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

C40结构的nbsi_2基硅化物具有优异的高温强度，在高温下表现出异常强化行为，并在1500℃左右出现峰值。Mo和W的加入使C11_b结构稳定，从而提高了C11_b的高温强度和峰值温度。相比之下，C11_b结构的MoSi_2的高温强度低于NbSi_2。然而，大量的滑移体系仍在运行，MoSi_2获得了良好的延展性。为了使这两种硅化物都具有良好的力学性能，研制了由C11_b和C40相组成的双相硅化物。如果采用传统的熔融和铸造工艺制备硅氧体，则C11_b和C40相形成片层，但片层的取向不容易控制。由于在NbSi_2-MoSi_2赝二元体系中存在包晶反应，因此很难得到单晶。利用浮区炉控制晶体生长速率，生长出C40结构的单晶。在1500℃长时间退火过程中，由C40相转变为C11_b相，形成一组片层。在片层结构中，C40和C11_b相之间的相界保持了良好的相干性，表现出良好的热稳定性。这些硅化物的力学性能很大程度上取决于片层边界与应力轴之间的夹角（φ）。当施加φ=45°应力时，双相硅化物具有良好的延展性。当φ=0时，双相硅化物表现出较高的高温强度，1400℃时屈服应力可达900MPa。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工作。

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.