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.
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.
