Superalloys: Chemistry, Process Development, Uses and Costs

Superalloys are also referred to as high performance alloys. To be identified as a superalloy it must exhibit great mechanical strength, have good surface stability, have creep resistance when under high temperatures, and resist both oxidation and corrosion. The base of a superalloy is typically cobalt, nickel, or nickel iron.

Chemical Development

The creep resistance of an alloy depends on the speed of the dislocations of the crystal structure. Slowing thespeed increases the creep resistance. To create superalloys chemical additions like titanium and aluminum canpromote gamma prime phase creations. A number of superalloys go through two phases of heat treatment thatwill cause both a primary and then secondary dispersion of square gamma particles. During this process thereare many other elements that may be present. Some examples of elements that may be present include cobalt,chromium, molybdenum, tantalum, tungsten, titanium, aluminum, carbon, hafnium, zirconium, and boronamong many others.

Process Development

Originally, superalloys were based in iron and cold wrought, before the 1940s. During the 1940s investmentcasting of base alloys made of cobalt increased the operating temperatures. Vacuum melting was developedduring the 1950s and allowed for finer control of the chemical compositions of the superalloys. It also reducedthe amount of contamination.

There are many superalloys present within gas turbine engines. Polycrystalline nickel based superalloys can beused for the disks within the high pressure turbine. This can then be used to create casting technology or powdermetallurgy.

Uses of Superalloys

There are many common uses for superalloys, but the most common use of these types of alloys is for gasturbine engines. The engines that use superalloys are typically subject to extremely high temperatures thatrequire a high amount of strength as well as good creep resistance and resistance to oxidation and corrosion.

Another use for superalloys is within turbocharger turbines. The superalloys used for turbocharger turbines istypically a steel shaft with electron beam welded to it. Mar-M247 and Inconel 713 are the common superalloysused for turbocharger turbines.

When corrosion is an issue, superalloys can be used. This includes saltwater and acidic environments wherestainless steel cannot be used.

Piston engines, both gasoline and diesel will typically use superalloys as well. The corrosion resistance offeredby the superalloy can be useful in the high temperatures found in diesel engines.

Other uses for superalloys include for gas turbines such as those used in both commercial and military aircraft,marine propulsion, and power generation, in nuclear reactors, submarines, space vehicles, racing and other highperformance vehicles, bomb casings, chemical processing vessels, and in heat exchanger tubing.

Cost of Superalloys

Most superalloys contain a high concentration of nickel, which is much more expensive than iron. Additionally,other expensive elements are sometimes used such as ruthenium and rhenium. The heat treatments and theelements used for alloying can make the process of creating superalloys hard to weld and machine. Thisdifficulty increases the cost, the number of parts that are rejected, and the rework that needs to be done.

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