What are superalloys commonly used for?
What are high temperature alloys?
Superalloys are a group of metals that have better resistance to creep, oxidation and corrosion than traditional alloys, while maintaining excellent mechanical properties even at high temperatures. In a sense, they act as high-performance alloys.
Superalloys, commonly used in gas turbine engine components in the aerospace industry, are based on nickel, cobalt or iron and offer far superior properties than traditional steel or aluminum alloys.
While most conventional iron-based alloys begin to lose significant material strength at 400 degrees Celsius (aluminum alloys even lower), many superalloys actually exhibit an increase in strength between 750 and 900 degrees Celsius.
Why are they important?
On a per-pound basis, superalloys are more expensive than steel, aluminum or stainless steel; they are also more complex to machine and mold into the desired shape. So why are they so widely used in certain applications?
Creep resistance
Creep is a material failure mode in which a component deforms at stress levels 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 extensive testing and analysis, it was discovered that long-term (albeit relatively mild) stress on the part can cause what is known as "cold flow," where the part plastically deforms below half its UTS. For more information on creep, check out our article.
It was further found that heat from any source accelerates creep. This is a potentially catastrophic problem for components in jet engines.
Superalloys enable parts such as turbine blades to operate under extreme centripetal forces and heat while maintaining their strength and, most importantly, their shape.
Long-term testing of non-creep-resistant materials before the use of superalloys produced interesting results, with turbine blades lengthening as they rotated, causing interference with the engine casing.
High temperature resistance
In applications such as aerospace engines and gas turbines, high temperature resistance is a must.
Not only do superalloys perform well at high temperatures, in some cases the UTS of the material increases, allowing for higher operating stresses or less mass. Both of these points are beneficial to the aviation industry.
Because superalloys have better heat resistance than conventional alloys, many improvements have been made in jet turbine engine design.
Jet turbines can produce such high thrust because they compress the intake air and fuel; the higher the compression, the more power the engine produces. As pressure and combustion rate increase, so does the amount of heat produced.
Thankfully, the heat and creep resistance of superalloys allows for higher combustion pressures, resulting in significantly higher efficiency. How many? In the U.S. alone, with more than 760 billion passenger air miles traveled in 2019, small efficiencies can equate to significant fuel savings.
