SUPERalloys
Introduction
Superalloys are heat resistant alloys of nickel, iron–nickel and cobalt that frequently operate at temperatures exceeding 1000 °F. However, some superalloys are capable of being used in load bearing applications in excess of 85% of their incipient melting temperatures. 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 elevated temperature strength and resistance to surface degradation is unmatched by other metallic materials.
Working at temperatures close to the melting point of the material and spinning hundreds of times per second while simultaneously supporting a load equivalent to the weight of a family car, the blades in a modern jet engine have to withstand what is arguably one of the most extreme environments any engineered substance could ever encounter. The combustion products routinely reach temperatures of 2000°C, and the environment inside the engine is highly corrosive and oxidizing.
The iron–nickel-base superalloys are an extension of stainless steel technology and generally are melted and cast to electrode / ingot shapes for subsequent fabrication to components. The iron–nickel-base superalloys usually are wrought, i.e., formed to shape or mostly to shape by hot rolling, forging, etc. On the other hand, after primary production by melting and ingot casting, the cobalt-base and nickel-base superalloys may be used either in wrought or cast form depending on the application or the alloy composition involved. The stainless steels, nickel–chromium alloys, and cobalt dental alloys which evolved into the superalloys used chromium to provide elevated-temperature corrosion resistance. A Cr2O3 layer on the surface proved very effective in protection against oxidation. Eventually, cast superalloys for the highest temperatures were protected against oxidation by chromium and aluminum. In our opinion, superalloys must contain chromium, probably at the level of 5% (some would argue 8%) or higher for reasonable corrosion resistance.
History
The term superalloy was first used shortly after World War II to describe a group of alloys developed for use in turbo superchargers and aircraft turbine engines that required high performance at elevated temperatures.
The first age-hardenable, high-temperature alloy dates back to about 1929 when various developers added titanium and aluminum to the standard 80% nickel/20% chromium resistance wire alloy. This was a precursor to the 80A nickel-base superalloy, developed in 1940-1944, but still in use today.
Little was done to advance the original age-hardenable alloys until the time period of 1935-1944 when World War II spurred demand for improved alloys that could be used in the early aircraft gas turbine engines. Alloy development activity exploded in the 1950’s and 1960’s to keep pace with the demands of the gas turbine engine industry. Progress in superalloy development not only made the jet engine possible, but allowed for constantly increasing thrust-to-weight ratios over the last 60 years.
The driving force behind their development has been the jet engine which has required ever higher operating temperatures. The use of the alloys has, however, extended into many other fields - all types of turbines, space vehicles, rocket motors, nuclear reactors, power plants, chemical equipment and possibly 20% of alloys have arisen for corrosion resistant applications.
But what sets modern-day materials apart from those tried and tested by time is that, today, we understand much more, at an atomic level, about why alloys work. Some of the theories developed over the last 50 years, including techniques like quantum mechanics, are now beginning to provide useful insights into how alloying will affect the structures and properties of the resulting materials, and while these methods are being perfected, there is also over sixty years of empirical data to fall back on, including statistical methods that can deduce how a new material might behave. That said, designing new alloys certainly isn't easy, because predicting how atoms of different elements will behave and interact with each other when they are mixed is extremely challenging.
Principal Alloys
A large number of alloys have been invented and studied; many have been patented. However, the many alloys have been winnowed down over the years, and only a few are extensively used. Alloy usage is a function of industry (gas turbines, steam turbines, etc.).
Iron-Nickel Base. The most important class of iron-nickel-base superalloys includes those strengthened by intermetallic compound precipitation in an fcc matrix. Other iron-nickel-base superalloys consist of modified stainless steels strengthened primarily by solid-solution hardening. Alloys in this last category vary from 19-9DL (18-8 stainless with slight chromium and nickel adjustments, additional solution hardeners, and higher carbon) to Incoloy 800H (21 wt% Cr, high nickel with small additions of titanium and aluminum). Other iron-nickel superalloys include Multimet, Discaloy, and Pyromet types.
Nickel-base superalloys are used in both cast and wrought forms, although special processing (powder metallurgy/isothermal forging) is frequently used to produce wrought versions of the more highly alloyed compositions (René 95, Astroloy, IN 100). An additional dimension of nickel-base superalloys has been the introduction of grain-aspect ratio and orientation as a means of controlling properties. In some instances, in fact, grain boundaries have been removed. Wrought powder metallurgy alloys and cast alloys such as MAR-M 247 have demonstrated property improvements due to control of grain morphology by directional recrystallization or solidification. Other nickel-base superalloys include Haynes, Inconel, Hastelloy, Nimonic, Incoloy, Pyromet, Refractaloy, Udimet, Unitemp, and Waspaloy.
Cobalt Base. The cobalt-base superalloys are invariably strengthened by a combination of carbides and solid-solution hardeners. The essential distinction in these alloys is between cast and wrought structures. Cast alloys are typified by X-40 and wrought alloys by L605. Other major types are Stellite and Haynes.
Structure and Properties
The superalloys are relatively ductile, although the ductilities of cobalt-base superalloys generally are less than those of iron-nickel- and nickel-base superalloys. Iron-nickel- and nickel-base superalloys are readily available in extruded, forged, or rolled form; the higher-strength alloys generally are found only in the cast condition. Hot deformation is the preferred forming process, with cold forming usually being restricted to thin sections (sheet).
Properties can be controlled by adjustments in composition and by processing (including heat treatment), and excellent elevated-temperature strengths are available in finished products. Appropriate compositions of all superalloy-base metals can be forged, rolled to sheet, or otherwise formed into a variety of shapes.
Superalloys contain a variety of elements in a large number of combinations to produce desired effects. Some elements go into solid solution to provide one or more of the following: strength (molybdenum, tantalum, tungsten, rhenium), oxidation resistance (chromium, aluminum), phase stability (nickel), and increased volume fractions of favorable secondary precipitates (cobalt). Carbides also are an important constituent of superalloys. They are particularly essential in the grain boundaries of cast polycrystalline alloys for production of desired strength and ductility characteristics. A discussion of the function of alloying elements in terms of microstructure would be incomplete without mention of the tramp elements. Elements such as silicon, phosphorus, sulfur, lead, bismuth, tellurium, selenium, and silver, often in amounts as low as the parts per million level, have been associated with property-level reductions, but they are not visible optically or with an electron microscope.
The principal microstructural variables of superalloys are the precipitate amount and its morphology, grain size and shape, and carbide distribution. Structure control is achieved through composition selection/modification and by processing. For a given nominal composition, there are property advantages and disadvantages of the structures produced by deformation processing and by casting. Cast superalloys generally have coarser grain sizes, more alloy segregation, and improved creep and rupture characteristics. Wrought superalloys generally have more uniform, and usually finer, grain sizes and improved tensile and fatigue properties.
Metallurgical Considerations
Nickel has an FCC crystalline structure, a density of 0.322lb/in., and a melting point of 2650 °F. While iron has a BCC structure at room temperature and cobalt a HCP structure at room temperature, both iron and cobalt-based superalloys are so highly alloyed that they have an austenitic γ FCC structure at room temperature. Therefore, the superalloys display many of the fabrication advantages of the FCC structure.
Nickel and iron–nickel based superalloys are strengthened by a combination of solid solution hardening, precipitation hardening and the presence of carbides at the grain boundaries. The FCC nickel matrix, which is designated as austenite (γ), contains a large percentage of solid solution element such as iron, chromium, cobalt, molybdenum, tungsten, titanium and aluminum. Aluminum and titanium, in addition to being potent solid solution hardeners, are also precipitation strengtheners. At temperatures above 0.9 Tm , which is in the temperature range for diffusion controlled creep, the slowly diffusing elements molybdenum and tungsten are beneficial in reducing high temperature creep.
The most important precipitate in nickel and iron–nickel based superalloys is γ’ FCC ordered Ni3(Al,Ti) in the form of either Ni3Al or Ni3Ti. The γ’ phase is precipitated by precipitation hardening heat treatments: solution heat treating followed by aging. The γ’ precipitate is an A3B type compound where A is composed of the relatively electronegative elements nickel, cobalt andiron, and B of the electropositive elements aluminum, titanium or niobium. Typically, in the nickel based alloys, γ’ is of the form Ni3(Al,Ti) , but if cobalt is added, it can substitute for some nickel as (Ni,Co)3(Al,Ti). The precipitate γ’ has only about an 0.1% mismatch with the γ matrix; therefore, γ’ precipitates homogeneously with a low surface energy and has extraordinary long term stability.
The coherency between γ’ and γ is maintained to high temperatures and has a very slow coarsening rate, so that the alloy overages extremely slowly even as high as 0. 7 Tm. Since the degree of order in Ni3(Al,Ti) increases with temperature, alloys with a high volume of γ’ actually exhibit an increase in strength as the temperature is increased up to about 1300°F.The γ/γ’ mismatch determines the γ’ precipitate morphology, with small mismatches (~0.05%) producing spherical precipitates and larger mismatches producing cubical precipitates as shown in Fig.1
Fig.1. Microstructure of Precipitation-Strengthened Nickel Based Superalloy
If appreciable niobium is present, the body centered tetragonal ordered γ’’ (Ni3Nb) precipitate can form. This is an important strengthening precipitate in some of the iron–nickel based superalloys, and forms the basis for strengthening the important alloy Inconel 718. Other less frequent precipitates include the hexagonal ordered ɳ(Ni3Ti) and the orthorhombic δ(Ni3Nb) phases which help to control the structure of wrought alloys during processing.
The compositions of commercial superalloys are complex (some contain as many as a dozen alloying elements), with the roles of various alloying elements shown in Table 1. Chromium and aluminum additions help in providing oxidation resistance. Chromium forms Cr2O3 on the surface, and when aluminum is present, the even more stable Al2O3 is formed. A chromium content of 5–30% is usually found in superalloys. Solid solution strengthening is provided by molybdenum, tantalum, tungsten and rhenium. Rhenium also helps to retard the coarsening rate of γ’ and is used in some of the latest cast nickel based alloys.
Cobalt helps to increase the volume percentage of helpful precipitates that form with additions of aluminum and titanium (γ’) and niobium (γ’’). Unfortunately, increasing the aluminum and titanium content lowers the melting point, thereby narrowing the forging range, which makes processing more difficult. Small additions of boron, zirconium and hafnium improve the mechanical properties of nickel and iron–nickel alloys. A number of alloying elements, although added for their favorable characteristics, can also form the undesirable σ, μ and Laves phases which can cause in-service embrittlement if the composition and processing are not carefully controlled.
The carbon content of nickel based superalloys varies from about 0.02 to0.2% for wrought alloys and up to about 0.6% for some cast alloys. Metallic carbides can form in both the matrix and at the grain boundaries. In high temperature service, the properties of the grain boundaries are as important as the strengthening by γ’ within the grains. Grain boundary strengthening is produced mainly by precipitation of chromium and refractory metal carbides. Small additions of zirconium and boron improve the morphology and stability of these carbides.
Table 1. Role of Alloying Elements in Superalloys
Alloy
Additions
|
Solid Solution Strengtheners
|
ɣ’
Farmers
|
Carbide
Farmers
|
Grain boundary
Strengtheners
|
Oxide Scale Formers
|
Chromium
Aluminum
Titanium Molebdenum
Tungsten
Boron
Zirconium
Carbon
Niobium
Hafnium
Tantalum
|
X
X
X
|
X
X
X
X
|
X
X
X
X
X
X
X
|
X
X
X
X
X
|
X
X
|
Carbides in superalloys perform three functions. First, grain boundary car-bides, when properly formed, strengthen the grain boundaries, prevent or retard grain boundary sliding and permit stress relaxation along the grain boundaries. Second, a fine distribution of carbides precipitated within the grains increases strength; this is especially important in the cobalt based alloys that cannot be strengthened by γ’ Precipitates. Third, carbides can tie-up certain elements that would otherwise promote phase instability during service. Since carbides are harder and more brittle than the alloy matrix, their distribution along the grain boundaries will affect the high temperature strength, ductility and creep performance of nickel based alloys. There is an optimum amount and distribution of carbides along the grain boundaries. If there are no carbides along the grain boundaries, voids will form and contribute to excessive grain boundary sliding. However, if a continuous film of carbides is present along the grain boundaries, continuous fracture paths will result in brittleness. Thus, the optimum distribution is a discontinuous chain of carbides along the grain boundaries, since the carbides will then hinder grain boundary sliding without adversely affecting ductility.
Multistage heat treatments are often used to obtain the desired grain boundary distribution, along with a mix of both small and large γ’ precipitates, for the best combination of strength at intermediate and high temperatures. Some of the important carbides are MC, M23C6, M6C and M7C3 . In nickel based alloys, M stands for titanium, tantalum, niobium or tungsten. MC carbides usually form just below the solidification temperature during ingot casting and are usually large and blocky, have a random distribution and are generally not desirable.
However, MC carbides tend to decompose during heat treatment intoother more stable carbides, such as M23C6 and/or M6C. In the M23C6 carbides,M is usually chromium but can be replaced by iron and to a smaller extent by tungsten, molybdenum or cobalt, depending on the alloy composition. M23C6 carbides can form either during heat treatment or during service at temperatures between 1400 and 1800°F. They can form from either the degeneration of MC carbides or from soluble carbon in the matrix. M23C6 carbides tend to precipitate along the grain boundaries and enhance stress rupture properties. M6C carbides form at temperatures in the range of 1500–1800°F.
They are similar to the M23C6 carbides and have a tendency to form when the molybdenum and tungsten contents are high. Although not nearly as prevalent as the other carbides, M7C3 carbides also precipitate on the grain boundaries, and are beneficial if they are discrete particles but detrimental if they form a grain boundary film. Over the years, a number of undesirable topologically closed-packed (TCP) phases have appeared either during heat treatment or in-service, the most important being σ, μ and Laves. These phases, which usually form as thin plates or needles, can lead to lower stress rupture strengths and a loss in ductility.
They also remove useful strengthening elements from the matrix, such as the refractory elements molybdenum, chromium and tungsten, which reduces both solid solution strengthening and the γ /γ’ mismatch. Modern computer modeling programs (e.g., Phacomp) are capable of predicting their occurrence so they can be avoided in alloy design. As with most high performance alloys, hydrogen, oxygen and nitrogen are considered detrimental and are held to very low levels.
Applications
Superalloys have been used in aircraft, industrial, and marine gas turbines, nuclear reactors, aircraft skins, spacecraft structures, petrochemical production, and environmental protection applications. Although developed for high-temperature applications, some are used at cryogenic temperatures. Orthopedic and dental prostheses have evolved from cobalt-base superalloy development. Applications continue to expand, although aerospace remains the predominant application.
The three industries that consume the most superalloy products are the:
· Aerospace industry—principally jet engine
· The land-based turbine business, and
· The chemical process industry
Superalloys are the primary materials used in the hot portions of jet turbine engines, such as the blades, vanes and combustion chambers, constituting over 50% of the engine weight.Superalloys are also used in other industrial applications where their high temperature strength and/or corrosion resistance is required. These applications include rocket engines, steam turbine power plants, reciprocating engines, metal processing equipment, heat treating equipment, chemical and petro chemical plants, pollution control equipment, coal gasification and liquification systems, and medical applications.
In general, the nickel-based alloys are used for the highest temperature applications, followed by the cobalt-based alloys and then the iron–nickel alloys.
Fig. 2. Stress Rupture Comparison of Wrought Superalloys
A relative comparison of their stress rupture properties is shown in Fig. 2.Superalloys are produced as wrought, cast and powder metallurgy product forms. Some superalloys are strengthened by precipitation hardening mechanisms, while others are strengthened by solid solution hardening. For jet engine applications, large grain cast alloys are preferred for creep and stress rupture limited turbine blade applications, while small grain forged alloys are preferred for strength and fatigue limited turbine disk applications. The large grain sizes help in preventing creep, while the smaller grain sizes enhance strength and fatigue resistance.
Processing
Superalloys have been used in cast, rolled, extruded, forged, and powder-processed forms. Sheet, bar, plate, tubing, shafts, airfoils, disks, and pressure vessels (cases) are some of the shapes that have been produced. Although developed for high-temperature use, some are used at cryogenic (low) temperatures and others at body temperature. Applications continue to expand but at lower rates than in previous decades. Aerospace usage remains the predominant application on a volume basis.
Processing is considered to be the art/science of rendering the superalloy material into its final form. Processing and alloying elements are interdependent. The general microstructural changes brought about by processing result from the overall alloy composition plus the processing sequence. The three most significant process-related microstructural variables, other than those resulting from composition/heat treatment interactions, are the size, shape, and orientation of the grains.
Grain size varies considerably from cast to wrought structure, generally being significantly smaller for the latter. Special processing, for example, directional solidification or directional recrystallization, can effect changes not only in grain size but also in grain shape and orientation, which significantly alter mechanical and physical properties. Corrosion reactions, however, are primarily functions of composition.
Powder techniques are being used extensively in superalloy production. Principally, high-strength gas turbine disk alloy compositions such as IN 100 or René 95, which are difficult or impractical to forge by conventional methods, have been powder processed. Inert atmospheres are used in the production of powders, often by gas atomization, and the powders are consolidated by extrusion or hot isostatic pressing (HIP). The latter process has been used either to produce shapes directly for final machining or to consolidate billets for subsequent forging. Extruded or HIPed billets often are isothermally forged to configurations for final machining.
Superalloys are used in the cast, rolled, extruded, forged and powder produced forms as shown in the overall process flow in Fig. 3. 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, and cast alloys are used where creep and stress rupture are important, such as turbine blades.
It should also be noted that the amount of alloying is so high in some superalloys that they cannot be produced as wrought products (which are produced from large diameter ingots); they must be produced as either castings or by powder metallurgy methods. In general, the more heat resistant the alloy, the more likely it is to be prone to segregation and brittleness, and therefore formable only by casting or by using powder metallurgy techniques.
Stress-corrosion cracking can occur in nickel- and iron-nickel-base superalloys at lower temperatures. Hydrogen embrittlement at cryogenic temperatures has been reported for such alloys. Furthermore, so-called inert environments—vacuum, for example, or gases such as helium or argon—may produce mechanical behavior substantially different from baseline uncoated properties, which usually are determined in static-air tests.
Two processes are used for the production of forged superalloys, ingot metallurgy and powder metallurgy. Ingot metallurgy often involves triple melt technology which includes three melting steps followed by homogenizing 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 ingot homogeneity and grain size Powder metallurgy (PM) is often required for high volume fraction γ’ strengthened alloys, such as René 95 and Inconel 100, which cannot be made by conventional ingot metallurgy and forging without cracking. The PM process (Fig. 6.5) includes (1) VIM to prepare the desired alloy composition, (2) remelting and atomizing to produce powder, (3) sieving to remove large particles and inclusions ( > 50–100μm in diameter), (4) canning to place the powder in a container suitable for consolidation, (5) vacuum degassing and sealing to remove the atmosphere and (6) hot isostatic pressing (HIP) or extrusion to consolidate the alloy to a billet. During HIP, small (< 20 μm) pores containing argon can be collapsed and tend to stay closed if subsequent processing temperatures are not significantly above the original HIP temperature. Billets are then subsequently forged to final part shape.
Creep failures are known to initiate at transverse grain boundaries, and it is possible to eliminate them in cast turbine blades to obtain further improve-ments in creep and stress rupture resistance. This can be achieved by directional solidified (DS) castings with columnar grains aligned along the growth direction, with no grain boundaries normal to the high stress direction. Further, by incorporating a geometric constriction in the mold or by the use of a seed crystal, it is possible to eliminate grain boundaries entirely and grow the blade as one single crystal (SX). The elimination of grain boundaries also removes the necessity for adding grain boundary strengthening elements, such as carbon, boron, zirconium and hafnium.
The removal of these elements raises the melting point and allows higher solution heat treatment temperatures with improvements in chemical homogeneity and a more uniform distribution of γ’ precipitates.
Fig. 3. Process Flow for Superalloy Components
Commercial Superalloys
The number of superalloys that have been developed and used over the years is large; a small number of these are listed in Tables 2. (wrought) and 3. (cast).Note that the designation in Table 2 breaks them into either solid solution or precipitation hardened. In reality, the solid solution strengthened alloys are strengthened both by solid solution hardening and by the presence of carbides, while the precipitation hardened alloys are strengthened by the combination of precipitates, solid solution hardening and the presence of carbides.
The iron–nickel based alloys, which are an extension of stainless steel technology, are generally wrought, where as the cobalt and nickel based alloys are both wrought and cast. The iron–nickel based alloys have high strengths below 1200°F and are more easily processed and welded than the nickel based alloys. Cobalt based superalloys have high melting points and high temperature capability at moderate stress levels, excellent hot salt corrosion resistance and better weldability than the nickel based alloys. However, they cannot compete with the nickel based alloys at high temperatures and high stress levels.
The most commercially important superalloy, Inconel 718, is listed as an iron–nickel based alloy even though it contains more nickel than iron. This classification fits with the traditional classification for this alloy, although many newer works list it as a nickel based alloy. Also note that for the cobalt based alloys, there are none listed as being precipitation hardened, because unfortunately, these alloys do not precipitation harden like the nickel and iron–nickel alloys. Also note that the composition of the cast alloys in Table 3 is generally more complex than for the wrought alloys.
Table 2.Nominal Compositions of Select Wrought Superalloys
Cr
|
Ni
|
Co
|
Mo
|
W
|
Nb
|
Ti
|
Al
|
Fe
|
C
|
Other
| |
Nickel Based
| |||||||||||
Solid Solution Hardened
| |||||||||||
Hasteloy X
|
22
|
49
|
<1.5
|
9
|
0.6
|
-
|
-
|
2
|
15.8
|
0.15
|
-
|
Inconel 625
|
21.5
|
61
|
-
|
9
|
-
|
3.6
|
-
|
0.2
|
2.5
|
0.05
|
-
|
Nimonic 75
|
19.5
|
75
|
-
|
-
|
-
|
-
|
0.4
|
0.15
|
2.15
|
0.12
|
<0.25Cu
|
Precipitation Hardened
| |||||||||||
Astroloy
|
15
|
56.5
|
15
|
5.25
|
-
|
-
|
3.5
|
4.4
|
<0.3
|
0.06
|
0.03 B, 0.06 Z
|
Inconel 100
|
10
|
60
|
15
|
5.25
|
-
|
-
|
4.7
|
5.5
|
<0.6
|
0.15
|
1.0 V, 0.06 Zr, 0.015 B
|
Inconel 70
|
16.0
|
4.5
|
-
|
-
|
-
|
-
|
1.75
|
2
|
0.2
|
37.5
|
1.0 V, 0.06 Zr, 0.015 B
|
Nimonic 95
|
19.5
|
53.5
|
18.0
|
-
|
-
|
-
|
2.9
|
2.0
|
<5.0
|
<0.15
|
+ B+ Z
|
René 95
|
14.0
|
61
|
8.0
|
3. 5
|
3.5
|
3.5
|
2.5
|
3.5
|
<0.3
|
0.16
|
0.01 B, 0.5 Z
|
Waspaloy
|
19.5
|
57
|
13.5
|
4.3
|
–
|
–
|
3
|
1. 4
|
<2
|
0.07
|
0.006 B, 0.09 Zr
|
Iron–Nickel Based
| |||||||||||
Solid Solution Hardened
| |||||||||||
19-9 DL
|
19.0
|
9
|
–
|
1.25
|
1.25
|
.4
|
0.3
|
-
|
66.8
|
0.30
|
1.10 Mn, 0.60 S
|
Haynes 55
|
22.0
|
21
|
20.0
|
3
|
2.5
|
0.1
|
0.1
|
0.3
|
29.0
|
0.10
|
0.50 Ta, 0.02 La, 0.002 Zr
|
Incoloy 802
|
21.0
|
32.5
|
-
|
-
|
-
|
-
|
-
|
.58
|
44.8
|
0.36
|
-
|
Precipitation Hardened
| |||||||||||
A-286
|
15.0
|
26
|
–
|
1.25
|
–
|
–
|
2.0
|
0.2
|
55.2
|
0.04
|
0.005 B, 0.3 V
|
Inconel 71
|
19.0
|
52.5
|
-
|
3
|
-
|
5.1
|
0.91
|
0.5
|
18.5
|
<0.08
|
<0.15 Cu
|
Incoloy 903
|
<0.1
|
38
|
15
|
0.1
|
-
|
3.0
|
1.4
|
0.7
|
41
|
0.0
|
-
|
Cobalt Based
| |||||||||||
Solid Solution Hardened
| |||||||||||
Haynes 25 (L605)
|
20.0
|
10
|
50.0
|
-
|
15
|
-
|
-
|
-
|
3.0
|
0.1
|
1.5 Mn
|
Haynes 188
|
22.0
|
22
|
37.0
|
-
|
14.5
|
-
|
-
|
-
|
<3
|
0.10
|
0.90 La
|
MP35-N
|
20.0
|
35
|
35.0
|
10
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
Table 3 Nominal Compositions of Select Cast Superalloys
C
|
Ni
|
Cr
|
Co
|
Mo
|
Fe
|
Al
|
B
|
Ti
|
Ta
|
W
|
Zr
|
other
| |
Nickel Based
| |||||||||||||
CMSX-2
|
–
|
66.2
|
8
|
4.6
|
0.6
|
-
|
5.6
|
-
|
1
|
6
|
8
|
6
|
-
|
Inconel 713C
|
0.12
|
74
|
12.5
|
-
|
4.2
|
-
|
6
|
0.012
|
0.8
|
1.75
|
-
|
0.1
|
0.9Nb
|
Inconel 738
|
0.17
|
61.5
|
16
|
8.5
|
1.75
|
-
|
3.4
|
0.01
|
3.4
|
-
|
2.6
|
0.1
|
2 Nb
|
MAR-M-24
|
0.15
|
59
|
8.25
|
10
|
0.7
|
0.5
|
5.5
|
0.015
|
1
|
3
|
10
|
0.05
|
1.5Hf
|
PWA 148
|
–
|
Bal
|
10
|
5
|
-
|
-
|
5
|
1.5
|
12
|
4
|
-
|
-
| |
René 41
|
0.09
|
55
|
19
|
11
|
10
|
-
|
1.5
|
0.01
|
3.1
|
-
|
-
|
-
|
-
|
René 80
|
0.17
|
60
|
14
|
9.5
|
4
|
-
|
3
|
0.015
|
5
|
-
|
4
|
0.03
|
-
|
René 80Hf
|
0.08
|
60
|
14
|
9.5
|
4
|
-
|
3
|
0.015
|
4.8
|
-
|
4
|
0.02
|
0.75Hf
|
René N
|
0.06
|
62
|
9.8
|
7.5
|
1.5
|
-
|
4.2
|
0.004
|
3.5
|
4.8
|
6
|
-
|
0.5Nb,
0.15Hf
|
Udimet 700
|
0.1
|
53.5
|
15
|
18.5
|
5.25
|
-
|
4.25
|
0.03
|
3.5
|
-
|
-
|
-
|
-
|
Waspaloy
|
0.07
|
57.5
|
19.5
|
13.5
|
4.2
|
1
|
1.2
|
0.005
|
3
|
-
|
-
|
0.09
|
-
|
Iron–Nickel Based
| |||||||||||||
Inconel 718
|
0.04
|
53
|
19
|
-
|
3
|
18
|
0.5
|
-
|
0.9
|
-
|
-
|
-
|
0.1Cu, 5Nb
|
Cobalt Based
| |||||||||||||
AirResist 215
|
0.35
|
0.5
|
19
|
63
|
-
|
0.5
|
4.3
|
-
|
-
|
7.5
|
4.5
|
0.1
|
0.1Y
|
FSX-414
|
0.25
|
10
|
29
|
52.5
|
-
|
1
|
-
|
0.01
|
-
|
-
|
7.5
|
-
|
-
|
Haynes 2
|
0.1
|
10
|
20
|
54
|
-
|
1
|
-
|
-
|
-
|
-
|
15
|
-
|
-
|
MAR-M 91
|
0.05
|
20
|
20
|
52
|
-
|
-
|
-
|
-
|
0.2
|
7.5
|
-
|
0.5
|
-
|
X-40
|
0.50
|
10
|
22
|
57.5
|
-
|
1.5
|
-
|
-
|
-
|
-
|
7.5
|
-
|
0.5Mn, 0.5 Si
|
Economics of superalloys
The cost of raw materials used in producing superalloys has become an increasingly important issue. The precious metal rhenium confers enhanced performance in Cannon-Muskegon's second and third generation single crystal alloys like CMSX-4 and CMSX-10 used extensively in gas turbine engines. Meanwhile, the effects of ruthenium additions are being studied across the world and new alloys containing ruthenium for increased temperature capability are under testing and evaluation. Unfortunately, the costs of these precious metal additions are staggering, representing increasing fractions of the total raw materials costs.
Rhenium (Re) has a density of 21.04 grams per cubic centimeter (surpassed only by platinum, iridium, and osmium) and a melting temperature of 3,180°C (surpassed only by tungsten and carbon). Its atomic number is 75 and its atomic weight is 186.207. It is extremely rare, present in the earth's crust at only 1 part per billion.1 It is basically a byproduct of a byproduct, being extracted from flue dusts from molybdenum sulphide concentrates, which are derived from purifying copper concentrates.
Superalloys that contain rhenium include CMSX-4, CMSX-10, CM186LC, CMSX486, PW1484, Rene N5, Rene N6, TMS-75, TMS-138, and TMS-162. Levels are typically in the range of 3% to 6%. According to Bhadeshia, rhenium is an expensive addition, but it provides enhanced creep resistance by promoting rafting, making lattice misfit more negative, and by reducing the overall diffusion rate in the nickel-base superalloys.
Rhenium (Re) has a density of 21.04 grams per cubic centimeter (surpassed only by platinum, iridium, and osmium) and a melting temperature of 3,180°C (surpassed only by tungsten and carbon). Its atomic number is 75 and its atomic weight is 186.207. It is extremely rare, present in the earth's crust at only 1 part per billion.1 It is basically a byproduct of a byproduct, being extracted from flue dusts from molybdenum sulphide concentrates, which are derived from purifying copper concentrates.
Superalloys that contain rhenium include CMSX-4, CMSX-10, CM186LC, CMSX486, PW1484, Rene N5, Rene N6, TMS-75, TMS-138, and TMS-162. Levels are typically in the range of 3% to 6%. According to Bhadeshia, rhenium is an expensive addition, but it provides enhanced creep resistance by promoting rafting, making lattice misfit more negative, and by reducing the overall diffusion rate in the nickel-base superalloys.
Ruthenium (Ru) has a melting temperature of 2,334°C, an atomic number of 44, and an atomic weight of 101.07. Ruthenium is ten times more rare than platinum and is difficult to refine and extract. As of January, 2007, rhenium occupied the eighth position (of precious metals) at a value of $5,500 per kg. According to the Metal-Pages website, the price per kg of rhenium (in the form of ammonium perrhenate) has increased from about $5,500 in January 2007 to more than $7,000 in May 2007!. Ruthenium occupied the eighth position in 1999 with a value of $1,225 per kg and has risen to the third position with its value of $21,540 per kg.
Nickel based superalloys
Nickel-Based superalloys are an unusual class of metallic materials with an exceptional combination of high temperature strength, toughness, and resistance to degradation in corrosive or oxidizing environments. These materials are widely used in aircraft and power-generation turbines, rocket engines, and other challenging environments, including nuclear power and chemical processing plants.
Intensive alloy and process development activities during the past few decades have resulted in alloys that can tolerate average temperatures of 1050°C with occasional excursions (or local hot spots near airfoil tips) to temperatures as high as 1200°C, which is approximately 90% of the melting point of the material. The underlying aspects of microstructure and composition that result in these exceptional properties are briefly reviewed here. Major classes of superalloys that are utilized in gas-turbine engines and the corresponding processes for their production are outlined along with their characteristic mechanical and physical properties.
As mentioned above, one of the main applications for nickel-based superalloys is gas-turbine-engine disc components for land-based power generation and aircraft propulsion. Turbine engines create harsh environments for materials due to the high operating temperatures and stress levels. Hence, as described in this article, many alloys used in the high-temperature turbine sections of these engines are very complex and highly optimized.
Gas turbines are complex machines, being employed in both aircraft engines or land-based power-generation applications. Small, intermediate, and large gas turbines are being developed rapidly for mobile land-based power units and large commercial aircraft applications.
The various parts within this type of power-generation system have specific and unique requirements. For example, the material used for the high-pressure turbine area of an engine reach the highest temperatures and is therefore one of the highest stressed parts of the engine, requiring very specialized nickel-based superalloy materials. The operating temperatures for the rim sections (near the gas flow path) of high-pressure turbine discs have continued to challenge materials and design engineers as temperatures now approach 760°C and even as high as 815°C for some specialized military applications. Turbine blades are attached to a disc which in turn is connected to the turbine shaft. The properties required for an aeroengine disc (Figure 4) are different from that of a turbine, because the metal is subjected to a lower temperature. The discs must resist fracture by fatigue. Discs are usually cast and then forged into shape and are typically polycrystalline.
Figure 4: Powder metallurgical aeroengine disc
One difficulty is that cast alloys have a large columnar grain structure and contain significant chemical segregation; the latter is not completely eliminated in the final product. This can lead to scatter in mechanical properties. One way to overcome this is to begin with fine, clean powder which is then consolidated.
The powder is made by atomization in an inert gas; the extent of chemical segregation cannot exceed the size of the powder. After atomization, Some discs are made from powder which is hot-isostatically pressed, extruded and then forged into the required shape.
The process is difficult because of the need to avoid undesired particles being introduced, for example, from the refractories used in the atomisation process, or impurities picked up during solidification. Such particles initiate fatigue and of courser, the failure of an aeroengine turbine disc can be catastrophic. In addition to the high temperature concerns, materials for modern turbine applications are driven by ever-growing commercial pressures. These pressures can be seen as demands rise for lower component costs, life-cycle costs, and maintenance costs. For lower acquisition costs, avenues such as alloys with reduced cobalt and alloys that result in higher processing yields are being pursued.
For lower life-cycle costs, alloys are being designed with longer service lives. Alloys with good stability and very low crack-growth rates that are easily inspected and monitored by nondestructive means are desired. Fuel efficiency and emissions are also key commercial and environmental drivers impacting turbine-engine materials. To meet these demands, modern nickel-based alloys offer an efficient compromise between performance and economics.
The essential solutes in nickel based superalloys are aluminium and/or titanium, with a total concentration which is typically less than 10 atomic percent. This generates a two-phase equilibrium microstructure, consisting of gamma (γ) and gamma-prime (γ'). It is the γ' which is largely responsible for the elevated-temperature strength of the material and its incredible resistance to creep deformation. The amount of γ' depends on the chemical composition and temperature, as illustrated in the ternary phase diagrams below.
The Ni-Al-Ti ternary phase diagrams show the γ and γ' phase field. For a given chemical composition, the fraction of γ' decreases as the temperature increases. This phenomenon is used in order to dissolve the γ' at a sufficiently high temperature (a solution treatment) followed by ageing at a lower temperature in order to generate a uniform and fine dispersion of strengthening precipitates.
Figure 5: The Ni-Al-Ti ternary phase diagrams show the γ and γ' phase field.
The γ-phase (Fig.6a) is a solid solution with a cubic-F lattice and a random distribution of the different species of atoms. Cubic-F is short for face-centered cubic. By contrast, γ' has a cubic-P (primitive cubic) lattice (Fig. 6b) in which the nickel atoms are at the face-centers and the aluminum or titanium atoms at the cube corners. This atomic arrangement has the chemical formula Ni3Al, Ni3Ti or Ni3 (Al, Ti). However, as can be seen from the (γ+γ')/γ' phase boundary on the ternary sections of the Ni, Al, Ti phase diagram, the phase is not strictly stoichiometric. There may exist an excess of vacancies on one of the sublattices which leads to deviations from stoichiometry; alternatively, some of the nickel atoms might occupy the Al sites and vice-versa. In addition to aluminum and titanium, niobium, hafnium and tantalum partition preferentially into γ'.
The γ phase forms the matrix in which the γ' precipitates. Since both the phases have a cubic lattice with similar lattice parameters, the γ' precipitates in a cube-cube orientation relationship with the γ (Fig. 6a and 6b). This means that its cell edges are exactly parallel to corresponding edges of the γ phase. Furthermore, because their lattice parameters are similar, the γ' is coherent with the γ when the precipitate size is small. Dislocations in the γ nevertheless find it difficult to penetrate γ', partly because the γ' is an atomically ordered phase. The order interferes with dislocation motion and hence strengthens the alloy.
Figure 6a: Crystal structure of γ
|
Figure 6b: Crystal structure of γ'.
|
The small misfit between the γ and γ' lattices is important for two reasons. Firstly, when combined with the cube-cube orientation relationship, it ensures a low γ/γ' interfacial energy. The ordinary mechanism of precipitate coarsening is driven entirely by the minimization of total interfacial energy. A coherent or semi-coherent interface therefore makes the microstructure stable, a property which is useful for elevated temperature applications.
The magnitude and sign of the misfit also influences the development of microstructure under the influence of a stress at elevated temperatures. The misfit is said to be positive when the γ' has a larger lattice parameter than γ. The misfit can be controlled by altering the chemical composition, particularly the aluminum to titanium ratio. A negative misfit stimulates the formation of rafts of γ', essentially layers of the phase in a direction normal to the applied stress. This can help reduce the creep rate if the mechanism involves the climb of dislocations across the precipitate rafts.