Superalloys: A Primer and History
The term "superalloy" was first used shortly after World War II to describe a group of alloys developed for use in turbosuperchargers and aircraft turbine engines that required high performance at elevated temperatures. The range of applications for which superalloys are used has expanded to many other areas and now includes aircraft and land-based gas turbines, rocket engines, chemical, and petroleum plants. 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 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:
- Gamma (): The continuous matrix (called gamma) is an face-centered-cubic (fcc) nickel-based 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.
- 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
Nickel-based superalloys are used in load-bearing structures to the highest homologous temperature of any common alloy system (Tm = 0.9, or 90% of their melting point). Among the most demanding applications for a structural material are those in the hot sections of turbine engines. The preeminence of superalloys is reflected in the fact that they currently comprise over 50% of the weight of advanced aircraft engines. The widespread use of superalloys in turbine engines coupled with the fact that the thermodynamic efficiency of turbine engines is increased with increasing turbine inlet temperatures has, in part, provided the motivation for increasing the maximum-use temperature of superalloys. In fact, during the past 30 years turbine airfoil temperature capability has increased on average by about 4°F per year. Two major factors which have made this increase possible are
- 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.
- Alloy development resulting in higher-use-temperature 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-of-the-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).
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.
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 water-cooled 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
- The alignment or elimination of any weak grain boundaries oriented transverse to the eventual loading direction.
- The low modulus associated with the <100> directions enhances thermal mechanical fatigue resistance in areas of constrained thermal expansionparticularly 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.
The first comprehensive book on superalloysand probably the best single source of information related to superalloysis, 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 Durand-Charre, 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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