Superalloys are a family of heat resistant alloys of nickel, iron-nickel, and cobalt that normally operate at temperatures exceeding 1000° F. They are required to exhibit combinations of high strength; good fatigue and creep resistance; good corrosion resistance; and the ability to operate at elevated temperatures for extended periods of time (i.e., metallurgical stability). Their combination of high temperature strength and resistance to surface degradation is unmatched by other metallic materials.
Strengthening of the nickel and iron-nickel based superalloys is due to the combination of solid solution hardening, precipitation hardening, and the
presence of carbides. Metallic carbides form in both the matrix, and at the grain boundaries, to help in providing high temperature stability. Cobalt based alloys are not precipitation strengthened; they are strengthened by a combination of solid solution strengthening and by carbides. The most important precipitate in nickel, and some iron-nickel based superalloys, is y' Ni3 (Al, Ti) in the form of either Ni3Al or Ni3Ti. The y' phase is precipitated by heat treatments. The important strengthening precipitate in the iron-nickel based alloy Inconel 718 is y'' (Ni3Nb).
Superalloys are used in cast, rolled, extruded, forged, and powder-produced forms. Wrought alloys are generally more uniform with finer grain sizes and superior tensile and fatigue properties, while cast alloys have more alloy segregation and coarser grain sizes but better creep and stress rupture properties. Accordingly, wrought alloys are used where tensile strength and fatigue resistance are important, such as disks, while cast alloys are used where creep and stress rupture are important, such as turbine blades. The amount of alloying is so high in some superalloys that they cannot be produced as wrought products; they must be produced as either castings or by PM methods. In general, the more heat resistant the alloy, the more likely it is to be prone to segregation and brittleness, and therefore producible only by casting or by using powder metallurgy techniques.
Two processes are used for the production of forged superalloys, ingot metallurgy, and PM. Ingot metallurgy often involves triple melt technology, which includes three melting steps, followed by annealing and hot working to achieve the desired compositional control and grain size. The three melting steps include (1) VIM to prepare the desired alloy composition; (2) ESR to remove oxygen containing inclusions; and (3) VAR to reduce compositional segregation that occurs during ESR solidification. Melting is followed by homogenizing and hot working to achieve the desired homogeneity and grain size.
Powder metallurgy is often required for high volume fraction y' strengthened alloys, such as René 95 and Inconel 100, which cannot be made by conventional ingot metallurgy and forging without cracking. The PM process includes (1) VIM to prepare the desired alloy composition; (2) remelting and atomizing to produce powder; (3) sieving to remove large particles and inclusions; (4) canning to place the powder in a container suitable for consolidation; (5) vacuum degassing and sealing to remove the atmosphere; and (6) HIP or extrusion to consolidate the alloy to a billet. Billets are then subsequently forged to final part shape.
Because of their strength retention at elevated temperatures, superalloys are more difficult to forge than most metals. The forgeability varies widely depending on the type of superalloy and its exact composition. For example, some of the iron-nickel based alloys, such as A-286, are similar to the austenitic stainless steels. At the other extreme, some superalloy compositions are intrinsically so strong at elevated temperatures that they can only be processed by casting or PM. In general, as the alloying content has been increased to obtain even greater elevated temperature strength, the forgeability has been degraded, i.e. the y' strengthened alloys are much more difficult to forge than the solid solution strengthened alloys.
Wrought superalloys can often be formed using techniques similar to stainless steels, although forming is more difficult. Like stainless steels, superalloys work harden rapidly during forming. The cobalt based alloys require greater forces than the nickel or iron based alloys. Forming presses are the same as those used for forming steel; however, because of their higher strength, more power is needed, usually 50-100% more power.
The hottest parts of the engine, the blades and vanes, are manufactured by investment casting. As the alloy content of nickel based superalloys was continually increased to obtain better creep and stress rupture capability, the alloys became increasingly difficult to forge. To allow even higher contents of alloying elements, it became necessary to change the fabrication process to casting. The original cast blades and vanes were fine grained polycrystalline structures made using conventional investment casting procedures. These blades were then heat treated to coarsen the grain structures to enhance creep resistance. Eventually it became possible to produce DS structures with columnar grains oriented along the longitudinal axis of the blade. The columnar grain structure enhances the elevated temperature ductility by eliminating the grain boundaries as failure initiation sites. The DS process also creates a preferred low modulus texture or orientation parallel to the solidification direction that helps in preventing thermal fatigue failures. An extension of the DS process is the development of the single crystal (SX) process, in which a single crystal grows to form the entire blade. The elimination of grain boundaries also removes the necessity for adding grain boundary strengthening elements, namely boron, hafnium, zirconium, and carbon. The removal of these elements raises the melting point, and allows a higher solution heat treatment temperature, with a consequent improvement in chemical homogeneity and more uniform distribution of y' precipitates.
Solution heat treating and aging is used to precipitation harden a great many of the nickel and iron-nickel based alloys. Solution treating temperatures range from about 1800-2250° F, or even up to 2400° F for single crystal alloys. Aging treatments are then used to strengthen precipitation-strengthened alloys by precipitating one or more phases (y' or y''). Aging treatments vary from as low as 1150° F to as high as 1900° F. Double aging treatments are used to produce different sizes and distributions of precipitates. A principal reason for double aging treatments, in addition to y' and y'' control, is the need to control grain boundary carbide morphology. Aging heat treatments normally range from 1600 to 1800° F with times from 4 to 32 h.
Superalloys are difficult to machine, perhaps second only to titanium in machining difficulty. Many of the same characteristics that make superalloys good high temperature materials also make them difficult to machine. General guidelines for machining superalloys are: conduct majority of machining in the softest state possible; use positive rake angles; use sharp cutting tools; use strong geometries; use rigid set-ups; prevent part deflection; use large lead angles; and when more than one pass is required, vary the depth of cut.
Because of the large amounts of y' strengthening in nickel and iron-nickel superalloys, they are considerably less weldable than the cobalt alloys. The y' strengthened alloys are susceptible to hot cracking during welding, or may crack after welding (delayed cracking). The susceptibility to hot cracking is a function of their aluminum and titanium contents, which forms y'. Cracking usually occurs in the HAZ and welding is usually restricted to wrought alloys with about 0.35 or less volume fraction of y'. Casting alloys with high aluminum and titanium contents are considered unweldable, because they will usually hot crack during the welding operation. Borderline alloys, such as René 41 and
Waspaloy, can usually survive the welding process but may crack later. One big advantage of Inconel 718 is its ability to be successfully fusion welded. Nickel brazing alloys produce joints with strength and oxidation resistance for service at temperatures up to 2000° F.
Superalloy engine blades are often coated to prevent environmental degradation, and more recently, to provide thermal barriers which allow even higher operating temperatures. Superalloy coatings are divided into two main categories: diffusion coatings are coatings that diffuse into the surface and react with alloying elements to form the protective coating, and overlay coatings that are deposited on the surface but only react with the substrate to the extent that an adherent bond is formed. While the diffusion and overlay coatings are applied to provide environmental resistance to oxidation and hot corrosion, ceramic TBC are applied to allow the turbine blades to operate at even higher temperatures. The TBC must be sufficiently thick, have a low thermal conductivity, have a high resistance to thermal shock, and contain a certain percentage of voids to provide more thermal insulation.
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