Superalloys for gas turbine engines
Superalloys are a group of nickel, iron–nickel and cobalt alloys used in aircraft turbine engines for their exceptional heat-resistant properties. Materials used in jet engines must perform for long periods of time in a demanding environment involving high temperature, high stress and hot corrosive gas. Many materials simply cannot survive the severe conditions in the hottest sections of engines, where the temperatures reach ~ 1300 °C. Superalloys, on the other hand, possess many properties required by a jet-engine material such as high strength, long fatigue life, fracture toughness, creep resistance and stress-rupture resistance at high temperature. In addition, superalloys resist corrosion and oxidation at high temperatures, which cause the rapid deterioration of many other metallic materials. Superalloys can operate at temperatures up to 950–1300 °C for long periods, making them suitable materials for use in modern jet engines.
Superalloys have played a key role in the development of high thrust engines since the 1950s when the era of jet-powered civil aviation and rocketry began. Jet aircraft would fly at slower speeds and with less power without superalloys in their engines. The most effective way of increasing the thrust of jet engines is by increasing their operating temperature. This temperature is limited by the heat resistance of the engine materials, which must not distort, soften, creep, oxidise or corrode. Superalloys with their outstanding high-temperature properties are essential in the development of jet engines.
The important role of superalloys in raising the maximum operating temperature of jet engines is shown in Fig. 12.1. This figure shows the improvement in creep resistance using an industry benchmark of the maximum temperature that materials can withstand without failing when loaded at 137 MPa (20 ksi) for 1000 h. Over the era of jet aircraft, the maximum temperature has risen over 50%. The benefits that the increased operating temperature has provided in engine power have been enormous. Over the past 20 years the thrust of the gas turbine engine has increased by some 60% while over the same period the fuel consumption has fallen by 15–20%. The impressive achievements in engine power and fuel efficiency have been accomplished in part by improvements in the material durability in the hottest sections of the engine, and in particular the high-pressure turbine blades.
12.1 Improvement in the temperature limit of superalloys for aircraft turbine engines. The operating temperature is defined as the creep life of the material when loaded to 137 MPa (20 ksi) for 1000 h.
Engine durability has also improved dramatically owing to advances in engine design, propulsion technology and materials. Improved durability allows the operator to better utilise the aircraft by increasing engine life and reducing maintenance inspections and overhauls. When the Boeing 707 first entered service in 1957 the engines were removed for maintenance after about 500 h of operation. Today, a modern Boeing 747 engine can operate for more than 20 000 hours between maintenance operations. This remarkable improvement is the result of several factors, including the use of materials with improved high-temperature properties and durability.
The development of superalloys together with other advances in engine technology has pushed the operating limit to ~ 1300 °C, resulting in powerful engines for large civil aircraft and high thrust engines for supersonic military fighters. The processing methods used to fabricate engine components have been essential in raising the operating temperatures of engines. The development of advanced metal casting and processing methods has been important in increasing properties such as creep resistance at high-temperature.
Figure 12.2 shows the materials used in the main components of a modern jet engine. Superalloys account for over 50% of the total weight. Superalloys are used in the hottest components such as the turbine blades, discs, vanes and combustion chamber where the temperatures are 900–1300 °C. Superalloys are also used in the low-pressure turbine case, shafts, burner cans, afterburners and thrust reversers. In general, nickel-based superalloys are used in engine components that operate above 550 °C. A problem with superalloys is their high density of 8–9 g cm−3, which is about twice as dense as titanium and three times denser than aluminium. Lighter materials are used whenever possible to reduce the engine weight. Titanium alloys are used to reduce the engine weight, but their use is restricted to components in the fan and compressor sections where the temperature is less than 550 °C. Titanium is used on the leading edges of carbon-fibre fan blades. Aircraft engines also contain aluminium alloys and fibre–polymer composites to reduce weight, although these materials can only be used in the coolest regions of the engine such as the fan blades and inlet casing, where the temperatures are less than 150 °C.
The use of superalloys in jet engines is explored in this chapter. The properties needed by materials used in engines are examined, such as high creep resistance and long fatigue life. The types of nickel, iron–nickel and cobalt superalloys are investigated, including their metallurgical properties that make them important engine materials. We also examine the advanced processing methods used to maximise the high-temperature properties of superalloys. The surface coating materials used to protect superalloys in the hottest regions of engines are also investigated.
Since the introduction of commercial jet engines in the 1950s, the aerospace industry has made on-going advances in engine technology to improve performance, power, fuel efficiency and safety. Figure 12.3 shows the improvements in the performance parameters of jet engines used in passenger aircraft. The maximum thrust at take-off has increased by 300% whereas the specific fuel consumption and engine weight per unit thrust has more than halved since the introduction of jet-powered airliners. In addition, modern jet engines are more durable, reliable, quieter and less polluting. These improvements in engine performance are the result of many factors, but without a doubt the development of nickel-based superalloys has been essential to the success.
The materials used in jet engines must survive arduous temperatures and withstand high stress for long periods. For example, turbine blades are designed to last at least 10 000 h of flying, which is equivalent to 8 million km of flight, at temperatures up to ~ 1200 °C. At this temperature the blades rotate at more 10 000 rpm which generates a speed of 1200 km h−1 at the blade tip and stress of about 180 MPa (or 20 tonne per square inch) at the blade root. To perform under such extreme conditions, materials used in the hot sections of jet engines must have some outstanding high temperature properties, which include:
Jet engines are complex engineering systems made using various types of metals, ceramics and composites, although superalloys are the key material to their high thrust and long operating life. There are many types of jet engines, with the most common on large passenger aircraft and military fighter aircraft being turbojets or turbofans. The basic difference between turbojets and turbofans is the way air passes through the engine to generate thrust. Turbojets operate by drawing all the air into the core of the engine. With turbofans the air passes through the engine core as well as by-passing the core. Most modern jet engines are turbofans, which are more fuel efficient than turbojets during flights at subsonic speeds and, for this reason, their operation is described.
Figure 12.4 shows the main sections of a turbofan engine. The first inlet section is the fan, which consists of a large spinning fan system that draws air into the engine. Air flowing through the fan section is split into two streams: one stream continues through the main core of the engine whereas the second stream by-passes the engine core. The by-passed air flows through a duct system that surrounds the core to the back of the engine where it produces much of the thrust that propels the aircraft forward. The temperatures within the fan section are not high, which allows titanium or composite materials to be used for their high specific stiffness, strength and fatigue life. Other considerations in the selection of materials in the fan region include resistance to corrosion, erosion and impact damage (from bird strike), making titanium alloys and composites suitable.
The air in turbofan engines passes through the engine core and enters the compressor section where it is compressed to high pressure. The compressor is made up of stages, with each stage consisting of compressor blades and discs, which squeeze the air into progressively smaller regions. As air is forced through the compressor section its pressure and temperature rapidly increase, thus requiring the use of heat-resistant materials. Hot compressed air then flows into the combustion chamber where it is mixed with jet fuel and ignited. This produces high-pressure gases that may reach a velocity of 1400 km h−1 and temperatures between 850 and 1500 °C. The combustion temperatures can exceed the melting point of the superalloys used in the combustion chamber. To survive, the superalloy engine components are protected with an insulating layer of ceramic material called a thermal barrier coating. The hot, high-pressure gases flow from the combustion chamber into the turbine section, which consists of bladed discs attached to shafts which run almost the entire length of the engine. The gases are allowed to expand through the turbine section which spins the blades. Power is extracted from the spinning turbine to drive the compressor and fan via the shafts. The turbine blades and discs are made using superalloys to withstand the hot gases coming from the combustion chamber. The gases that pass through the turbine are combined in the mixer section with the colder air that by-passed the engine core. The hot gases then flow into the rear-most section called the nozzle where high thrust is generated to propel the aircraft forward.
Nickel-based superalloy is the most used material in turbine engines because of its high strength and long fatigue life combined with good resistance to oxidation and corrosion at high temperature. Nickel-based superalloy is the material of choice for the hottest engine components that are required to operate above 800 °C. Without doubt, one of the most remarkable properties of nickel superalloys that is utilised in jet engines is their outstanding resistance against creep and stress rupture at high temperature. (The creep and stress rupture properties of materials are explained in chapter 22). Creep is an important material property in order to avoid seizure and failure of engine parts. Creep involves the plastic yielding and permanent distortion of materials when subjected to elastic loads. Most materials experience rapid creep at temperatures of 30–40% of their melting temperature. For example, aluminium and titanium alloys, which are used in the cooler regions of jet engines, creep rapidly above 150 and 350 °C, respectively. Nickel superalloys resist creep so well they can be used at 850 °C, which is over 70% of their melting temperature (Tm = 1280 °C). Very few other metallic materials possess excellent creep resistance at such high temperatures. The exceptional creep and stress rupture resistance of nickel superalloys means that engines can operate at higher temperatures to produce greater thrust. The outstanding creep and stress rupture resistance of nickel-based superalloys is shown in Fig. 12.5. Compared with the materials used in aircraft structures, aluminium, titanium and magnesium alloys, the stress rupture strength of nickel-based alloys is outstanding.
Nickel superalloys contain at least 50% by weight of nickel. Many of the superalloys contain more than ten types of alloying elements, including high amounts of chromium (10–20%), aluminium and titanium (up to 8% combined), and cobalt (5–15%) together with small amounts of molybdenum, tungsten and carbon. Table 12.1 gives the composition of several nickelbased superalloys used in jet engines.
The functions of the alloying elements are summarised in Table 12.2. The elements serve several important functions, which are to:
|Chromium||Solid solution strengthening; corrosion resistance|
|Molybdenum||Solid solution strengthening; creep resistance|
|Tungsten||Solid solution strengthening; creep resistance|
|Cobalt||Solid solution strengthening|
|Niobium||Precipitation hardening; creep resistance|
|Aluminium||Precipitation hardening; creep resistance|
|Carbon||Carbide hardening; creep resistance|
Superalloys are not named or numbered according to any system; they are usually given their name by the company that developed or commercialised the alloy. Of the many alloys, the most important for aerospace is Inconel 718 which accounts for most of the nickel superalloy used in jet engines. For example, superalloy 718 accounts for 34% of the material in the General Electric CF6 engine (shown in Fig. 12.2) used on the Boeing 787. Alloy 718 is a high-strength, corrosion-resistant alloy that is used at temperatures up to about 750 °C. Hastelloy X and Inconel 625 are often used in combustion cans and Inconel 901, Rene 95 and Discaloy are used in turbine discs. Nickel-based superalloys are available in extruded, forged and rolled forms. The higher strength forms are generally only found in the cast condition, such as directional and single crystal castings. The alloys PWA1480 and PWA1422 are special types of superalloys used in turbine blades that are produced by single crystal (SX) and directional solidification (DS) methods, respectively. These casting methods for producing blades are explained in chapter 6.
Nickel superalloys derive their strength by solid solution hardening or the combination of solid solution and precipitation hardening. Superalloys that harden predominantly by solid solution strengthening contain potent substitutional strengthening elements, such as molybdenum and tungsten. These two alloying elements have the added benefit of having low atomic diffusion rates in nickel, and move very slowly through the lattice structure at high temperatures. Creep is controlled by atomic diffusion processes, with the creep rate increasing with the diffusion rate of the alloying elements. Therefore the slow movement of alloying elements impedes the creep of nickel.
Solid solution-hardened nickel alloys have good corrosion resistance, although their high-temperature properties are inferior to precipitation hardened alloys. Figure 12.6 shows the creep-rupture strength of nickel-based superalloys hardened by means of solid solution strengthening or precipitation strengthening. Here, the stress necessary to cause tensile rupture in 100 h is given over a range of temperatures. Nickel superalloys hardened by precipitation strengthening provide the best high-temperature performance and, for this reason, are used in jet-engine parts such as blades, discs, rings, shafts, and various compressor components. Precipitation-hardened nickel alloys are also used in rocket motor engines.
Precipitation-hardened nickel alloys contain aluminium, titanium, tantalum and/or niobium that react with the nickel during heat treatment to form a fine dispersion of hard intermetallic precipitates. The most important precipitate is the so-called gamma prime phase (γ′) which occurs as Ni3Al, Ni3Ti, or Ni3(Al,Ti) compounds. These precipitates have excellent long-term thermal stability and thereby provide strength and creep resistance at high temperature (Fig. 12.7). A high volume fraction of γ′ precipitates is needed for high strength, fatigue resistance and creep properties (typically above 50%).
The γ′ precipitates are formed by a heat-treatment process that involves solution treatment followed by thermal ageing to dissolve the alloying elements into solid solution. Solution treatment is performed at 980–1230 °C. Single-crystal nickel alloys are solution treated at higher temperatures (up to 1320 °C). Following solution treatment and quenching, the nickel alloy is aged at 800–1000 °C for 4–32 h to form the γ′ precipitates. The ageing temperature and time is determined by the application of the superalloy. Lower ageing temperatures and shorter times produce fine γ′ precipitates for engine parts requiring strength and fatigue resistance, such as discs. Higher temperatures produce coarse γ′ precipitates desirable for creep and stress rupture applications, such as turbine blades.
Precipitation-hardened nickel alloys contain other thermally stable compounds (in addition to γ′) which also contribute to the high-temperature properties. Titanium, tantalum, niobium and tungsten react with carbon to form several types of hard carbide precipitates: MC, M23C6, M6C and M7C3 where M stands for the alloying element. These carbides perform three important functions:
Small amounts of boron, hafnium and zirconium are often used as alloying elements in nickel. These elements combine with other elements to pin grain boundaries, thereby reducing their tendency to slide under load and thereby increasing the creep strength. Niobium is an important alloying element for the precipitation hardening of nickel alloys. The most commercially important superalloy is Inconel 718, and it is strengthened mainly by niobium precipitates (Ni3Nb).
The newest types of superalloys contain rare earth elements (such as yttrium or cerium) at concentrations of 2.5 to 6% to increase the high-temperature creep strength by precipitation and solid solution hardening. However, rare earth elements are expensive to use, they increase the density of the superalloy, and they can produce casting defects. Ruthenium is increasingly being used instead of, or in combination with, the rare earth elements to achieve similar improvements in high-temperature mechanical performance without the problem of casting defects. The platinum group of elements (platinum, iridium, rhodium, palladium) are also being used to increase the operating temperature limit to 1100–1150 °C, but these are expensive.
Some grades of nickel superalloys contain submicrometre-sized oxide particles, such as ThO2 or Y2O3, to promote higher elevated temperature tensile and stress rupture properties. For example, superalloy MA754 is produced by powder metallurgy involving mechanical alloying to introduce about 1% by volume of a fine dispersion of nano-sized oxide particles.
A problem with using metals at high temperature is rapid oxidation which quickly breaks down and degrades the material. Nickel alloys also contain 10–20% chromium to provide oxidation resistance through the formation of a protective surface oxide film composed of Cr2O3 or NiCr2O4. Nickel alloys rapidly degrade by oxidation without this surface film, which must be stable at the high operating temperatures of gas turbine engines.
Iron–nickel superalloys are used in gas turbine engines for their structural properties and low thermal expansion at high temperature. Iron–nickel alloys expand less than nickel or cobalt superalloys at high temperature, which is an important material property for engine components requiring closely controlled clearances between rotating parts. Iron–nickel alloys are generally less expensive than nickel- and cobalt-based superalloys, which is another advantage. The main uses for Iron–nickel alloys in jet engines are blades, discs and casings.
The composition of several Iron–nickel alloys used in jet engines is given in Table 12.3, and most contain 15–60% iron and 25–45% nickel. Iron–nickel superalloys are hardened by solid solution strengthening and precipitation strengthening. Aluminium, niobium and carbon are used as alloying elements to promote the formation of hard intermetallic precipitates or carbides that are stable at high temperature. The precipitates are similar to those present in nickel-based superalloys, and include γ′ Ni3(Al,Ti), γ′′(Ni3Nb) and various types of carbides. The precipitates provide Iron–nickel alloys with good resistance against creep and stress rupture at elevated temperature. Chromium is used to form an oxide surface layer to protect the metal from hot corrosive gases and oxidation.
Cobalt superalloys possess several properties which make them useful materials for gas turbine engines, although they are more expensive than nickel superalloys. Cobalt alloys generally have better hot-corrosion resistance than nickel-based and iron–nickel alloys in hot atmospheres containing lead oxides, sulfur and other compounds produced from the combustion of jet fuel. Cobalt alloys have good resistance against attack from hot corrosive gases, which increases the operating life and reduces the maintenance of engine parts. However, comparison between nickel and cobalt alloys must be treated with some caution because there are wide differences in hot corrosion resistance within each group of superalloys. That is, certain nickel superalloys also have excellent resistance against hot corrosion. Cobalt alloys also have good stress rupture properties, although not as good as precipitation-hardened nickel-based alloys (Fig. 12.6).
Cobalt superalloys contain about 30–60% cobalt, 10–35% nickel, 20–30% chromium, 5–10% tungsten, and less than 1% carbon. The composition of some cobalt alloys used in jet engine components is given in Table 12.4. The main functions of the alloying elements are to harden the cobalt by solid solution or precipitation strengthening. The precipitates that form in cobalt alloys do not provide the same large improvement in high-temperature strength as nickel alloys and, for this reason, the resistance of cobalt alloys against creep and stress rupture is inferior to precipitation-hardened nickel-based and Iron–nickel alloys. Cobalt alloys are generally used in components that operate under low stresses and need excellent hot-corrosion resistance.
Turbine blades contain rows of hollow aerofoils for cooling to increase the engine operating temperature. Cool air flows through the holes, which are located just below the surface, to remove heat from the superalloy. The aerofoils are remarkably effective at cooling, which allows increased operating temperature and associated improvements in engine efficiency. To increase the operating temperature even further, the hottest engine parts are coated with a thin ceramic film to reduce heat flow into the superalloy. The film is called a thermal barrier coating, which has higher thermal stability and lower thermal conductivity (1 W m−1 K−1) than nickel superalloy (50 W m−1 K−1). The use of the coating allows higher operating temperatures (typically at least 170 °C) in the turbine section. The coating provides heat insulation and this lowers the temperature of the superalloy engine component. Thermal barrier coatings can survive temperatures well in excess of the melting temperature of the superalloy itself, and also provide protection from the effects of thermal fatigue and creep and the oxidising effect of sulfates and other oxygen-containing compounds in the combustion gases.
Thermal barrier coatings are complex multilayered structures, as shown in Fig. 12.8. The coating can be applied using various deposition methods, with the most common being electron-beam physical vapour deposition (EBPVD). In this process, a target anode material consisting of zirconium oxide (ZrO2) and yttrium oxide (Y2O3) is bombarded with a high-energy electron beam under vacuum. The electron beam heats the anode material to high temperature which causes atoms and oxide molecules from the target to transform from the solid into gas phase. The gaseous atoms and molecules then precipitate as a thin solid layer of the anode material onto a substrate, such as a nickel superalloy component. The coating is deposited on the surface to a thickness of about 0.1–0.3 mm, which is sufficient to provide heat protection to the underlying metal.
The most common coating material is yttria-stabilised zirconia (YSZ), which is based on zirconia doped with 7% yttria. The YSZ is bonded to the surface via intermediate layers which improve the adhesion strength properties. An intermediate bond coat with a chemical composition MCrAlY (where M = Co, Ni or Co + Ni) or NiAl–Pt are often used. The bond coat also provides oxidation and corrosion resistance to the underlying superalloy component. Thermal barrier coatings are used on engine components in the combustion chamber and turbine sections, including high-pressure blades and nozzle guide vanes. However, YSZ TBCs are unsuitable for use on loaded rotating components because their low tensile strength and toughness causes them to crack and spall.
The aerospace industry is continually developing new materials to increase the operating temperature limit of gas turbine engines. As mentioned in chapter 9, titanium aluminides are being developed for engine applications. A group of refractory intermetallics based on metal silicides (Mo5Si3, Nb5Si3, Ti5Si3) retain high strength to about 1300 °C, which is 200 °C higher than single-crystal nickel superalloys. Nb5Si3 is attracting the greatest interest of the silicides, although it is prone to oxidation at high temperature. Other types of advanced materials such as eutectic solidified ceramics, ceramic matrix composites (e.g. carbon/carbon), and silicon nitride are also being evaluated as high-temperature engine materials. For the foreseeable future, however, nickel-based superalloys are likely to be the dominant structural material for high-temperature components in jet engines.
Superalloys have played a central role in the development of jet engine technology. The development of superalloys with better high-temperature and hot-corrosion properties together with advances in engine design and propulsion technology has resulted in great improvements in engine performance. Over the past 20 years, the thrust of jet engines has increased by more than 60% whereas the fuel consumption has fallen by 15–20%, and these improvements are, in part, the result of improvements in the high-temperature properties of superalloys.
A variety of high-performance materials is used in modern jet engines. Aluminium and carbon-fibre composites are used in the coolest sections of engines (operating at temperatures below about 150 °C), such as the fan and inlet casing, to minimise weight. Titanium (α + β and β) alloys are used in engine components with operating temperatures below about 550 °C, which includes parts in the fan and compressor sections. Superalloys are used for components that operate above 550 °C, such as the blades, discs, vanes and other parts found in the combustion chamber and other high-temperature engine sections.
Materials used in the hottest engine components, such as high-pressure turbine blades and discs, must have high strength, fatigue life, fracture toughness, creep resistance, hot-corrosion resistance and low thermal expansion properties. Nickel-based superalloys are the material of choice of these engine components because of their capability to operate at temperatures up to 950–1200 °C for long periods of time.
Nickel-based superalloys used in jet engines have a high concentration of alloying elements (up to about 50% by weight) to provide strength, creep resistance, fatigue endurance and corrosion resistance at high temperature. The types and concentration of alloying elements determines whether the superalloy is a solid solution-hardened or precipitation-hardened material. Precipitation-hardened superalloys are used in the hottest engine components, with their high-temperature strength and creep resistance improved by the presence of γ′ [Ni3Al, Ni3Ti, Ni3(Al,Ti)] and other precipitates that have high thermal stability.
The casting process is important in the production of heat-resistant superalloy engine components. The creep resistance of materials is improved by minimising or eliminating the presence of grain boundaries that are aligned transverse to the load direction. Superalloys are cast using directional solidification which produces a columnar grain structure with few transverse grain boundaries or single crystal casting which eliminates all grain boundaries.
Iron–nickel superalloys are used in jet engines for their high-temperature properties and low thermal expansion. These superalloys, which contain 15–60% iron and 25–45% nickel, are used in blades, discs and engine casings that require low thermal expansion properties.
Cobalt superalloys are used in jet engine components that require excellent corrosion resistance against hot combustion gases. The alloys contain 30–60% cobalt and high concentrations of nickel, chromium and tungsten which provide good resistance against lead oxides, sulfur oxides and other corrosive compounds in the combustion gas.
Thermal barrier coatings are a ceramic multilayer film applied to the superalloy surface to increase the operating temperature of the engine. The coating is an insulating layer that reduces the heat conducted into the superalloy. Yttria-stabilised zirconia (YSZ) is the most common coating material, and is used on engine components in the combustor chamber and turbine sections, including high-pressure blades and nozzle guide vanes.
Winston, M.R., Partridge, A., Brooks, J.W. The contribution of advanced high-temperature materials for future aero-engines Proceedings of the Institute of Mechanical Engineers. Part L: Journal of Materials: Design and Applications. 2001; 215:63–73.