A demanding set of requirements
Aerospace is a field where small differences in material choice add up. Every kilogram of structure is a kilogram that cannot be payload, fuel or passengers, so low weight matters. At the same time, parts must be reliable, survive repeated loading over many years, and tolerate environments ranging from cold, thin air to engine heat and atmospheric re-entry.
No single material meets all of that. Instead, a vehicle is a collection of regions with different demands, and each region uses the materials that fit. The sections below look at them in turn. Specifics vary by aircraft and program, so treat this as a general picture.
Airframes: aluminum, titanium and composites
For decades, the fuselage and wings of most airliners were built mainly from aluminum alloys. Aluminum is light, easy to form and machine, and well understood, including its behavior under repeated stress cycles. Its strength drops at elevated temperature, so it suits regions that do not get very hot.
Titanium alloys are heavier than aluminum but stronger, resist corrosion well, and keep useful strength to higher temperatures. They are often used in landing gear components, engine mounts and structure near hot zones. A further reason for titanium is compatibility with carbon-fiber structures, since some other metals can corrode when in contact with carbon fiber.
Composites, particularly carbon fiber in an epoxy-type resin, have taken a growing share of modern airframes. Their appeal is high stiffness and strength for low weight, and the ability to tailor fiber direction to the loads. The fibers themselves are covered in carbon fiber, and the structures built from them in advanced composites. Many composite panels use sandwich structures, where a lightweight core separates thin skins to give high bending stiffness at low mass.
Composites bring challenges too: they can be damaged by impact in ways that are not always visible, they are harder to repair than metal, and inspection is more involved. Checking them without damaging them relies on methods such as nondestructive evaluation.
Engines: where heat dominates
Gas turbine engines are among the hardest materials environments in engineering. Air is compressed, mixed with fuel and burned, and the hot gas then drives turbine blades. The material in the hottest section has to withstand temperatures close to or even above the melting point of the alloy itself, aided by cooling.
- Fan and compressor. Titanium alloys are common in the cooler front sections for their strength-to-weight ratio. Composite fan blades and casings are also used in some engines.
- Combustor and turbine. Nickel-based superalloys are the traditional choice, because they retain strength, resist creep (slow deformation under load and heat) and resist oxidation. Blades are often cast as single crystals to avoid weak grain boundaries.
- Protective coatings. Ceramic layers, discussed in thermal barrier coatings, insulate metal parts so they can survive in hotter gas. Internal cooling passages work alongside them.
- Ceramic matrix composites. These materials are lighter than metal alloys and can tolerate higher temperatures, which is why ceramic matrix composites are used in some stationary turbine parts. Their use in hot sections continues to develop.
The broader question of which materials hold strength at heat is compared in the article on high-temperature materials.
Spacecraft and re-entry
Spacecraft face different problems. In orbit, materials must handle vacuum, wide temperature swings, radiation and, for some parts, atomic oxygen in low orbit. Outgassing, where a material releases vapor in vacuum, can contaminate optics and sensors, so material selection includes this factor.
During launch and re-entry, thermal protection becomes critical. A vehicle returning from orbit encounters very intense heating. Different approaches have been used:
- Ablative materials that char and erode to carry heat away.
- Insulating ceramic tiles or blankets, as used on some reusable designs.
- Carbon-based heat shields; carbon-carbon composites have been used on high-heat areas such as leading edges because they hold strength at very high temperatures.
Structures for rockets and satellites often use aluminum-lithium alloys, titanium and carbon composites. Propellant tanks add requirements such as compatibility with very cold or reactive fuels.
Why these materials, and the properties behind them
Several properties repeat across aerospace choices.
- Specific strength and stiffness. Strength and stiffness relative to density are more informative than raw strength, because weight is the constraint. Composites and titanium rank well here.
- Fatigue resistance. Aircraft see countless load cycles, so fatigue behavior and crack growth are central to design and inspection schedules.
- Corrosion resistance. Moisture, de-icing fluids and salt air attack some alloys, so coatings, surface treatments and material choice all matter. This is a subject in its own right under corrosion resistance.
- Thermal behavior. Parts joined together expand differently with heat, so mismatched materials can build stress. This is the focus of thermal expansion.
- Damage tolerance. Rather than assuming parts never crack, designers assume small flaws exist and plan to detect them before they grow dangerous.
Hard materials in manufacturing aerospace parts
Making aerospace parts is itself a materials challenge. Titanium and nickel alloys are tough on cutting tools, and carbon composites are abrasive and can delaminate when drilled. Hard tool materials such as cemented carbide and diamond-coated tools are used for these jobs, depending on the workpiece. Diamond tends to suit composites and non-ferrous metals better than some steels and superalloys, a limit discussed elsewhere on this site.
Looking ahead
Aerospace materials development tends to move gradually, because qualifying a new material for flight takes extensive testing and certification. Ongoing directions include lighter composites, more heat-tolerant ceramics and coatings, and metals produced by additive manufacturing. Which of these become routine will depend on cost, repairability, certification and performance in service, and predictions here carry real uncertainty.