Part II Engineering Materials
6.4 Superalloys
Superalloys constitute a category that straddles the ferrous and nonferrous metals.
Some of them are based on iron, whereas others are based on nickel and cobalt. In fact, many of the superalloys contain substantial amounts of three or more met- als, rather than consisting of one base metal plus alloying elements. Although the tonnage of these metals is not signifi cant compared with most of the other metals discussed in this chapter, they are nevertheless commercially important because they are very expensive; and they are technologically important because of what they can do.
The superalloys are a group of high-performance alloys designed to meet very demanding requirements for strength and resistance to surface degradation (corro- sion and oxidation) at high service temperatures. Conventional room temperature strength is usually not the important criterion for these metals, and most of them possess room temperature strength properties that are good but not outstanding.
Their high temperature performance is what distinguishes them; tensile strength, hot hardness, creep resistance, and corrosion resistance at very elevated tempera- tures are the mechanical properties of interest. Operating temperatures are often in the vicinity of 1100°C (2000°F). These metals are widely used in gas turbines—
jet and rocket engines, steam turbines, and nuclear power plants—systems in which operating effi ciency increases with higher temperatures.
The superalloys are usually divided into three groups, according to their principal constituent: iron, nickel, or cobalt:
➢ Iron-based alloys have iron as the main ingredient, although in some cases the iron is less than 50% of the total composition.
➢ Nickel-based alloys generally have better high temperature strength than alloy steels. Nickel is the base metal. The principal alloying elements are chromium and cobalt; lesser elements include aluminum, titanium, molybdenum, niobium (Nb), and iron. Some familiar names in this group include Inconel, Hastelloy, and Rene 41.
➢ Cobalt-based alloys consist of cobalt (around 40%) and chromium (perhaps 20%) as their main components. Other alloying elements include nickel, molyb- denum, and tungsten.
In virtually all of the superalloys, including those based on iron, strengthening is accomplished by precipitation hardening. The iron-based superalloys do not use martensite formation for strengthening. Typical compositions and strength prop- erties at room temperature and elevated temperature for some of the alloys are presented in Table 6.15.
6.4
TABLE • 6.15 Some typical superalloy compositions together with strength properties at room temperature and elevated temperature.
Chemical Analysis, %a
Tensile Strength at Room Temperature
Tensile Strength at 870°C (1600°F)
Superalloy Fe Ni Co Cr Mo W Otherb MPa lb/in2 MPa lb/in2
Iron-based
Incoloy 802 46 32 21 ⬍2 690 100,000 195 28,000
Haynes 556 29 20 20 22 3 6 815 118,000 330 48,000
Nickel-based
Incoloy 807 25 40 8 21 5 1 655 95,000 220 32,000
Inconel 718 18 53 19 3 6 1435 208,000 340 49,000
Rene 41 55 11 19 1 5 1420 206,000 620 90,000
Hastelloy S 1 67 16 15 1 845 130,000 340 50,000
Nimonic 75 3 76 20 ⬍2 745 108,000 150 22,000
Cobalt-based
Stellite 6B 3 3 53 30 2 5 4 1010 146,000 385 56,000
Haynes 188 3 22 39 22 14 960 139,000 420 61,000
L-605 10 53 20 15 2 1005 146,000 325 47,000
Compiled from [11] and [12].
aCompositions to nearest percent.
bOther elements include carbon, niobium, titanium, tungsten, manganese, and silicon.
References
[1] Bauccio. M. (ed.). ASM Metals Reference Book, 3rd ed. ASM International, Materials Park, Ohio, 1993.
[2] Black, J, and Kohser, R. DeGarmo’s Materials and Processes in Manufacturing, 11th ed., John Wiley & Sons, Hoboken, New Jersey, 2012.
[3] Brick, R. M., Pense, A. W., and Gordon, R. B.
Structure and Properties of Engineering Ma- terials, 4th ed. McGraw-Hill, New York, 1977.
[4] Carnes, R., and Maddock, G., “Tool Steel Selec- tion,” Advanced Materials & Processes, June 2004, pp. 37–40.
[5] Encyclopaedia Britannica, Vol. 21, Macro- paedia. Encyclopaedia Britannica, Chicago, 1990, under section: Industries, Extraction and Processing.
[6] Flinn, R. A., and Trojan, P. K. Engineering Ma- terials and Their Applications, 5th ed. John Wiley & Sons, New York, 1995.
[7] Guy, A. G., and Hren, J. J. Elements of Physical Metallurgy, 3rd ed. Addison-Wesley, Reading, Massachusetts, 1974.
[8] Hume-Rothery, W., Smallman, R. E., and Haworth, C. W. The Structure of Metals and Alloys. Institute of Materials, London, 1988.
[9] Keefe, J., A Brief Introduction to Precious Metals,” The AMMTIAC Quarterly, Vol. 2, No. 1, 2007.
[10] Lankford, W. T., Jr., Samways, N. L., Craven, R.
F., and McGannon, H. E. The Making, Shap- ing, and Treating of Steel, 10th ed. United States Steel Co., Pittsburgh, 1985.
[11] Metals Handbook, Vol. 1, Properties and Selection: Iron, Steels, and High Performance Alloys. ASM International, Metals Park, Ohio, 1990.
[12] Metals Handbook, Vol. 2, Properties and Selec- tion: Nonferrous Alloys and Special Purpose
Problems 157
Materials, ASM International, Metals Park, Ohio, 1990.
[13] Moore, C., and Marshall, R. I. Steelmaking. The Institute for Metals, The Bourne Press, Ltd., Bournemouth, U.K., 1991.
[14] Wick, C., and Veilleux, R. F. (eds.). Tool and Manufacturing Engineers Handbook, 4, Vol. 3, Materials, Finishing, and Coating.
Society of Manufacturing Engineers, Dear- born, Michigan, 1985.
Review Questions
6.1 What are some of the general properties that distinguish metals from ceramics and polymers?
6.2 What are the two major groups of metals?
Defi ne them.
6.3 What is an alloy?
6.4 What is a solid solution in the context of alloys?
6.5 Distinguish between a substitutional solid solution and an interstitial solid solution.
6.6 What is an intermediate phase in the context of alloys?
6.7 The copper-nickel system is a simple alloy sys- tem, as indicated by its phase diagram. Why is it so simple?
6.8 What is the range of carbon percentages that defi nes an iron-carbon alloy as a steel?
6.9 What is the range of carbon percentages that defi nes an iron-carbon alloy as cast iron?
6.10 Identify some of the common alloying ele- ments other than carbon in low alloy steels.
6.11 What are some of the mechanisms by which the alloying elements other than carbon strength- en steel?
6.12 What is the predominant alloying element in all of the stainless steels?
6.13 Why is austenitic stainless steel called by that name?
6.14 Besides high carbon content, what other alloy- ing element is characteristic of the cast irons?
6.15 Identify some of the properties for which aluminum is noted?
6.16 What are some of the noteworthy properties of magnesium?
6.17 What is the most important engineering prop- erty of copper that determines most of its applications?
6.18 What elements are traditionally alloyed with copper to form (a) bronze and (b) brass?
6.19 What are some of the important applications of nickel?
6.20 What are the noteworthy properties of titanium?
6.21 Identify some of the important applications of zinc.
6.22 What important alloy is formed from lead and tin?
6.23 (a) Name the important refractory metals.
(b) What does the term refractory mean?
6.24 (a) Name the four principal noble metals.
(b) Why are they called noble metals?
6.25 The superalloys divide into three basic groups, according to the base metal used in the alloy.
Name the three groups.
6.26 What is so special about the superalloys? What distinguishes them from other alloys?
6.27 What are the three basic methods by which metals can be strengthened?
Problems
6.1 (A) For the copper-nickel phase diagram in Figure 6.2, fi nd the compositions of the liquid and solid phases for a nominal composition of 60% Ni and 40% Cu at 1316°C (2400°F).
6.2 For the preceding problem, use the inverse lever rule to determine the proportions of liquid and solid phases present in the alloy.
Answers to Problems labeled (A) are listed in the Appendix at the back of the book.
6.3 Using the lead-tin phase diagram in Figure 6.3, determine the liquid and solid phase composi- tions for a nominal composition of 30% Sn and 70% Pb at 250°C (482°F).
6.4 For the preceding problem, use the inverse lever rule to determine the proportions of liquid and solid phases present in the alloy.
6.5 (A) Using the lead-tin phase diagram in Figure 6.3, determine the liquid and solid phase compositions for a nominal composition of 90% Sn and 10% Pb at 204°C (400°F).
6.6 For the preceding problem, use the inverse lever rule to determine the proportions of liquid and solid phases present in the alloy.
6.7 In the iron-iron carbide phase diagram of Figure 6.4, identify the phase or phases present at the following temperatures and nominal compositions: (a) 650°C (1200°F) and 2%
Fe3C, (b) 760°C (1400°F) and 2% Fe3C, and (c) 1095°C (2000°F) and 1% Fe3C.
Ceramics
Chapter Contents
7.1 Structure and Properties of Ceramics