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

点击翻译按钮获取中文摘要

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.