Chapter 6
6.8 High Temperature Materials for Aeroengines
Yutaka Koizumi and Hiroshi Harada
High Temperature Materials Center and The Rolls-Royce Centre of Excellence for Aerospace Materials, NIMS
Due to the ever increasing demands on oil and the resulting increase in the price, the improvement of specific fuel consumption (SFC) has become the hottest issue in aeroengine industries and airline companies. The improvement in SFC is required also for the CO2 reduction to mitigate global warming. Fig.6.8.1 shows an improvement in SFC in the last 40 years1). Further improvement is still needed for the next generation airliners and, to realise this, new materials especially materials with high temperature capabilities are desired for increasing peak temperature of Carnot cycle of the engine.
Ni-base superalloys have been used as most of the turbine
components. Among them, Ni-base single crystal (SC) superalloys are the materials being used as the first stage blades in hightemperature turbines where the operating temperature and the stress are the highest. Fig.6.8.2 shows the history of improvement in temperature capability of Ni-base superalloys. The “Target” shown in this figure is that of the “High Temperature Materials 21 Project ” being conducted in NIMS (Phase 2: 2006-2010). Typical chemical compositions of Ni-base superalloys for turbine blades are also presented in Table 6.8.1.
Fig.6.8.1 A History of Improvement in Specific Fuel Consumption |
Fig.6.8.2 History of improvement in temperature capability of Ni- |
of Aeroengins. |
base superalloys. |
Table 6.8.1 Chemical Composition of Ni-base Superalloys for Aeroengine Turbine Blades (wt%, bal. Ni).
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Materials Outlook for Energy and Environment
Ni-base superalloys evolved from forged/wrought alloys to conventionally cast (CC) alloys, directionally solidified (DS) alloys, and then single-crystal (SC) alloys. Single crystal superalloys have also evolved from 1st generation (0Re) to 2nd generation (2-3Re), 3rd generation (5-6Re), 4th generation (5-6Re and 2-3Ru), and then 5th generation (5-6Re and 5-6Ru) alloys. So far 1st to 3rd generation SC superalloys are used practically, typically CMSX-102), a 3rd generation alloy, in the latest fan jet engine Trent 900 used for Airbus 380. The 4th generation alloys have been developed in the US, France and Japan. The highest temperature capability, 1100 °C, has been reached by NIMS with 5th generation alloys TMS-162 and TMS-1963, 4), which are the only 5th generation alloys available at present in the world. In NIMS superalloys, especially 4th and 5th generation alloys, an interfacial dislocation network on γ and γ’ phases are designed to be finer (20 nm) to prevent dislocation cutting through the interface to suppress creep deformation.
When Ni-base superalloys are used as turbine blades in the high-temperature turbines, thermal barrier coatings (TBC) and air cooling systems inside the blades are essential, as shown in Fig.6.8.3. As a new development in this field, in 2006, “EQ coating” was invented as a bond coat material at NIMS5). Because of the composition carefully designed to be in an equilibrium state with the substrate superalloy, the interface can maintain the phase stability for the entire life of the blade, eliminating the usual problem so-called secondary reaction zone (SRZ) causing shorter life of the blade.
As the turbine temperature increases, the temperature capabilities of turbine discs, which are produced through cast and wrought (C&W) or powder metallurgy (P/M) processes, are also required to be improved. Fig.6.8.4 shows the improvement of turbine disc superalloys in terms of tensile strength at 650 °C6). Here, TMW-4 is a C&W Ni-Co-base alloy developed in NIMS7), exhibiting same property as P/M processed alloys. Because of the lower process cost with C&W, TMW-4 alloy has a highest cost-
Fig.6.8.3 Thermal Barrier Coated Turbine Blade for an Aeroengine (Provided by Rolls-Royce plc).
Fig. 6.8.4 Improvement in Strength of Turbine Disc Superalloys.
performance compared with other disc superalloys.
So-called alternative materials to Ni-base superalloys are also being investigated. Ir or Pt group metals base “refractory superalloys”, composed of γ and γ’ phases as Ni-base superalloys have been proposed by NIMS8). They have potentials for ultra-high temperature use, because of the high melting points and useful creep strength even at 1750 °C, as well as the better oxidation property than conventional refractory metals and alloys based on W, Mo, Ta, and Nb. The cost and density are the factors to limit the range of applications. Nb/Nb-silicide and Mo/Mo-silicide base alloys are also widely investigated. Cr-base alloys also have an attention. Cr-Re and Cr-Ag alloys with good ductilities for structural materials have been reported9). In spite of these activities, it is fair to say that it will take a long time for these alternative materials to possibly substitute the Ni-base superalloys, except that TiAl base alloys are planned to be used as low-pres- sure turbine blade materials for weight reduction.
References
1)Rolls-Royce Plc “The Jet Engine” (ISBN 0902121235) (2005) 84.
2)G.L.Erickson, Superalloys 1996, Ed. By R.D.Kissinger et al. (1996) 35.
3)J.X.Zhang, T.Murakumo, Y.Koizumi, T.Kobayashi, H.Harada, S.Masaki, Jr.: Met.Mat.Trans. A 33A (2002) 3741.
4)Y. Koizumi, J. X. Zhang, T. Kobayashi, T. Yokogawa, H. Harada, Y. Aoki, M Arai: Japan Inst. Metals, 67 (2003) 468.
5)A.Sato, K.Kawagishi, H.Harada: Met. Mat. Trans. A, Phy. Met. Mat. Sci. 37A(3), (2006) 789.
6)H. Hatsutori, M.Takekawa: Netsusyori 44 (2004) 209.
7)Y. Gu, H. Harada, C. Cui, D. Ping, A. Sato and J. Fujioka: Scripta Materialia 55 (2006) 815.
8)Y. Mitarai, Y. Gu and H. Harada: Platinum Metals rev. 46
(2002) 74.
9)Y. Gu, H. Harada and Y.Ro: JOM, September (2004) 28.
Chapter 6
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Chapter 6. Materials for Energy Transmission and Conversion