Superalloys: A Primer and History
INTRODUCTION
The term "superalloy" was first used shortly after World War
II to describe a group of alloys developed for use in
As a supplement to The
Minerals, Metals & Materials turbosuperchargers and aircraft turbine engines that required
Society's site dedicated to the high performance at elevated temperatures. The range of
9th International Symposium
applications for which superalloys are used has expanded to
on Superalloys, this page was
developed by Randy Bowman many other areas and now includes aircraft and land-based
gas turbines, rocket engines, chemical, and petroleum plants.
of NASA Lewis Research
Center.
They are particularly well suited for these demanding
applications because of their ability to retain most of their
strength even after long exposure times above 650°C (1,200°F). Their versatility stems
from the fact that they combine this high strength with good low-temperature ductility
and excellent surface stability.
Superalloys are based on Group VIIIB elements and usually consist of various
combinations of Fe, Ni, Co, and Cr, as well as lesser amounts of W, Mo, Ta, Nb, Ti, and
Al. The three major classes of superalloys are nickel-, iron-, and cobalt-based alloys.
NICKEL-BASED SUPERALLOYS
Nickel-based alloys can be either solid solution or precipitation strengthened. Solid
solutioned strengthened alloys, such as Hastelloy X, are used in applications requiring
only modest strength. In the most demanding applications, such as hot sections of gas
turbine engines, a precipitation strengthened alloy is required. Most nickel-based alloys
contain 10-20% Cr, up to 8% Al and Ti, 5-10% Co, and small amounts of B, Zr, and C.
Other common additions are Mo, W, Ta, Hf, and Nb (often still referred to as
"columbium" although the name "niobium" was adopted by the International Union of
Pure and Applied Chemistry in 1950 after more than 100 years of controversy). In broad
terms, the elemental additions in Ni-base superalloys can be categorized as being i)
formers (elements that tend to partition to the matrix, ii) ' formers (elements that
partition to the ' precipitate, iii) carbide formers, and iv) elements that segregate to the
grain boundaries. Elements which are considered formers are Group V, VI, and VII
elements such as Co, Cr, Mo,W, Fe. The atomic diameters of these alloys are only 3-13%
different than Ni (the primary matrix element). ' formers come from group III, IV, and V
elements and include Al, Ti, Nb, Ta, Hf. The atomic diameters of these elements differ
from Ni by 6-18%. The main carbide formers are Cr, Mo, W, Nb, Ta, Ti. The primary
grain boundary elements are B, C, and Zr. Their atomic
diameters are 21-27% different than Ni.
The major phases present in most nickel superalloys are
as follows:
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Gamma ( ): The continuous matrix (called
gamma) is an face-centered-cubic (fcc) nickelbased austenitic phase that usually contains a high percentage of solid-solution
elements such as Co, Cr, Mo, and W.
Gamma Prime ( '): The primary strengthening phase in nickel-based superalloys
is Ni3(Al,Ti), and is called gamma prime ( '). It is a coherently precipitating phase
(i.e., the crystal planes of the precipitate are in registry with the gamma matrix)
with an ordered L12 (fcc) crystal structure. The close match in matrix/precipitate
lattice parameter (~0-1%) combined with the chemical compatability allows the '
to precipitate homogeneously throughout the matrix and have long-time stability.
Interestingly, the flow stress of the ' increases with increasing temperature up to
about 650oC (1200oF). In addition, ' is quite ductile and thus imparts strength to
the matrix without lowering the fracture toughness of the alloy. Aluminum and
titanium are the major constituents and are added in amounts and mutual
proportions to precipitate a high volume fraction in the matrix. In some modern
alloys the volume fraction of the ' precipitate is around 70%. There are many
factors that contribute to the hardening imparted by the ' and include ' fault
energy, ' strength, coherency strains, volume fraction of ', and ' particle size.
Extremely small ' precipitates always occur as spheres. In fact, for a given
volume of precipitate, a sphere has 1.24 less surface area than a cube, and thus is
the preferred shape to minimize surface energy. With a coherent particle,
however, the interfacial energy can be minimized by forming cubes and allowing
the crystalographic planes of the cubic matrix and precipitate to remain
continuous. Thus as the ' grows, the morphology can change from spheres to
cubes (as shown in this figure) or plates depending on the value of the
matrix/precipitate lattice mismatch. For larger mismatch values the critical
particle size where the change from spheres to cubes (or plates) occurs is reduced.
Coherency can be lost by overaging. One sign of a loss of coherency is directional
coarsening (aspect ratio) and rounding of the cube edges. Increasing directional
coarsening for increasing (positive or negative) mismatch is also expected.
Carbides: Carbon, added at levels of 0.05-0.2%, combines with reactive and
refractory elements such as titanium, tantalum, and hafnium to form carbides
(e.g., TiC, TaC, or HfC). During heat treatment and service, these begin to
decompose and form lower carbides such as M23C6 and M6C, which tend to form
on the grain boundaries. These common carbides all have an fcc crystal structure.
Results vary on whether carbides are detrimental or advantageous to superalloy
properties. The general opinion is that in superalloys with grain boundaries,
carbides are beneficial by increasing rupture strength at high tempeature.
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Topologically Close-Packed Phases: These are generally undesirable, brittle
phases that can form during heat treatment or service. The cell structure of these
phases have close-packed atoms in layers separated by relatively large interatomic
distances. The layers of close packed atoms are displaced from one another by
sandwiched larger atoms, developing a characteristic "topology." These
compounds have been characterized as possessing a topologically close-packed
(TCP) structure. Conversely, Ni3Al (gamma prime) is close-packed in all
directions and is called geometrically close-packed (GCP).
TCPs ( , µ, Laves, etc.) usually form as plates (which appear as needles on a
single-plane microstructure.) The plate-like structure negatively affects
mechanical properties (ductility and creep-rupture.) Sigma appears to be the most
deleterious while strength retention has been observed in some alloys containing
mu and Laves. TCPs are potentially damaging for two reasons: they tie up and '
strengthening elements in a non-useful form, thus reducing creep strength, and
they can act as crack initiators because of their brittle nature.
APPLICATIONS
RELATED LINKS
Superalloy-Related Companies
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Axel Johnson Metals
Cannon-Muskegon Corp.
Carpenter Technology
Corporation
Chromalloy
Dynamet
Haynes International
Howmet Corp.
INCO
Ladish
PCC Airfoils
Special Metals
Teledyne Allvac
Utica Corporation
Wyman-Gordon
Turbine Engine Manufacturers
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AlliedSignal Aerospace
Allison Engine Company
CFM International
Daimler-Benz Aeorspace
General Electric Aircraft Engines
Nickel-based superalloys are used in load-bearing
 Lycoming
 Pratt & Whitney (P&W Canada)
structures to the highest homologous temperature of
 Rolls Royce/BMW Rolls Royce
any common alloy system (Tm = 0.9, or 90% of their
 Snecma
melting point). Among the most demanding
 Volvo Aero Corporation
applications for a structural material are those in the
 Westinghouse
hot sections of turbine engines. The preeminence of
superalloys is reflected in the fact that they currently Another listing of manufacturers is
comprise over 50% of the weight of advanced
available from Gas-Turbines.
aircraft engines. The widespread use of superalloys
in turbine engines coupled with the fact that the
thermodynamic efficiency of turbine engines is
Turbine Engine Information
increased with increasing turbine inlet temperatures
has, in part, provided the motivation for increasing
 How a Jet Engine Works (from
NASA Lewis)
the maximum-use temperature of superalloys. In
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Gas Turbine Primer (from Gasfact, during the past 30 years turbine airfoil
Turbines)
temperature capability has increased on average by
about 4°F per year. Two major factors which have
made this increase possible are
Other Interesting Sites
1. Advanced processing techniques, which
improved alloy cleanliness (thus improving
reliability) and/or enabled the production of
tailored microstructures such as directionally
solidified or single-crystal material.
2. Alloy development resulting in higher-usetemperature materials primarily through the
additions of refractory elements such as Re,
W, Ta, and Mo.
About 60% of the use-temperature increases have
occurred due to advanced cooling concepts; 40%
have resulted from material improvements. State-ofthe-art turbine blade surface temperatures are near
2,100°F (1,150°C); the most severe combinations of
stress and temperature corresponds to an average
bulk metal temperature approaching 1,830°F
(1,000°C).
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General Science
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Elements
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Structures
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Technology
Although superalloys retain significant strength to
temperatures near 1800°F, they tend to be susceptible to environmental attack because of
the presence of reactive alloying elements (which provide their high-temperature
strength). Surface attack includes oxidation, hot corrosion, and thermal fatigue. In the
most demanding applications, such as turbine blade and vanes, superalloys are often
coated to improve environmental resistance.
PROCESSING
The material and casting technique improvements that have taken place during the last 50
years have enabled superalloys to be used first as equiaxed castings in the 1940s, then as
directionally solidified (DS) materials during the 1960s, and finally as single crystals
(SC) in the 1970s. Each casting technique advancement has resulted in higher use
temperatures.
In DS processing, columnar grains are formed parallel to the growth axis. In nickel-based
alloys, the natural growth direction is along the <100> crystallographic direction. This
morphology is accomplished by pouring liquid metal into a mold that contains a watercooled bottom plate. Solidification first occurs at the bottom plate, after which the mold
is slowly withdrawn from the furnace, allowing the metal inside to directionally solidify
from bottom to top. The exceptional properties of DS and SC alloys is due to
1. The alignment or elimination of any weak grain boundaries oriented transverse to
the eventual loading direction.
2. The low modulus associated with the <100> directions enhances thermal
mechanical fatigue resistance in areas of constrained thermal expansion—
particularly turbine vanes. In general, the lack of transverse grain boundaries
coupled with the lower modulus can result in 3-5 times improvement in rupture
life.
SC casting were developed during the 1970s and were a spin-off from the technological
advances made in the DS casting processes. SC casting are produced in a similar fashion
to DS by selecting a single grain, via a grain selector. During solidification, this single
grain grows to encompass the entire part. Single crystals obtain their outstanding strength
through the elimination of grain boundaries that are present in both equiaxed and
directionally solidified materials. In addition, the elimination of grain boundary
strengtheners such as C, B, Si, and Zr raises the single crystal's melting point. By
increasing the alloy's melting point, the homogenization heat-treat temperature can be
increased without fear of incipient melting, thus allowing for more complete solutioning
of the ' and thereby increasing alloy strength and maximum use temperature.
FURTHER READING
The first comprehensive book on superalloys—and probably the best single source of
information related to superalloys—is, appropriately, Superalloys, published in 1972 by
John Wiley & Sons. Since the book's original publication, it has become widely regarded
as the standard reference in the field of superalloys. The 1987 edition, Superalloys II,
although based on the original version was thoroughly updated to reflect the latest
developments in the field.
Another good reference is "The Microstructure of Superalloys" by Madeleine DurandCharre, published by Gordon and Breach Science Publishers in 1998. With more than
100 illustrations, the 140-page text explains all the transformation mechanisms involved
in the formation of microstructures during solidification and heat treatments
(crystallization paths, segregation, crystal orientation, precipitation, TCP, coarsening and
rafting, etc.). It includes up-to-date information and data such as phase diagrams and
crystallographic structures. The nearly 300 references provide a valuable resource for
further investigation.
Additional information can also be gleaned from the following conference sites:
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The 4th International Symposium on 718, 625, 706, and Derivatives
The 9th International Symposium on Superalloys
The content of this site was developed by Randy Bowman (randy.bowman@lerc.nasa.gov); your feedback is welcome.