Ask someone what future materials are likely to be used in aircraft, chances are that you’ll get something along the lines of Carbon fiber / Titanium. While this is partly true, It is also an outdated statement.

Carbon Fiber has been around for decades and has been used extensively in many airliners. For instance, Each Boeing 787 contains approximately 35,000 kg of carbon fiber reinforced polymer (CFRP), made with 23,000 kg of carbon fiber.

First, let’s look back at the materials we have been using up to now.

From the time the Wright brothers made their first flight at kitty hawk to the advent of the first world war there wasn't a whole lot of development in the aviation industry.  This was partly due to the aviation industry being a fledgling industry as a whole, and partly because aircraft were seen as not much more than a novelty (much like jet-packs are seen today). In fact, Marshal Ferdinand Foch, Who was a French military strategist and would become a commander in the French army for the first world war, said:

Today Aircraft in all shapes and sizes have infiltrated the military of many countries to such an extent that to imagine an army without an array of aircraft is a laughable proposition.

The aircraft of yore used commonly available materials such as Timber for wings, Sitka spruce, and Irish linen were two famous kinds of wood that were used. The airplane wings would be covered in Fabric and sometimes, Dope. Dope is actually Nitrocellulose and Cellulose acetate mixed together. If the name Nitrocellulose strikes you as familiar, It is because it is also used to manufacture gun cotton, which is a liquid explosive.

Yeah, Definitely not a great idea to cover your aircraft wings with a potentially flammable material.

Necessity is the mother of all invention, and this phase saw the most rapid development of aircraft in human history. in about 20 years, Aircraft went from novelty items to airplanes like the North American P-51 Mustang and Super-marine Spitfire which could take out entire armies from the air.

Y alloy, The first of the alloys made its debut during the world war I period. Y alloy was conceived by the British National Physical Laboratory (NPL). Duralumin, An aluminum alloy which contains 4% copper was already known at this point. Duralumin had good strength characteristics and was being used in zeppelins. Aircraft of that time were still largely constructed of wood, But the engine (particularly the pistons) needed an alloy that would retain the strength of Duralumin at high temperatures.

Thus, Y alloy was born. It contained Aluminum and 4% copper like duralumin, but also 2% nickel and 1.5% magnesium. Y alloy proved to be a success!

Aircraft grade Aluminum made its debut in 1935, Though it did not gain much popularity until about 10 years later. 6061 is the most common aircraft grade aluminum in use. 6061 contains magnesium and silicon which give it good heat resistant treatment properties.

The aluminum age reached its peak in the 1970s, with up to 70% of some aircraft being made up of aluminum and its various alloys. By then, Other materials had begun to make an appearance- a little Titanium here, some fiberglass there, But they were used in small quantities limited to 3-5% of the aircraft. The times have changed, A typical fighter jet of today, for example, might contain as little as 20% aluminum.

So which materials are changing the status quo that aluminum held on to for 40 years? What makes these materials so special? What are their drawbacks? Let’s find out.

The engine of an aircraft is one of its most complex parts and with the advent of lean burn engines temperatures inside can reach up to 2100 C. This necessitates alloys which can handle even more heat than before without losing their structural integrity– This is where Heat Resistant Super alloys come into the picture.

Titanium Aluminide (TiAl) and Aluminum-Lithium (Al-Li), Materials which have been around since the 1970’s, are only recently gaining traction. Ti-Al retains its strength and corrosion resistance up to Temperatures of 600 C, similar to nickel alloy but it is easier to work with. Most significantly, Ti Al can go a long way in improving fuel consumption and increasing thrust to weight ratio as it is only half the weight of conventional alloys. For example, The GEnx engines used on the Boeing 787 have low-pressure turbine blades made up of Ti Al (Alloy 48-2-2).

Pratt and Whitney have also come out with their PW1000G engine. This engine also contains a newly discovered, Beta stabilized form of Ti-Al.

Aluminium-Lithium (Al-Li)

Earlier, 7075 Aluminium was extensively used during world war II by the Japanese Imperial Japanese Navy. It was manufactured in secret in 1943 by a Japanese company, Sumitomo metal. It is an Alloy of Aluminium and Zinc being its chief ingredients. It was famed chiefly for its good machinability. One reason why it fell out of use was due to its relatively high cost and only average resistance to corrosion.

Enter Aluminium-Lithium. As you might know, Lithium is the least dense metal on the periodic table. Lithium replaces Aluminum atoms in the crystal lattice without disturbing its structure. Even adding 1% Lithium reduces the alloys weight by 3% while increasing its stiffness by 5%. This effect can be seen up to the solubility limit of Lithium in Aluminum, which is 4.2%

Introducing another metal into a lattice causes strain in the lattice, which blocks atomic dislocations and thus makes the lattice harder, Thus allowing less of it to be used.

Al-Li can also form precipitation in the form of Al3Li metastable phase, This grants the resulting metal corrosion resistance.

To sum it up, Al-Li ‘s high strength, high stiffness, low density, damage resistance, corrosion resistance and weld friendly nature make it the alloy of choice for Fuel and Oxidiser Tanks of SpaceX’s Falcon 9 rocket. Its use is also being investigated into by NASA for many research projects. Airbus is also currently using the AA2050.

Titanium 5553 (Ti Triple 5 3)

Titanium is generally an outstanding material for aerospace applications because it is lightweight, strong and has excellent corrosion resistance. Titanium has been in use since the 50s for military aircraft. Ti-6Al-4V is the major alloy in such applications, with one major exception being the SR 71 Blackbird manufactured by Lockheed Martin, which used the Beta alloy of Titanium.

The main drawback is that Titanium and it’s older alloys have a working temperature limited to about 520 C. New Titanium Composites are trying to overcome these temperature limitations, with Titanium 5553 emerging at the forefront. Titanium 553 is an alloy  (5553 stands for  Ti-5Al- 5V-5Mo-3Cr ) with aluminum, Vanadium, Molybdenum, and Chromium) which has the much-improved strength and hardness properties compared to its predecessors. This has lead to it extensively being used in the Boeing 787.

The Sukhoi S 27 Beirkut, With forward swept wings

For many years, Aircraft designers conceived designs that could not be built due because the materials to build them did not exist. They called such materials “Unobtainium” (Yes, Before Avatar! ).

One very famous example is NASA’s space shuttle, which would have been impossible to construct if it wasn’t for the composite ceramic tiles on its body to protect it from heat during re-entry. Some experimental aircraft like Grumman’s X-29 and Russian Sukhoi S-27 Beirkut have forward swept wings that are impossible to construct unless specific composites are not used, else the wings would be bent out of shape.

Closer to home, The Light aircraft Tejas, developed at the CSIR NAL Labs is 45% composites! The recently re-engineered SARAS, Which is a multirole light aircraft being developed for both military and civilian applications has Carbon Fiber composite (CFC) wings.

So what exactly is a composite? A composite can be thought of as a material that contains a stronger load carrying metal embedded in a somewhat more brittle substance. The Harder substance is called the reinforcement while the weaker material is called the matrix. The advantage is that the matrix is easy to shape, while the reinforcement provides strength to the aircraft.

The most commonly used composites in aircraft beginning from the world war era to date are Fiberglass, Carbon Fiber reinforced polymer and Aramid fiber.

Fiber Glass: Basically consists of a plastic matrix reinforced with glass fibers. It is lightweight, strong and robust. Its disadvantage is that the raw materials are expensive, and it is less hard than carbon fiber, although it is less brittle.

Carbon Fiber Reinforced Polymer: Is extremely strong and lightweight, and as the name suggests, It contains Carbon fibers. The composite may contain other fibers like Aramid. Examples are Kevlar (Used to make bulletproof vests), Twaron (Used in automotive industries), aluminum and glass fibers.

Aramid Fiber is a class of heat-resistant synthetic fibers. They are used in aerospace and military applications. With each passing generation of aircraft, the number of composites used increases. For example, The Boeing 707 had Fiberglass that accounted for 2% of its weight. Today, many aircraft cross 50% of fiberglass. Modern military aircraft like the F-22 have as much as 60% of their body built out of composite materials.

Composites have many advantages over conventional metal alloys-

advanced composites do not corrode. Corrosion and fatigue cracking is a significant cause of failure in aircraft.

They have good impact resistance. Failure occurs often in joints and composite materials advocate 1 piece designs which have fewer parts to assemble and thus, fewer failure points. Composites can be shaped in ways that are impossible for metals without the extensive use of fasteners and joints.

Ceramic matrix composites consist of Ceramic fibers in a ceramic matrix. They were developed because conventional ceramics had very low crack resistance- loads could easily induce microscopic cracks that would lead to the material losing structural integrity.  The use of long Ceramic strands increased the crack resistance as well as the elongation and thermal shock resistance. Carbon (C), Silicon Carbide (SiC), Alumina (Al2O3) and mullite (Al2O3- SiO2) are often used for CMCs.

The possible uses of CMCs in the Aero Industry are in the hot sections of the engines, Turbine disks etc.

They have a metal matrix with an oxide, carbide or nitride reinforcement. Though they are not very tough, they have potential applications in heavily loaded surfaces like Helicopter rotor blades, Turbine fan blades and floor supports. The F-16 uses single layered Silicon carbide fibers in a titanium matrix for its landing gear.

They are used in tools such as Tungsten Carbide cutting tools which has a Tungsten Carbide matrix with cobalt embedded.

Carbon nanotubes are being looked into as the next generation of aircraft materials. They are at least as strong as carbon fibers, But they are much more flexible and it has the added advantage of being an EMI shielding material. EMI stands for Electromagnetic Interference. Aircraft occasionally build up charge from lightning strikes while flying in the air. These can cause damage to internal circuitry. While modern aircraft have metal to protect them from damage, Carbon Nanotubes offer a better solution by being basically the weight of a coat of paint, compared to a much heavier metal.

Carbon Nano-tubes allow for futuristic applications like self-deicing wings, Wings that can function as an antenna and other applications that sound like being straight out of a science fiction novel!

Boeing and GM together developed a metal micro-lattice which they describe as being “99% air” and as being the lightest metal in the world. The material only weighs as much as 1/10th the weight of carbon fiber and is actually slightly lighter than air itself!

Boeing plans to use this material on its space rockets in about 5 years, and in civilian aircraft in about a decade. Boeing describes it as an “Open Polymer cellular structure.”

Should the micro-lattice become widespread in use, It opens up avenues for huge cost savings in aircraft, as it is very light and strong while simultaneously being very flexible.

SSMs upon heating revert to their pre-deformed shape. They usually consist of copper/nickel-based alloys. They have potential applications in variable jet intakes where conventional designs have scope for weight reduction.

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