Introduction
The development of a new nickel-based superalloy for use in the high-pressure turbine is turbine disk of a modern gas protracted one of the most expensive and exercises undertaken by gas- turbine manufacturers.plete development cycle .A typical tom- can take more than 10 years. This delay arises not only because of the extensive mechanical testing and validation required to ensure the safety of the critical part, but also because of the difficulty of finding a material chemistry that provides the correct degree of balance between mechanical properties and long-term material stability. The conventional approach to this latter problem is to use a combination of experience, empirical wisdom, and a few basic calculation tools based on electron-valency theory (PHACOMP).' There is thus an urgent need to develop a tool that reliably summarizes the known body of knowledge about the stability of Ni-based superalloys and provides a method that can quickly and reliably guide the alloy developer through the initial alloy-chemistry selection phase of development.
Calculation of phase equilibria usingcalculated phase-diagram (CALPHAD) techniques z has now matured to the point-where it is possible for the alloy developer to quickly and reliably calculatephase equilibrium in a matter of minutes without an in-depth knowledge of thermodynamics. There thus exists a tool that enables the definition of alloy chemistries so that specific assumptions can be tested and the final chemistry selected without having to carry out the traditional manufacturing, processing, testing, and examination of a large raft of experimental chemistries. This article aims to show how such a tool has been used and high-lights the success of this approach in the field of Ni-based superalloy development.
Background
The gas turbine has its origins in the pioneering work of Sir Frank Whittle in the late 1930s. The world's first practical jet engine, the Whittle Wl, flew in the Gloster E28/39 on May 14, 1941. This engine weighed approximately 1,200 lbs and developed about 860 lbs of thrust.The engine was primarily made of chromium steel, and the peak gas temperature was about 6000C. The modern successor to this engine in civil applications is the high-bypass turbofan, which powers aircraft such as the Boeing 777 and the Airbus 330.Typical of this type of engine is the Rolls-Royce Trent family of engines (Figure 1), which typically weigh 16,000 lbs (75 tons) and can produce in excess of 110,000 lbs of thrust.Gas temperatures in these engines are in excess of 12000C. To achieve the equivalent thrust, one Trent would have to be replaced by 127 Whittle Wls. Conversely, to equal the power-to-weight ratio of the Whittle W1, the Trent would have to weigh 68 tons. These increases in performance have been achieved through improvements not only in engineering and aerodynamics but also in materials and the conditions that they can tolerate without catas-trophic, rapid degradation or failure. One of the best examples of such materials are Ni-based superalloys that are used to make the high-pressure turbine disk. In this application, these alloys (Table I) operate at a peak metal temperature of 6600C and rotate in excess of 10,000 rpm. The component is thus very heavily loaded and exposed to very high temperatures for a significant portion of its life, which may be 10,000 h or more.Furthermore, gas-turbine design engineers always seek to increase the operating temperature of the engine to improve its overall thermo-dynamic efficiency.This leads to a requirement for alloys with an ever-increasing temperature capability. Stability of the microstructure and good mechanical properties at high temperatures are thus essential, as failure of such a part cannot be contained,and damage to the aircraft on this scalemay have catastrophic consequences.
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