Nickel-based superalloys have come to dominate the high temperature stages of the gas turbine engine, from the high pressure compressor through the combustor and turbine stages to the exhaust outlet. Their success is due to their unique combination of mechanical strength and resistance to oxidation and corrosion at elevated temperatures.
Two components which have driven the development of the nickel-based superalloys are the high pressure turbine blade and disc.
The High Pressure Turbine Blade
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The high pressure turbine blade sits in the harsh environment behind the combustor and rotates at high speed in order to extract energy from the high temperature gas stream.
It is required to withstand centrifugal loads of up to ten tonnes while operating at temperatures in excess of the melting point of the alloy. Therefore, turbine blade research and development has always focused on increasing temperature capabilities and its evolution illustrates the hand in hand advancement of materials and manufacturing techniques.
Operation in this environment makes severe demands on both the mechanical properties and environmental stability of the blade system and is only possible through the close integration of design, materials and manufacturing.
A step change in temperature capability was realised through the introduction of directional solidification, eliminating transverse grain boundaries, a source of weakness in a creep dominated application. It was then a natural progression to the complete elimination of grain boundaries via single crystal casting and the continued drive for more temperature capability has led to successive generations of alloy with ever more exotic alloying additions.
With increasing blade operating temperatures, the intrinsic resistance of the metal to environmental attack is no longer sufficient. Protective coatings are therefore required to provide a thermal barrier and/or impart the necessary oxidation and corrosion resistance, see Coatings. The net effect of all the advances in blade technology outlined above, coupled with alloy development, has been to increase metal temperatures by approximately 300°C over the last 50 years: a figure that can be doubled when the temperature of the gas stream itself is considered.
The High Pressure Turbine Disc
Turbine discs operate at lower temperatures than blades, as they are not in the direct gas path exiting the combustor, however, they must attain the most stringent levels of mechanical integrity. The failure of a turbine disc may not be contained by the engine casing and could seriously hazard the aircraft. The development of disc alloys has traditionally been driven by the requirements of the high pressure turbine disc. The consistent objective is a hotter disc with an equivalent cyclic life, requiring highly alloyed, higher strength materials. Traditionally the manufacture of turbine discs has been via a cast and wrought route.
However, for advanced nickel-based superalloys with high alloy contents segregation at the ingot stage becomes problematic. The solution has been to move to a powder processing route involving the atomisation of a molten stream of metal in an inert atmosphere. The resultant rapid solidification and fine powder size restricts segregation. Consolidation is then achieved by hot isostatic pressing (HIPping) followed by extrusion to provide a fully dense billet for subsequent isothermal forging.
Components produced with a single microstructure throughout invariably result in a trade-off in design and/or component life because of the relationship between various critical mechanical properties and grain size. Therefore, further temperature capability and weight optimisation is achieved by producing dual-microstructure components, whereby a fine grain size is retained in the lower temperature bore region to maximise tensile and fatigue strength whilst the hot rim of the disc is selectively grain coarsened to enhance creep and fatigue crack growth resistance. By selectively developing this coarse grain microstructure the temperature capability of the alloy is improved by up to 30°C.