Based Duplex Silicides for Turbine Blade Operating at Ultra High-Temperature

具有C40结构的NbSi2基硅化物具有良好的高温强度，在高温下表现出反常的强化行为，并在1500℃左右出现一个峰值。作为二硅化物，Mo和W的加入有望稳定C11b结构，导致高温强度和峰值温度的提高。相反，具有C11b结构的MoSi2的高温强度低于NbSi2。然而，MoSi2具有大量的滑移系统，并获得了良好的塑性。为了利用这两种硅化物的良好力学性能，开发了由C11b和C40相组成的双相硅化物。如果采用传统的熔炼和铸造工艺制备硅苷，C11_b和C40相形成片层，但片层取向不易控制。在NbSi_2-MoSi_2伪二元系中，由于存在包晶反应，获得单晶非常困难。利用浮区炉控制晶体生长速度，生长出C40结构的单晶。在1500℃长时间热处理过程中，C40相向C11b相转变，形成了单一的片层组织。在层状结构中，C40和C11b相之间的相界保持了良好的共格，并表现出良好的热稳定性。这些硅化物的力学性能强烈地依赖于片层边界与应力轴之间的夹角(φ)。当应力为φ=45°时，双相硅化物具有良好的延性。当φ=0时，双相硅化物具有较高的高温强度，1400℃时的屈服应力达到900 Mpa。如果层状结构与涡轮叶片的纵向对齐，这些叶片可以在超高温下运行。

NbSi_2-based silicides with the C40 structure had superior high-temperature strength and showed anomalous strengthening behavior at high-temperatures accompanied by a peak around 1500℃. Addition of Mo and W which are expected to stabilize the C11_b structure as a disilicide induced an increase in high-temperature strength and peak temperature. In contrast, MoSi_2 with the C11_b structure showed the lower high-temperature strength than NbSi_2. However, a large number of slip systems were operative and good ductility was obtained for MoSi_2. In order to use good mechanical properties of both silicides, duplex silicides composed with the C11_b and C40 phases were developed. If the silisides were prepared by conventional melting and casting processes, the C11_b and C40 phases form lamellae but it is not easy to control the orientation of lamellae. It is very difficult to obtain single crystals because there is a peritectic reaction in the NbSi_2-MoSi_2 psuedo binary system. Single crystals with the C40 structure were growned by controlling the crystal growth rate using the floating zone furnace. During annealing at 1500℃ for long period, the phase transformation from the C40 to the C11_b phase occurred forming a single set of lamellae. In the lamellar structure, the phase boundary between the C40 and C11_b phases maintained a good coherency and displayed good thermal stability. The mechanical properties of these silicides depended strongly on the angle (φ) between the lamellar boundary and stress axis. When stress is applied at φ=45°, good ductility was obtained in the duplex silicides. At φ=0, the duplex silicides showed high strength at high temperatures and 900MPa for the yield stress was obtained at 1400℃. If the lamellar structure is aligned to the longitudinal direction of the turbine blades, these blades can be operative at ultra high temperatures.