We’ve been using metal alloys for over 10,000 years, ever since tin and copper were blended together to form a new, better material ─ bronze.
Nowadays, almost all metals we use are alloys of some kind, as they are far superior to their constituent elements alone.
With the advent of gas turbines, engineers required materials with superior mechanical properties and high temperature resistance, a group collectively referred to as superalloys.
What are superalloys?
Superalloys are a group of metals that offer better creep, oxidation and corrosion resistance than traditional alloys, whilst retaining excellent mechanical properties, even at high temperatures. In a sense, they act as high performance alloys.
Commonly used in the aerospace industry for components in gas turbine engines, superalloys use a base of nickel, cobalt or iron to offer far superior properties versus traditional alloys of steel or aluminum.
Whilst most traditional alloys based on iron start to see a significant decline in material strength at 400 degrees Celsius (with aluminum alloys even lower), many superalloys actually exhibit an increase in strength between 750 and 900 degrees.
Why are they important?
Per pound, superalloys are more expensive than steel, aluminum or stainless steels; they are also more complex to work and mold into a desired shape. So why are they so widely used in certain applications?
Creep is a material failure mode whereby a component deforms at a stress level well below its ultimate tensile strength.
When it was first discovered, much was known about ultimate tensile strength, stress/strain and fatigue, but this new phenomenon seemed to defy traditional metallurgy.
After significant testing and analysis, it was found that a prolonged (although relatively mild) stress on a part could cause what is also known as ‘cold-flow’ ─ the part would deform plastically at less than half its UTS. For more information about creep, check out our article here.
It was further discovered that heat, from any source, accelerates creep. For components in jet engines this is a potentially catastrophic issue.
Superalloys enable parts such as turbine blades to operate at the extremes of centripetal force and heat, all while retaining their strength and most importantly, their shape.
Before the use of superalloys, long term tests of non-creep resistant materials resulted in interesting results, with turbine blades lengthening whilst spinning, causing interference with engine casings.
Resistance to high temperatures
In applications such as aerospace engines and gas turbines, resistance to high temperatures is a must.
Not only do superalloys perform well at elevated temperatures, in some cases the UTS of the material can increase, allowing for even higher operating stresses or less mass. Both points are a plus for aviation.
Because superalloys resist heat better than traditional alloys, many improvements have been made to the design of jet turbine engines.
Jet turbines can generate such high levels of thrust due to the compressing of intake air and fuel; the higher the compression, the more power the engine can produce. As the pressure and combustion rate increase, so does the heat produced.
Thankfully, superalloys’ resistance to heat and creep have allowed for higher combustion pressures, significantly increasing efficiency. How much? With over 760 billion passenger air miles covered in 2019, in the US alone, small efficiencies can equate to big savings on fuel.
What makes them unique
Superalloys stand apart from most other metals as being exotic, only used for very specific applications. But what really makes them special?
Lattice structure and precipitation hardening
At an atomic level, the elements that make up metals form lattice structures. What does this mean? The atoms are arranged in perpendicular rows and grids, unlike plastics. The natural tendency of metals to form lattice structures is both beneficial and a hindrance, affording them strength, but only in certain planes.
Alloying a metal (along with other benefits such as oxidation resistance) helps solidify this lattice structure and increases the force required to cause atomic planes to slip, by blocking these slip planes.
Superalloys, in particular nickel-based superalloys, go one step further. The formation of a two-phase equilibrium microstructure affects how the alloying elements arrange themselves and gives them protection from multiple different modes of failure.
This technique for hardening materials is known as precipitation hardening, and is used in many alloys of aluminum, stainless steel and nickel.
Examples of superalloys
Nickel, iron and cobalt are the primary categories of superalloys, with many subsections inside each. Let’s take a quick look at a few:
This alloy is a combination of molybdenum and chromium, creating excellent corrosion resistance, as well as retaining good weldability and resistance to acid.
There are many variations of Hastelloy available, including the commonly used C276, Hastelloy B, B-2 and so on. Each grade has slightly different characteristics and levels of alloying elements to accommodate a wide range of applications
For more on Hastelloy, read our full guide here
Inconel is sometimes used as a high temperature alternative to stainless steel. It improves upon stainless steel with its ability to stay acid and corrosion resistant up to extremely high temperatures.
Chromium is the most abundant alloying element within Inconels, but like most superalloys, there are many different variations available.
For more on Inconel, read our full guide here.
Properties of superalloys
Close control over both the alloying elements and heat treatment can afford metallurgists precision in determining a material’s properties. The same is true for superalloys, with each sub-group often having a surprising variation in mechanical or chemical properties; we’ll only cover some of the general ones.
- High thermal resistance
- Strong corrosion resistance
- High strength
- Good machinability
- Weldability in some cases
- Shape memory
- Low thermal expansion
- Superior thermal resistance than nickel-based alloys
- Higher hot corrosion resistance compared to both nickel and iron alloys
- Greater thermal fatigue resistance at elevated temperatures
- Better weldability
- Poor machinability in some alloys
- Low strength in high temperatures compared to nickel-based alloys
- Excellent wear resistance compared to nickel
- Good creep resistance
- Generally better weldability
- Good machinability
Composition of superalloys
Hastelloy, a commonly used nickel superalloy, has many variations of the chemical composition depending on the necessary application.
Hastelloy C22 has the following composition:
- 56% nickel
- 22% chromium
- 13% molybdenum
- Other: lower amounts of iron, tungsten and cobalt
Inconel 625 differs from a typical Inconel, containing more nickel and less molybdenum; it has the following composition:
- 58-71% nickel
- 21-23% chromium
- 8-10% molybdenum
- 5% iron
- Other: lower amounts of niobium, cobalt and manganese
Cobalt-based superalloys are typically composed of cobalt and chromium. X-40, a non-ferrous cobalt based alloy is used mainly for its excellent creep resistance.
A typical composition Alloy X-40 is:
- 54% cobalt
- 24% chromium
- 10% nickel
- 7.5% tungsten
- 0.5% carbon
A-286 is a frequently used iron-based superalloy. It can be thought of as an evolution of stainless steel, offering the usual corrosion resistance, only up to an extreme of 700 degrees Celsius.
A-286 is like all stainless steels in that it is primarily composed of iron and chromium. These are the other elements present:
- 53% iron
- 26% nickel
- 15% chromium
- 7.5% tungsten
- 2.15% aluminum
Processing of superalloys
In order to create these new multi-phased materials, a lot of changes had to be made to the ordinary steel and alloy making processes. In addition to new processing, tighter control on process parameters was necessary.
Nickel-based superalloys are most commonly cast into a usable shape. There are a limited number of applications that cold form from shapes from ingots, but these generally provide lesser material properties over the casting process.
Directionally Solidified casting (DS)
Developed in the 1960s, this type of casting differs dramatically from the traditional casting of aluminum or iron.
Firstly, pours are completed inside a furnace, allowing the temperature to be closely controlled. The molds are then slowly removed from the high heat in accordance with the specifications of the material and property requirements.
Directionally solidified molds are designed to incorporate one inner face significantly colder than the rest. This is accomplished using water cooled heat exchangers.
The purpose of this complex process is to control from where and in what direction the grains form. The cooled face of the mold, usually the bottom, causes the whole part to cool from bottom to top.
Being able to control the direction of grain boundaries allows engineers and metallurgists to create parts that are extremely strong in certain directions. This is an extremely useful tool in the aerospace industry when crafting jet turbine engine components.
The very first Directionally Solidified turbine blades were produced by Pratt and Whitney in 1969 – these were used in the engines of the famous SR-71 Blackbird!
Single Crystal casting (SC)
In continuation of the benefits of DC casting, single crystal casting monitors the process with even more intricacy, resulting in nearly 95% of the part being formed from a singular metallic crystal.
All of the problems caused by grain boundaries are eliminated this way such as slip, cavitation and inclusions. One of the biggest advantages this gives is a huge increase in creep resistance.
SC casting is performed using molds even more complex than DC casting. In addition to the water cooled plate, the molds feature a series of bends – the purpose of which is to reduce the amount of crystal structure formations.
Arranging these into a helical spiral, with knurling details to the walls, Pratt & Whitney engineers started creating castings with almost no secondary crystallization.
The criticality of temperature control here cannot be understated. Even a slight irregularity in the temperature of the mold wall can cause many crystallization sites, immediately ruining the entire pour.
What are they used for?
The use of superalloys is widespread. While the materials themselves are often used in relatively small amounts, they can achieve large improvements in efficiency and strength of a component.
- Nuclear power generation
- Chemical processing
- Heat treating
Probably the most well known and well documented use of nickel-based superalloys is in turbine engines. The high temperature strength and creep resistance of the material has revolutionized the aerospace industry, providing incredible efficiency gains in both commercial and passenger flight.
There are multiple other applications of a similar nature to turbine blades, where creep and oxidation resistance is a must. These include automotive turbocharger turbines, power plant steam turbines and more.
Superalloys are used in static components too, with many processing systems utilizing their high strength and corrosion resistance for valves, vessels, bolts and ducting. These environments are often places where stainless steel would traditionally be used.
Superalloys in the future
Having cemented their reputation in the aviation industry, the next logical step for superalloy use is in space travel.
With many of the same challenges faced by sub-stratospheric flight, namely the drive for weight reduction/efficiency, high temperature variation and high stress, engines used in space stand to benefit from the implementation of superalloys.
Being able to save weight through stronger materials is extremely important. We’ve already witnessed this with the construction of almost every space launch device since the 1960s.
As commercial space exploration continues to gain traction with the likes of SpaceX and Blue Origin, fuel and space saving will only grow in importance.