Structural Materials in Aerospace Systems
Dipankar Banerjee
Materials Challenges for the 21st Century feature

[March 2001 Issue]

When you hear something about a new material, write it down because it will be the best thing you'll ever hear about it. -Jim Williams quoting Bob Sprague of General Electric This is my dream for the taking. Take it, don't turn away. -Simon and Garfunkel

Surely one of the defining events of the past century was humankind's first flight in 1903. A related epoch-making technology was the enabling of rocket propulsion, the associated developments of space flight and missile-delivery systems, and the deployment of satellites and space stations. As we begin this new millennium, our immediate goals seem well defined: Passenger aircraft at Mach 2 or greater with a range of 5000-8000 miles would revolutionize commercial transport, while a reusable, single-stage, transatmospheric, hypersonic vehicle will open up new horizons in transportation. Fighter capability will expand to very short take-off and landing aircraft with operational capability beyond 50,000 ft at supersonic speeds, with the high agility that thrust vectoring can provide. The expansion of space-based communication and sensing systems, coupled with our ultimate dreams of space exploration and colonization, will require highly durable structures capable of withstanding very unique environments whose effects we have gained experience of with extensive satellite deployment, deep-space missions such as the Voyager, and space platforms such as the Mir. In this article, we shall examine the demands made of structural materials in such systems in three sections, the first covering propulsion systems; the second, airframe structures; and the third dealing with space-based platforms.

Propulsion Systems

Propulsion systems of interest in this century will be the conventional air-breathing jet engines to supersonic speeds of Mach 2 or 3 for passenger aircraft, ramjets for speeds ranging from Mach 2-5, and scramjets for speeds beyond this to power light, reusable, hypersonic transport. Reusable rocket engines with significantly improved durability over the current versions used in the space shuttle will be developed, while nuclear-powered engines will continue to be used in deep-space exploration. While the common catchwords for the use of materials in such systems are "hotter" and "lighter," their actual use varies from many thousands of hours in civil transport to a few seconds or minutes in tactical missiles; therefore, approaches to materials development and usage also vary significantly for these different applications.

The turbojet, the turbofan, or the turboprop power aircraft today, the former accelerating a small mass of air to high velocities while the latter judiciously mixes the benefits of the former with the advantages of accelerating a large fraction of air to smaller velocities. The evolution of the jet engine (itself just about 60 years old) has been defined by the improvement of the temperature capability of materials in terms of both their specific strength or toughness and modulus, as well as durability defined by creep and fatigue and resistance to turbine combustion products. The allowable compressor outlet temperature and the turbine entry temperature directly affect the efficiency of the engine, demanding materials that perform at higher and higher temperatures, while density defines the thrust-to-weight ratio and specific fuel consumption. Turbomachinery rotates at speeds exceeding 10,000 rpm, leading to high stresses that can alternate concurrently with temperature variations. The emergence of titanium alloys in the 1950s that could be used in the compressors and the nickel-based superalloys in the 1940s for the turbines set the current industry standard. The temperature capability of titanium alloys has evolved from about 300°C, possible with the workhorse Ti-6Al-4V alloy, to about 550-600°C today. This increase has in part been affected by a unique application of the Cottrell atmosphere through the addition of Si to retard dislocation glide and climb processes over a temperature range in which creep processes are critical. The development of Ni-based alloys has proceeded along two major paths, one for very strong, fatigue- and fracture-resistant materials for turbine discs with a relatively lower temperature capability (700-750°C), the other directed toward high-temperature capability (1100°C) through cast alloys for turbine blading. The industry norm for the former was set by the emergence of alloy 718 in the late 1950s. More recently evolved versions obtain higher strengths through increased alloying, while offsetting the attendant disadvantages of segregation in large ingots through the use of powders as starting materials. The development of the cast superalloys presents a fascinating metallurgical story that contains all of the elements of classical physical metallurgy, starting with the discovery of precipitation-hardening, the use of solid-solution strengthening with refractory elements, the strengthening of grain boundaries, the elimination of disadvantageously oriented grain boundaries by directional solidification during the investment casting process (which is an evolution of the ancient Indian lost-wax process for casting temple bronzes), and finally the elimination of grain boundaries themselves through single-crystal growth processes. Design features incorporating complex internal cooling profiles were permitted by innovations in the casting process, and this, together with very recently developed thermal-barrier coatings based on insulating ceramic layers, permits the use of cast Ni-based superalloys at turbine entry temperatures that exceed the melting point of the metal! The evolutionary improvement in alloy properties has approached a natural asymptote. In the case of Ni-based alloys these limits are set by the melting point of the material (the cast alloys have an intrinsic temperature capability in structural performance at an astonishing 80-85% of their melting point). In the case of titanium alloys, the limit is set by the approach to the transition to the high-temperature bcc allotropic modification of titanium that exhibits anomalously high diffusivity.

The intermetallics present a fascinating option for combining excellent temperature capability and relatively low densities with their high melting point and stiffness and intrinsically lower diffusivities. These properties also imply (because of the greater directionality in their bonding in relation to metals) that they are relatively brittle. Of the intermetallics, the aluminides are the most amenable to ductilization or toughening using plasticity as a conventional stress relaxation mechanism in arresting cracks. The remarkable discovery of the boron effect in ductilizing Ni3Al at levels of a few atoms in a million fueled extraordinary advances in toughening and processing intermetallics, so that today there are several additional engineering alloys based on the compounds Ti3Al (or Ti2AlNb) and TiAl that can be regarded as potential structural materials providing significant density benefits while matching the performance of conventional Ni-based alloys in the application temperature range of 600°C to about 800°C, while alloys based on NiAl provide density and thermal-conductivity-based benefits over the cast Ni-based superalloys.

The operating temperatures in turbomachinery for supersonic civil transport are not much higher than during takeoff than in conventional civil aeroengines or during high-performance missions of military aircraft. However, materials will see such peak temperatures for much longer durations, say tens of thousands of hours in conventional aircraft as compared with thousands of hours in military aircraft. The reliability of materials operating at high temperatures will therefore be a key factor. Additional issues for power plants in high-speed civil transport are noise pollution as well as the emission of NOx. The Concorde engine produces 20 g of NOx for every kg of fuel burnt, and its jet velocity is about 900 m/s. An acceptable goal is about 5 g of NOx for every kg of fuel. Apart from the combustion process itself, the mixing of cooling air in the combustor and turbine of such engines (required to maintain Ni-based alloys at their operational temperature capability) with hot gases from combustion results in the formation of NOx. Thus, uncooled materials operating at today's turbine and combustor temperatures exceeding 1200°C will be developed to meet these requirements. The challenge is to find materials with high enough melting points and adequate toughness that also form stable oxide scales on oxidation. The only oxides that possess low oxygen-transport properties and low vapor pressures at temperatures above 1200°C are Al2O3, SiO2, and BeO (and possibly the spinels of yttrium aluminum garnet). The materials that permit selective oxidation to form these scales are MoSi2, SiC, Si3N4, and the beryllides. The use of ceramics and very high temperature intermetallics also implies that plasticity will not be adequate for crack-growth resistance. In such brittle materials, toughness is governed simply by the size and density of pre-existing flaws in the material, which in turn depend on the volume of the material. Two key developments paved the way for advances in this area. The first involved the understanding and applications of alternative toughening mechanisms that do not rely on plasticity but are based instead on microcracking, crack-branching or -bridging, and other related mechanisms. The second involves sophisticated applications of the ancient art of converting an organic material such as wood to inorganic materials such as charcoal by decomposition. Thus, organometallic polymers such as polycarbosilane were first processed to SiC fiber in the mid-1970s. Alternatively, methyltrichlorosilane is decomposed in the gaseous form to deposit SiC coatings on tungsten or carbon fiber. Together, these have led to the synthesis of composites based on SiC fiber embedded in a SiC matrix, titanium alloys and their intermetallics reinforced with continuous fibers of SiC, or second-phase toughened silicides. Titanium matrix composites reinforced with SiC have matured to the point of being considered for use in integrated blade and disc rotors in replacement of Ni-based superalloys and other engine applications, while SiC/SiC composites are used in relatively low-temperature applications as convergent-divergent nozzles of fighter aircraft. The use of these materials at higher temperatures for long durations in oxidizing or moisture-containing environments presents unique challenges that will have to be overcome, while the cost of fibers and composite consolidation processes precludes application for low-volume requirements. The control of noise will demand very large exhaust systems, whose weight can be controlled only through the use of the titanium aluminides, or SiC-fiber-reinforced composites of intermetallics, SiC or Si3N4, or both.

In contrast to power plants for aircraft, rocket engines represent the use of materials at very high temperatures for relatively short durations. Solid fuels are used in tactical missiles with a time scale of application measured in seconds to minutes. Both solid and liquid propellants are used in strategic missiles and rockets for payload boosters and auxiliary engines and as booster engines in fighter aircraft, with a time scale of application ranging from hours to tens of hours. The materials of choice for these short-term applications are those that possess the highest specific strength and modulus at very high temperatures-the refractory metals and carbon/carbon or carbon/SiC composites. The key parts of rocket engines are the combustion chambers, turbomachinery in liquid-fueled engines, and the supersonic nozzle. Solid-fuel casings are made primarily out of aluminum or steel, depending upon the specific impulse required. These will be increasingly replaced by graphite/polymer composite structures. The thrust chamber sees temperatures ranging from 1500°C to about 3500°C, the solid propellants providing the higher temperatures. The combustion products can be oxidizing and reducing as well as corrosive and erosive, with material loss measured in mils per hour rather than mils per year. Thermal transients are extremely high, with heating rates of the order of 900-5000°C/s. The combustion pressure can approach 300 atm. Thrust-chamber materials must therefore be cooled actively, or by film cooling, radiation, or ablation. Carbon/carbon composites, continuous carbon fibers arranged in two- or higher-dimensional weaves held together by a matrix infiltrated by liquid or gaseous phases and then carbonized, offer the highest strength at temperatures above 1500°C. Such materials are used for short-term applications in the thrust chambers of tactical missiles. For longer durations of use, actively cooled combustion-chamber materials consist of copper alloy liners in casings of Ni-based alloys, as in the space shuttle. In liquid-propelled engines, the coolant is usually the fuel-for example, hydrogen in cryogenic engines-and the very steep thermal gradients impose thermal strains of the order of 2-3% each time the engine is fired. In a reusable engine, this could be 500 times or more. Moreover, the thickness of the copper liner has to be as low as 0.25 mm to maintain surface temperatures below 1000°C. Thus, the liner must have high strength, low cycle-fatigue resistance, high coolant-embrittlement resistance, and possess a thermal conductivity at least as high as that of copper. These materials will also find use in the actively cooled structures of ramjets and scramjets. The space shuttle engine, for example, uses a copper-silver-zirconium alloy. However, the simplest of thrust-chamber structures is realized by radiative cooling. While niobium coated with silicide is a current option, advanced radiatively cooled structures may consist of layered refractory structures such as iridium/rhenium combinations coated with zirconia.
The efficiency of liquid-propelled rocket engines depends on the pressure at which the fuel is injected into the thrust temperature. The turbopumps that achieve this operate at high temperatures and use cast Ni-based superalloys. Tomorrow's turbopumps, or jet turbines that integrate with ramjets and scramjets, will use SiC composites consisting of three-dimensional preforms of silicon carbide fiber reinforced with a matrix of silicon carbide.

Materials in Airframes

In their first flight in 1903, the Wright brothers used an airframe structure made of wood, wire, and fabric. Skin temperatures of airframes rise rapidly with flight velocity, especially at the leading edges of the structure, inlet ducts, and lower surfaces. Aluminum was first used in the early 1900s and continues to be used today extensively in civil transport, but also defines an upper limit to the speed of such carriers. The development of aluminum alloys has proceeded in two distinct directions, one for the tension-dominated sections of the airframe that use primarily the 2000 (Al-Cu-Mg) series alloys, the other for the compression-dominated sections that use the 7000 (Al-Zn-Mg) series alloys. The 2024 alloy of the former class has been used for over 60 years! It has only very recently been supplanted by improved alloys with higher strength, toughness, and fatigue properties.

The 7000 alloys possess lower fatigue lives under tensile loading because of lower fatigue-crack-growth performance. The early part of aluminum alloy development in the 7000 alloys focused essentially on durability-related issues such as improved stress-corrosion resistance and exfoliation-corrosion resistance in thick-section products in short transverse directions, and such development was pursued at the expense of strength. More recent development has realized increasing strength while retaining properties associated with durability. As in steels, titanium, and nickel, further improvements will saturate. Li and Be are unique among the spectrum of alloying additions to Al in that they decrease density while increasing the modulus. Al-Li alloys have not lived up to their initial promise of widespread use in airframes, partly because of limitations in short transverse ductility and delamination of plates, and partly because of the use of polymer matrix composites in originally envisaged applications of sheet products.

In the latter part of the 1900s, polymer matrix composites have found increasing use in secondary and then primary structures of both civilian and military aircraft, significantly supplanting aluminum alloys. Today's polymer matrix composites use glass, carbon, aramid, or boron fibers as the reinforcement, while the matrix can be either thermoplastic or thermosetting. The first high-performance carbon fibers emerged in the mid-1960s from the conversion of polyacrylonitrile precursors and paved the way for the widely used graphite-epoxy composites. The most common matrix resins are thermosetting epoxy resins, which can be used to temperatures as high as 175°C and are compatible with all common reinforcements. These also possess fatigue strengths significantly superior to aluminum alloys. Bismaleimides and other thermosetting polyimides offer increasing temperature capability from 250-315°C. The thermosets can be processed at lower temperatures as monomers or oligomers with low viscosity and then cured to form stable, cross-linked structures. Thermoplastics such as PEEK [poly(ether ether ketone)] are processed beyond their melting point and are tougher than the thermosets, but soften with temperature because of the lack of cross-linking. They are also less resistant to the environment. Thus, the inverse correlation between high temperature capability and toughness common to metals, intermetallics, and ceramics appears in polymeric matrix materials in the form of the degree of cross-linkage of the chain structures. Blending thermoplastics with thermosets can expand the envelope of strength and toughness. The cost of processing polymer matrix composites is significantly higher than equivalent metal structures. The cost arises from the use of labor-intensive hand lay-up to make the parts, expensive autoclaves to achieve curing and the elimination of voids and porosity, and the use of mechanical fasteners to join many small parts. Low-cost processes such as resin transfer molding have been improved to allow a high volume fraction of fiber. The embedding of smart sensors and actuators adds an additional dimension to the use of composite materials in terms of the health monitoring of structures made of such materials. The polymer matrix composites present extraordinary opportunities for the integration of materials synthesis and design at various levels in the hierarchy of structures ranging from the molecular architecture of the matrix and fiber to microscopic engineering of the fiber shape and three-dimensional fiber arrays.

At supersonic speeds (Mach 2-3), the highest temperatures approach or exceed 200°C, requiring airframe materials made out of high-strength beta titanium alloys, as in the SR71. For supersonic civil transport that will succeed the Concorde, materials of airframes must withstand these temperatures for greater than 60,000 h and must weigh about 20% less than the aluminum alloys used in the Concorde for a 330-passenger capacity aircraft with a 5000-mile range. Toughened, high-temperature, polymer matrix composites will be developed to withstand the lower end of such thermal environments without environmental degradation, while titanium will be used as honeycomb structures and superplastically formed and diffusion-bonded sandwiches. Hybrid laminates of titanium alloys, aluminum alloys, and reinforced polymers will emerge to provide combinations of fatigue strength and toughness beyond the capability of the constituent materials.

The re-entry phase of reusable launch vehicles would result in temperatures as high as 1600-1700°C at the nose and inlet ducts, and upper skin temperatures that can range from 600-900°C. A variety of additional requirements of materials also exist. The external aerodynamic flux creates a plasma environment of dissociated species on nose caps and control surfaces at low pressures that can induce volatilization. Materials must have a high emissivity to radiate heat away into cooler areas and must not catalyze the recombination of dissociated species, which is an exothermic reaction. The acoustic loads generated during launch must be damped. Protected C/C, C/SiC, or SiC/SiC composites as large parts will carry the thermostructural loads on hot sections, while SiO2 or Al2O3 fibrous material will be used as insulators. At lower temperatures, honeycomb and sandwich structures of titanium or nickel-based superalloys may form the thermal protection system, and these metallic materials will be extensively used as fasteners.

Materials in Space

Several uncommon effects are associated with the use of materials in outer space. The vacuum of free space can lead to outgassing. The resulting gas cloud can lead to corona discharges. Condensation of the gas cloud modifies thermo-optical and electrical properties and alters radiation effects. Radiation-UV, protons and electrons, and cosmic rays-can modify the properties of materials at a molecular level. The temperature of structures is determined by heat input from the sun or reflection from a planet and emissivity into the sink of space. Cycling leads to thermal fatigue, and low temperatures cause condensation of gas clouds, while high temperatures lead to increased outgassing. Very high velocity impact from micrometeoroids and space debris results in cratering or complete penetration. Atomic oxygen in low earth orbits can lead to oxidation, corrosion, and erosion. These effects must be considered in the context of long lifetimes; a ten-year lifetime for communication satellites is common, space stations must be designed to function over 30 years, and solar missions will last for more than a decade. Coatings for the external environment for thermo-optical control and corrosion protection will constitute a key challenge. While much of the supporting structure of space platforms is made of conventional aerospace-grade aluminum alloys, two key issues determine the choice of structural materials for beams, struts and columns for satellites, antennae, and space platforms. High-dimensional stability in thermal cycling (160-93°C) arises from the need to maintain precise alignments in communication and sensing systems. Stiffness, rather than strength, is the key design factor in the zero-gravity environment. Carbon or aramid-fiber polymer matrix composites emerge as preferred materials on both these counts. Both of these fibers exhibit a negative coefficient of thermal expansion (CTE) and this offsets the positive CTE of the matrix so that there are virtually no thermally induced dimensional changes over the temperature range of interest.

Beyond the Immediate Horizon

The time cycle from conception to application of aerospace materials is notoriously long. As an example, the first studies on the intermetallic TiAl were carried out in the 1950s. Renewed efforts in the 1970s led to the development of the first "engineering" alloys of TiAl in the 1980s, and component manufacturing programs and ground tests of turbine blades were conducted in the mid-1990s. TiAl still has not flown. Part of the problem is cost-related. "Revolutionary" materials developed toward the end of the last century can only find niche applications, in contrast to alloys such as Ti-6Al-4V and INCO718, and therefore economies of scale do not come into operation in determining flyaway costs. The conservatism inherent in aerospace applications makes the prediction of materials usage perhaps a little bit easier than, for example, in the field of information technology. Nevertheless, earlier predictions on the use of Al-Li alloys, intermetallics, and ceramics have often been overly optimistic. Recall then that Praveen Chaudhari, in his article on materials for information technology in this series [MRS Bulletin 25 (7) (July 2000), p. 55], talks about human beings in constant electronic touch with other human beings and information sources. Remote-controlled surveillance will evolve to remote-controlled active operations. Virtual reality will dominate! Will the societal need for staffed travel cease? Will transportation be confined to goods and materials, and without the constraints inherent in the safety of passenger transportation systems? Will new materials be used far more rapidly than ever before?

The increasing use of composites will allow new structural paradigms to evolve based on biological, hierarchical materials, which meet complex, multifunctional requirements. Such structures will be organized in discrete scales ranging from the molecular to the microscopic to the macroscopic, with specified interactions between the different scales. A new engineering involving the design and processing of such structures is visible on the time horizon.

We have, in the preceding parts of this article, talked more of materials than processes. We find it interesting, in conclusion, to imagine in the more distant future the gradual integration of processing of parts and synthesis of materials into combined operations. The seeds of such operations are already evident; in high-rate powder-based deposition processes that combine melting, refining, atomization, and spraying onto mandrels, in rapid-prototyping techniques involving laser forming, in combined superplastic forming and diffusion bonding, and in situ combustion synthesis reactions to produce the desired combination of phases and microstructures. We believe too that robust joining techniques for similar and dissimilar materials will enhance such an integration process by eliminating the need for mechanical fastening of separately manufactured parts. We have no doubt that manufacturing techniques being explored for microelectromechanical systems will find an echo in their larger counterparts.

Key Research Issues for the Aluminides
The application of the aluminides will depend on our ability to extend their application spectrum, and thus reduce scale-related costs. For this to happen,

- significant successes in ductilizing the aluminides must be extended to toughness at high strain rates, or impact resistance; and
- processability to a range of mill forms and products will be established, especially for TiAl alloys.

The Challenge of High-Temperature Composites
SiC-based composites will be the structural material of this century. In such composites, interface control is crucial in order to

- provide "weak" interfaces that allow debonding in brittle matrix composites;
- provide adequate frictional stresses against fiber pull-out; and
- prevent reactions between dissimilar matrix/fiber combinations.

The nature of the optimization is well captured in the use of a pyrolytic carbon coating used to "weaken" SiC fiber/SiC matrix interfaces. The coating is, however, the site for environmental degradation. The exploration of the properties of materials such as SiC modified by "alloying" has barely begun, and the use of organometallic polymer precursors provides routes to the synthesis of "alloyed" ceramics that may well open up new paradigms in structural materials development based on ceramics.

Materials in Rocket Engines
- The short-term oxidation resistance of carbon-based composites will be developed
through the "alloying" of carbon with carbides of Hf, Ta, or Si, and regenerative or self-sealing glass coatings.
- Cu-graphite or Cu-Nb composites will emerge for durable, actively cooled liners in thrust chambers.
- Radiatively cooled nozzles will be made of layered refractory-metal structures of Ir and Re with zirconia coatings.
- Three-dimensional SiC/SiC impellers will be used in turbopumps.

The Future of Aluminum
- Polymer matrix composites outperform aluminum alloys in specific strength, modulus and fatigue, albeit at flyaway costs that can be 50-100 times higher than that of aluminum alloys.
- New options for stronger aluminum alloys are emerging from amorphous compositions, from precipitation-strengthening by quasicrystalline phases, or from the Al-Be system.
- High-temperature strength can be derived from rapidly solidified alloys based on the Al-Fe system, or through scandium additions to conventional alloys.
- Laminates of aluminum and polymer composites provide exceptional fatigue-crack-growth resistance and damage tolerance at high specific strength levels.

Improved Polymer Matrix Composites
- Textile engineering will be highly automated to allow precise control over the fiber space and dimensionality of woven preforms. This will permit designed-in directionality of properties.
- Oriented, rigid rod molecules incorporated in flexible backbones or tough matrices strengthened by carbon nanotubes will offer improved combinations of hot strength and toughness.
- New matrix formulations and impregnation processes will emerge for efficient penetration into complex macroscopic shapes. Irradiation-based curing processes will be used for large parts (requiring expensive autoclaves), complex shapes, and localized curing to permit joining and repair.

Dipankar Banerjee graduated from the Indian Institute of Technology at Madras and obtained his doctorate degree from the Indian Institute of Science in Bangalore, India in 1979. He has since worked at the Defence Metallurgical Research Laboratory in Hyderabad, India, where he is currently the director. He has spent short spells as a postdoctoral fellow at Carnegie Mellon University, and as a visiting scientist at General Electric's Corporate Research and Development Laboratory at Schenectady, the University of Paris-Sud, and Los Alamos National Laboratory. His research interests lie in the structure of materials in relation to their properties, especially in titanium, its alloys and intermetallics.

Materials Challenges For The Next Century presents a series of articles speculating on the role of materials in society in the coming century and beyond.