Service Temperatures for Superalloys

The superalloys, as has been noted, consist of alloys of iron-nickel-, nickel-, and cobaltbase that are destined generally for use above about 1000 F (540 C) and below the melting points of the alloys, which usually are at or above about 2200 F (1204 C). Some superalloys also find use in space applications where subzero and cryogenic temperatures are an issue. The bulk of this text presumes that the application will be at elevated temperature.

Wrought nickel- and iron-nickel-base alloys, in general, have temperature limitations of about 1500 F (816 C). Above that temperature, cast alloys generally are used. The majority of superalloys are strengthened by the production of secondary phases (precipitates), and the upper temperature limit for alloy use is governed by the base (nickel- or iron-nickel-base), the volume/type of precipitate, and the form (cast or wrought).

It is commonly understood in the superalloy industry that certain alloy types are used for specific temperatures of application. For example, most wrought nickel- and ironnickel- base superalloys are used only to about 1200 to 1300 F (649 to 704 C). The range of such alloys actually starts below 1000 F (540 C), frequently as low as 800 F (427 C), and the wrought alloys are particularly useful in gas turbines when titanium alloys might be inappropriate. Cast alloys are used across the temperature range but particularly at the highest temperatures, especially as in gas turbine engines.

Superalloys usually are processed to optimize one property in preference to others.
The same composition, if used in cast and wrought state, may have different heat treatments applied to the different product forms. Even when a superalloy is used in the same product form, process treatments may be used to optimize one property over others. For example, an alloy such as Waspaloy was being produced in wrought form for gas turbine disks. By adjustment of processing conditions, principally heat treatment, substantial yield strength improvements (a desirable effect) were achieved in the wrought product at the expense of creep-rupture strength.

Basic Metallurgy of Superalloys

Iron, nickel, and cobalt are generally facecentered cubic (fcc-austenitic) in crystal structure when they are the basis for superalloys. However, the normal room-temperature structures of iron and cobalt elemental metals are not fcc. Both iron and cobalt undergo transformations and become fcc at high temperatures or in the presence of other elements alloyed with iron and cobalt. Nickel, on the other hand, is fcc at all temperatures. In superalloys based on iron and cobalt, the fcc forms of these elements thus are generally stabilized by alloy element additions, particularly nickel, to provide the best properties.

The upper limit of use for superalloys is not restricted by the occurrence of any allotropic phase transformation reactions but is a function of incipient melting temperatures of alloys and dissolution of strengtheningphases. Incipient melting is the melting that occurs in some part of the alloy that, when solidified, is not at equilibrium composition and thus melts at a lower temperature than that at which it might otherwise melt. All alloys have a melting range, so melting is not at a specific temperature even if there is no nonequilibrium segregation of alloy elements. Superalloys are strengthened not only by the basic nature of the fcc matrix and its chemistry but also by the presence of special strengthening phases, usually precipitates. Working (mechanical deformation, often cold) of a superalloy can also increase strength, but that strength may not endure at high temperatures.

Some tendency toward transformation of the fcc phase to stable lower-temperature phases occasionally occurs in cobalt-base superalloys. The austenitic fcc matrices of superalloys have extended solubility for some alloying additions, excellent ductility, and (iron-nickel- and nickel-base superalloys) favorable characteristics for precipitation of uniquely effective strengthening phases.

Pure iron has a density of 0.284 lb/in.3 (7.87 g/cm3), and pure nickel and cobalt have densities of about 0.322 lb/in.3 (8.9 g/cm3). Iron-nickel-base superalloys have densities of about 0.285 to 0.300 lb/in.3 (7.9 to 8.3 g/ cm3); cobalt-base superalloys, about 0.300 to 0.340 lb/in.3 (8.3 to 9.4 g/cm3); and nickel base alloys, about 0.282 to 0.322 lb/in.3 (7.8 to 8.9 g/cm3). Superalloy density is influenced by alloying additions: aluminum, titanium, and chromium reduce density, whereas tungsten, rhenium, and tantalum increase it. The corrosion resistance of superalloys depends primarily on the alloying elements added, particularly chromium and aluminum, and the environment experienced.

The melting temperatures of the pure elements are as follows: nickel, 2647 F (1453 C); cobalt, 2723 F (1495 C); and iron, 2798 F (1537 C). Incipient (lowest) melting temperatures and melting ranges of superalloys are functions of composition and prior processing. Generally, incipient melting temperatures are greater for cobalt-base than for nickel- or iron-nickel-base superalloys. Nickel-base superalloys may show incipient melting at temperatures as low as 2200 F (1204 C). Advanced nickel-base single-crystal superalloys having limited amounts of melting-point depressants tend to have incipient melting temperatures equal to or in excess of those of cobalt-base superalloys.