To understand the capbilities of modern jet engines, it is not sufficient to explain how jet engines work. The issue is rather how the most sophisticated materials of the day allow operation in the extreme conditions of temperature, mechanical loading, and corrosive resistant that is required for a global economy dependent on affordable long-range jet transportation. The design of jet engines is fundamentally limited by the state of metallurgical engineering available at the time the engine was developed. The status of metallurgy is what we will attempt to expose, at leasetto some extent.
From the Heinkel HeS3, used in the world’s first jet plane in 1939 to the newest GE 90115B which provides the motive force for the Boeing 777, jet engines and their materials have evolved to provide vastly improved performance and reliability. These remarkable advances can primarily be attributed to advances in high-temperature metallurgy.
During operation, the components in a jet engine are exposed to one of the harshest environments encountered in any engineered product. High temperatures, corrosive gases, vibrations and high mechanical loads that would humble any.conventional materials are everyday toil for jet engine materials. This extreme environment can lead to failure of the most robust materials in much the same way that a paperclip that is bent back and forth breaks because of metal fatigue.
What metallurgically engineered materials make a jet engine a practical driver of transportation, rather than a showcase for modern technology? Most of the high-performance parts of a modern jet engine are made of superalloys.
Discovered in the 1950s, superalloys quickly became the industry standard owing to their excellent mechanical and thermal properties combined with outstanding corrosion resistance. Nickel-based superalloys are precipitation-hardened, giving the superalloy excellent creep resistance (long operating times) and strength which increases with temperature. Due to these unique material properties, Ni-based superalloys are still the primary material choice for high-temperature jet engine components.
Modern Ni-based superalloys contain many additives such as tungsten, tantalum, molybdenum, or rhenium to increase their maximum operating temperature. A typical nickel-base superalloy (Inconel-100) has a density of 7.8 g/cc and is composed of 62% (by weight) nickel, 15% chromium, 10% cobalt, 5% each of titanium and aluminum, 3% molybdenum, and trace amounts of carbon, boron, zirconium, and vanadium. Inconel-100 melts at 1135 °C, and can be used as temperatures as high as 1000 °C. In contrast, conventional carbon steels lose much of their strength by 500 °C, and have (by comparison) very little corrosion resistance.
Unlike most of the engine components, the fan, which pulls large quantities of air into the engine, does not experience high temperatures. It does, however, face the special challenge of surviving encounters with rain, snow, hail, and the occasional bird strike. Thus, when selecting materials for the fan impact toughness as important as strength. By comparison, low carbon steels are hardly corrosion resistant, and lose most of their strength once an operating temperature of
Fans in most modern turbojet engines are made of titanium alloy fan blades mounted to a superalloy fan disk. Originally solid fan blades were used, but a recent approach to reducing fan weight is being championed by Rolls Royce, who are making fan blades of hollow titanium, often filled with a titanium honeycomb, which greatly reduces its parasitic weight. The two halves of a hollow fan blade are cast of titanium alloy (often Ti-4Al-6V), welded together, then machined to the desired dimensions and polished. The fan disk, which includes mounting notches for the blades, is usually machined from a single titanium alloy forging.
A new approach to fan design appears in the GE 90 series of commercial turbofan engines. Here the fan is made of advanced composites, with titanium added to the leading and trailing edges of the blades to even out internal stresses. This fan is 10 ½ feet in diameter, and has a maximum rated rotational speed of 2550 rpm, so that the blade tips are supersonic. Centripetal force pulls the fan blades apart with over 10000 g’s of acceleration. When a fan fails under this enormous stress, it falls into high-velocity pieces, so measures must be taken to contain shrapnel. With composite materials, however, the main failure mechanisms result in production of a tangled mess of fibers and composite, which is much easier to safely confine, leading to weight savings of as much as 700 pounds for the largest engines.
The compressor accepts the air from the fan (over a ton per second in the GE 90115B at take-off), and compresses the air to a pressure 40 or more times the inlet pressure, which also increases the temperature to as high as 700 °C. This compressed air is then injected into the combustion chamber, where it is mixed with jet fuel and ignited. The turbine then spins in the exhaust of the combustion chamber, generating the power needed to drive the fan and the compressor. Modern turbines operate at inlet temperatures as high as 1600 °C, and can produce in excess of 100,000 horsepower. The combination of these conditions requires remarkably strong components. Compressors and turbines share many design and manufacturing techniques, so these will be described together.
The most common manufacturing technique for compressors or turbines is to form a compressor or turbine disk using superalloys and hot isostatic pressing. Such a disk is rather like a large notched wheel to which the blades are attached. In hot isostatic pressing, a fine grained metal powder is injected into a flexible mold which is evacuated of air. The mold is then compacted at high temperature by surrounding it with a high pressure gas. Process conditions are about 25000 psi and 1200 °C. Diffusion of atoms between the particles results in a fine grain polycrystalline compact. This compact is nearly fully dense, and is near net shape, so that only minor finishing operations need be carried out. The compressor or turbine blades are then cast, and the two assembled into the final compressor or turbine assemblies. In some cases one-piece
assemblies called blisks are made, often using friction welding to affix the blades to the disk.
Blades for compressors having outlet temperatures below about 350 °C can be made of high temperature titanium alloys such as Ti6Al4V. Compressor blades for higher temperature operation are generally made of nickel-base superalloy. As it is extremely difficult to cool the compressor blades, operating temperatures are currently limited to about 700 °C, although if necessary some of the methods used to make turbine blades could expand this envelope to some extent.
Perhaps the most interesting area of jet engine metallurgy is the manufacture of the turbine blades. Solid turbine blades cast from a suitable superalloy can be used for turbine inlet temperatures up to about 900 °C. Adopting hollow blades with cooling channels allows excess air from the compressor to be circulated within the blade and through small holes onto the blade surfaces. Further improvement can be obtained by coating the blades with a thermal barrier. The thermal barrier coating is composed of thermally insulating materials, often ceramics and refractory metals, which can withstand high temperature and corrosive atmosphere, and improve the effectiveness of the blade cooling system. The net improvement in operating temperatures is substantial, but still not enough to allow operation at modern turbine inlet temperatures.
The high-temperature mechanical properties of the superalloy itself had to be improved. One approach is directional solidification, developed in the 1960s. Directional solidification produces long columnar grains along the loading direction of the turbine blades – that is, aligned from the base of the blade to the tip. The relative lack of grain boundaries transverse to this loading direction results both in increased strength and retention of that strength to higher temperatures.
Directionally solidified components are made in an induction heated mold, with an initial temperature of about 1500 °C. This mold contains a water-cooled copper plate at the bottom of the mold. The superalloy is poured into the mold, where it begins to solidify near the copper plate. The copper plate has a knurled surface, so that a large number of evenly spaced crystallites appear in the initial freezing process. The mold is then slowly cooled from the bottom up, so that the superalloy crystals grow in alignment with the thermal gradient, along the axis of the blade. The speed of cooling is critical to obtaining the desired microstructure, requiring several hours to produce an average turbine blade.
Although directionally solidified superalloys are a considerable improvement over superalloys produced using traditional casting or hot isostatic casting, they still have defects which limit their performance in gas turbines. Single crystal turbine blades, in which such defects are reduced to the maximum practical extent, were developed in the 1970s, largely through the efforts of the research labs of Pratt & Whitney.
The fabrication of a single crystal turbine blade is rather similar to the directional solidification process, where the crystal grows in a carefully controlled one-dimensional thermal gradient. However, in this case the growth is started in a lower chamber called the starter. The growth process in the starter is the same as in directional solidification, where columnar crystals form at a chill plate. The starter chamber narrows at the top to form a helical tube known as the pigtail. As the superalloy grows, only a few crystallites can enter the pigtail. Because the pigtail curves, most crystallites will grow into the pigtail wall and then stop, as there is no space left into which they can grow. In the end, only one crystallite emerges from the pigtail. The blade grows from this lone crystallite, forcing the turbine blade into a single crystal structure. This is essentially the same method used to grow very large silicon crystals for use in microelectronics.
Because single crystals have no grain boundaries, grain boundary strengthening elements are not needed. As a result, the superalloys used in single crystal fabrication have a simpler composition than traditional superalloys. For example, CMSX-6 is 70.4% nickel, 10% chromium, 5% cobalt, 4.8% aluminum, 4.7% titanium, 3% molybdenum, 2% tantalum, and 0.1% hafnium. Designing such a superalloy is an artform, combining basic concepts of metallurgy with a great deal of experimentation aimed at optimizing specific metallurgical properties. Recently developed superalloys for single crystal blades also have several percent of rhenium and ruthenium to further increase operating temperatures.
The combustor has the difficult task of burning large quantities of fuel, supplied through fuel spray nozzles, with enormous volumes of air supplied by the compressor, and releasing the resulting heat in such a manner that the air expands to deliver a smooth stream of uniformly heated gas into the turbine. This must be accomplished without melting anything important, and with the minimum loss in pressure and with the maximum heat release possible within the limited space available.
The combustor for a high performance jet engine is made of superalloys, which often receive special ceramic coatings, such as alumina and zirconia. These coatings serve to insulate the inner surface of the combustor from the intense heat of the burning fuel-air mixture, and also provide an important degree of erosion protection.
Jet engine metallurgy has shown rapid progress over the past 70 years, and the rate of improvement continues to this day. The sum of small continuous improvements is an important part of this evolution, but it has also depended on dramatic moments of insight and discovery. The advent of new classes of high-temperature structural materials, such as metal matrix composites and boron fiber reinforced metals and ceramics, should form the basis for a fascinating story into the future.