Recommended Reading

[1] Metallic Materials Properties Development and Standardization, U.S. Department of Transportation, Federal Aviation Agency, D0T/FAA/AR-MMPDS-01, January 2004.

[2] ASM Handbook Vol. 1: Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, 1990.

[3] ASM Handbook Vol. 4: Heat Treating, ASM International, 1991.

[4] ASM Handbook Vol. 6: Welding, Brazing, and Soldering, ASM International, 1993.

[5] ASM Handbook Vol. 14: Forming and Forging, ASM International, 1988.

[6] ASM Handbook Vol. 16: Machining, ASM International, 1990.

References

Davis, D.P., "Structural Steels", in High Performance Materials in Aerospace, Chapman & Hall, 1995, pp. 151-181.

Smith, W.F., "Alloy Steels", in Structure and Properties of Engineering Alloys, McGraw-Hill, Inc., 2nd edition, 1993, pp. 125-173.

Smith, W.F., "Stainless Steels", in Structure and Properties of Engineering Alloys, McGraw-Hill, Inc., 2nd edition, 1993, pp. 288-332.

Morlett, J.O., Johnson, H., Troiano, A., Journal of Iron and Steel Institute, Vol. 189, 1958, p. 37.

Parker, E.R., Metallurgical Transactions, 8A, 1977, p. 1025. Imrie, W.M., Royal Society London Philosophical Transactions, A282, 1976, p. 91. "Forging of Carbon and Alloy Steels", in ASM Handbook Vol. 14: Forming and Forging, ASM International, 1988.

Sprague, L.E., "The Effects of Vacuum Melting on the Fabrication and Mechanical Properties of Forging", Steel Improvement and Forge Company, 1960.

Philip, T.V., McCaffrey, T.J., "Ultrahigh Strength Steels", in ASM Handbook Vol. 1: Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, 1990. Davis, J.R., "Carbon and Alloy Steels", in Alloying: Understanding The Basics, ASM International, 2001, p. 190.

Somers, B.R., "Introduction to the Selection of Carbon and Low-Alloy Steels", in ASM Handbook Vol. 6: Welding, Brazing, and Soldering, ASM International, 1993. Totten, G.E., Narazaki, M., Blackwood, R.R., Jarvis, L.M., "Factors Relating to Heat Treating Operations", in ASM Handbook Vol. 11: Failure Analysis and Prevention, ASM International, 2002.

Becherer, B.A., Withford, T.J., "Heat Treating of Ultrahigh-Strength Steels", in ASM Handbook Vol. 4: Heat Treating, ASM International, 1991.

Callister, W.D., "Phase Transformations", in Fundamentals of Materials Science and Engineering, John Wiley & Sons, Inc., 5th edition, 2001, p. 345.

Guy, A.G., "Heat Treatment of Steel", in Elements of Physical Metallurgy, Addison-Wesley, 2nd edition, 1959, pp. 465-494.

Philip, T.V., McCafferty, T.J., "High Fracture Toughness Steels", in ASM Handbook Vol. 1: Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, 1990.

"AerMet 100 Alloy" datasheet, Carpenter Technology Corporation, 1995.

Dahl, J.M., "Ferrous-Base Aerospace Alloys", Advanced Materials & Processes, May 2000, pp. 33-36.

Rohrbach, K., Schmidt, M., "Maraging Steels", in ASM Handbook Vol. 1: Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, 1990. Schmidt, M., Rohrbach, K., "Heat Treatment of Maraging Steels", in ASM Handbook Vol. 4: Heat Treating, ASM International, 1991.

Harris, T., Priebe, E., "Forging of Stainless Steels", in ASM Handbook Vol. 14: Forming and Forging, ASM International, 1988.

[22] Washko, S.D., Aggen, G., "Wrought Stainless Steels: Fabrication Characteristics", in ASM Handbook Vol. 1: Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, 1990.

[23] Kosa, T., Ney, R.P., "Machining of Stainless Steels", in ASM Handbook Vol. 16: Machining, ASM International, 1990.

[24] Pollard, B., "Selection of Wrought Precipitation-Hardening Stainless Steels", in ASM Handbook Vol. 6: Welding, Brazing, and Soldering, ASM International, 1993.

This Page is Intentionally Left Blank

Chapter 6

Superalloys

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).1 Their combination of elevated temperature strength and resistance to surface degradation is unmatched by other metallic materials.

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. Typical applications are shown in Fig. 6.1. 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 petrochemical plants, pollution control equipment, coal gasification and liquification systems, and medical applications.2

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.

Fan Ti Alloy

Low Pressure Compressor Ti or Ni Alloy ts

High Pressure

Turbine

Ni Alloy Combustion '

Low Pressure Turbine Ni Alloy

Fan Ti Alloy

Low Pressure Compressor Ti or Ni Alloy

High Pressure

Turbine

Ni Alloy Combustion '

Low Pressure Turbine Ni Alloy

Inconel 718 Turbine

Turbine Blades Ni Alloy

Turbine Exhaust

Case Ni Alloy

Inlet Case Al Alloy

Accessory Section Al or Fe Alloy

Fig. 6.1. Typical Material Distribution in Jet Engine

Turbine Blades Ni Alloy

Turbine Exhaust

Case Ni Alloy

Inlet Case Al Alloy

Accessory Section Al or Fe Alloy

Fig. 6.1. Typical Material Distribution in Jet Engine

Matrix Club Reading

1000 1200 1400 1600 1800 2000 2200 1000 Temperature (° F)

Fig. 6.2. Stress Rupture Comparison of Wrought Superalloys3

Solid Solution Strengthened Nickel and Iron-Nickel Alloys

1000 1200 1400 1600 1800 2000 2200 1000 Temperature (° F)

Fig. 6.2. Stress Rupture Comparison of Wrought Superalloys3

A relative comparison of their stress rupture properties is shown in Fig. 6.2. 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.

6.1 Metallurgical Considerations4-6

Nickel has an FCC crystalline structure, a density of 0.322 lb/in.3, 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 y 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.1 The FCC nickel matrix, which is designated as austen-ite (y), contains a large percentage of solid solution elements 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 0.6 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 y' FCC ordered Ni3(Al,Ti) in the form of either Ni3Al or Ni3Ti. The y' phase is precipitated by precipitation hardening heat treatments: solution heat treating followed by aging. The y' precipitate is an A3B type compound where A is composed of the relatively electronegative elements nickel, cobalt and iron, and B of the electropositive elements aluminum, titanium or niobium. Typically, in the nickel based alloys, y' 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 y' has only about an 0.1% mismatch with the y matrix; therefore, y' precipitates homogeneously with a low surface energy and has extraordinary long-term stability. The coherency between y' and y 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 y' actually exhibit an increase in strength as the temperature is increased up to about 1300° F. The y/y' mismatch determines the y' precipitate morphology, with small mismatches (~ 0.05%) producing spherical precipitates and larger mismatches producing cubical precipitates as shown in Fig. 6.3.

Precipitation Hardening Microstructure
Fig. 6.3. Microstructure of Precipitation-Strengthened Nickel Based Superalloy

If appreciable niobium is present, the body centered tetragonal ordered y'' (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 ^ (Ni3Ti) and the orthorhombic 8 (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 6.1. 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 y' 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 (y') and niobium (y''). 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 favourable characteristics, can also form the undesirable a, p 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 to 0.2% 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 y' within the grains. Grain boundary strengthening is

Table 6.1 Role of Alloying Elements in Superalloys7

Alloy

Solid Solution

Í

Carbide

Grain Boundary

Oxide Scale

Additions

Strengtheners

Formers

Formers

Strengtheners

Formers

Chromium

Was this article helpful?

0 0

Post a comment