What is a High Temperature Alloy?
A high-temperature alloy is a metal alloy that can perform at higher temperatures than ordinary steel or iron. These materials are used in power generation, aerospace, specialty engineering and electronics.
Refractory metallic elements such as molybdenum, tungsten and titanium are frequently with nickel alloy powder used in high-temperature alloys. They enhance hardenability, strength and toughness as well as resistance to wear and corrosion.
Characteristics
High-temperature alloys are primarily characterized by their ability to maintain strength above room temperature, usually between 500degF and 2200degF. These alloys possess strong oxidation resistance, corrosion resistance and fatigue resistance properties, making them a desirable metal for the aerospace, energy, defense and chemical industries.
In addition, these alloys have the capability to form protective chromia (Cr2O3) scales that provide excellent resistance to hot corrosion and spallation. These alloys are commonly used in gas turbine blades, rotor hubs and other parts that are exposed to long-term exposure to thermal cycling conditions.
A wide variety of these materials are now available and are being tailored to meet specific service requirements. This includes improvements in alloy cleanliness, better homogeneity, tighter inspection limits and greater workability.
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Applications
Typical high temperature alloy applications require a combination of mechanical strength, microstructural stability and corrosion/oxidation resistance. These include components used in industrial gas turbines, jet engines and other applications with temperatures in the range of 800-1200degC.
Traditionally, nickel base superalloys have been the prime materials for these applications. However, aluminide based intermetallic compounds such as TiAl-base alloys have emerged as candidates for high temperature applications since they contain high aluminium contents and can form protective alumina scale.
However, they are brittle at room temperature and lack good oxidation properties with stainless steel powders. In addition, a conflict between high fracture strength and high ductility also inhibits their engineering application.
A new alloy design strategy was developed to tackle these issues. An AlCoCrFeNi2.1 (atomic portion) eutectic high-entropy alloy was designed with a fine lamellar fcc/B2 structure to balance the dual goals of both high ductility and fracture strength at room temperature.
Materials
A high-temperature alloy consists of materials that are highly heat resistant. The main elements used in a high-temperature alloy are nickel, cobalt and iron.
The high temperature properties of these metals are enhanced by refractory elements such as molybdenum and tungsten. These elements form hard, stable carbides which enhance heat resistance, strength, toughness and corrosion and wear resistance.
They are frequently used in a range of high-temperature applications including turbine blades, mufflers, calciners and medical grade wire. For example, HAYNES 230 (r) alloy tubes are much longer lasting than competitive products, which sag in two to three months or scratch the wire.
Precipitation hardening alloys dominate high strength applications in the 1000 - 1500degF (540 - 815degC) temperature range. These alloy systems consist of an austenitic matrix strengthened by intermetallic precipitates (e.g., nickel, aluminum or titanium) and solid solution strengthening elements such as molybdenum.
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Processing
High-temperature alloys are characterized by their combination of mechanical strength, microstructural stability and corrosion/oxidation resistance. Typical materials for high temperature applications include nickel-based superalloys, cobalt-based alloys and intermetallic compounds.
For hot end components in aircraft jet engines, nickel-based alloys are often used. Besides nickel-based alloys, titanium-based alloys and refractory metal-based alloys are also used in the manufacture of the hot end parts in industrial gas turbines.
In addition to these conventional alloys, some new high temperature Aluminum alloys and titanium powder made by advanced powder processing are in use for high performance pistons and Aerospace airframe. These aluminum alloys have outstanding thermal stability at service temperatures and perform even better than Titanium or Steel.
These alloys are mainly precipitation hardened, where the austenitic matrix is strengthened by a mixture of nickel and solid solution strengthening elements. These precipitation-forming elements are usually either aluminum, titanium or columbium (Ni3 Al, Ni3 Ti and Ni3 Cb). This requires a continual increase in the nickel content to promote the formation of these intermetallic precipitates.