1. Introduction
Superalloys are designed to be used at temperatures of 540oC and higher , typically up to 0.7 x their absolute melting temperature, which may be up to around 1000oC. At 760oC most superalloys have a yield strength in excess of 500MPa.
They are also corrosion resistant. About three quarters of superalloys are used in aerospace, typically about one half of the weight of a modern jet engine is made up of superalloys.

There are 3 types:

Iron Nickel based - these are an extension of stainless steel technology and are frequently wrought. Typically contain 20-40% Ni, 15-20% Cr, 30-50% Fe, plus Mo, Al, Ti.

Nickel based - may be wrought or cast. Typically more than 40% Ni, 10-20% Cr, up to 10% Al and Ti, 5-10% Co and small amounts of B, Zr and C. Mo, W and Nb are also commonly added.

Cobalt based - may be wrought or cast. Typically contain 35-60% Co, 20-30% Cr, up to 35% Ni with some W and Mo.

Appropriate compositions of all types can be forged, rolled or formed. The more highly alloyed compositions are normally processed as castings. Fabricated structures may be built up by welding and brazing, but highly alloyed compositions containing a large amount of hardening phase are difficult to weld.

As well as jet engines, these alloys are also used for fixtures and jigs to be used in furnaces.

Most superalloys are propriertary, the following shows the main types and trade names:

Nickel chromium alloys, 'Inconel', International Nickel Co.
Nickel iron alloys, 'Ni-Resist', International Nickel Co.
Nickel based and Cobalt based, 'Hastelloy', Haynes International.

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David J Grieve, 23rd May 2005.