Rep~t4trEe New England Association of Chem
Murray Kaufmon General Electric Company West Lynn, Massachusetts
High Temperature Alloys for
Small Gas Turbines
T h e efficiency, fuel consumption, and power-to-weight ratio of a gas turbine are improved by increasing the operating temperature. Furthermore, a small gas turbine offers a theoretical power-toweight advantage over a larger one according to the ' l S / 2 law" (power is proportional to the square of the diameter, weight is proportional to the cube). These factors combine to make a continual demand for materials ~vhichcan withstand the higher temperatures, and also can meet other exacting requirements of small parts. For example: minimum thicknesses of small parts are often limited by handling and fabrication procedures rather than by strength; dimensional tolerances are smaller and more difficult to maintain; oxidation and surface corrosion affect a larger percentage of the cross-section; and dimensional stability, especially where warping can occur, ishighly important. It therefore becomes most important to learn the structure and behavior of alloys used in small gas turbines.
thermal cycling, erosion, and other forces which are difficult t,o predict or calculatte--and frequently they are the most important factors in determining the effective life of a part. An example of a high-temperat,ure component is the combustion liner shown in the photograph. Usually these combustion liners, the afterburner liners, and the turbine diaphragm bands or shrouds are fabricated from sheet material; the turbine nozzles may be fabricated from sheet or may be cast; and the turhine buckets are cast or forged. Since the mechanicnl properties and the cost of fabrication vary with the manufacturing process, it is important to know about the euitahility of an alloy for each typc of processing and t,n know it,s propt.rtirs.
Components Requiring High-Temperature Alloys
If a temperature of 1500°F (815'C) is chosen as the dividing point between "medium" and "high" temperatures, there are four major locations which demand attention in a discussion of high-temperature alloys: the combustion area, the initial turbine stages, the afterburner (if present), and the exhaust nozzle. In Table 1 are given the approximate temperatures and Table 1. Approximate Temperatures and Stresses Encountered in Typical High-Temperature Components Maximum
Stress Combustion liner 11-85 engine1
Turbine buckets Afterburner liner and
High-Temperature Alloys 1!100"F (10
stresees to be expected in modern engines. Naturally, t,he exact temperatures and stresses encountered in a specific part will depend on many fact~ora,esperially the mechanical design and the amount of cooling used. In addition to the st,at.ic or dynamic stresses, the romponents are also subjected to possible hot spots, Presented at the NEACT Meeting, May, 1961, Worcester, Mass., and in part to the ASME Aviation Conference, IMareh, 11161.
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The alloys used in high-temperature applications are usually classified arcording to their chemical composition: high iron alloys with appreciable nickel and chromium. nirkel-base alloys, and cobalt-base alloys. The word "base" indicates that the named element is present in greater quantity than any other single constituent, but not necessarily more than 50%. Table 2 shows the nominal compositions of some of the commonly used alloys in each classification. A more complete listing (with average properties) was compiled by Simmons and Krivohok ( 1 ) .
Aside from having the ability to be fabricated into the desired part configuration, the two most important factors influencing the selection of an alloy for high temperature application are its oxidation resistance and it.s mechanical properties. Oxidation resistance depends on the formation of an adherent surface oxide which restricts further attack. Chromium, nickel, and aluminum aid in the production of such a film, and all the alloys listed in Table 2 contain sufficient amounts to provide good resistance at least up to 1800°F, and some (notably Inco 702 and 713) over 2000°F. The mechanical properties are largely determined by the matrix material (base metal with other elements in solution), by the grain size and orientation, and by the presence of non-dissolved particles (which by their size, composition, and distribution further affect the properties). All of the alloys rely on strengthening of the matrix by solution formation. A coarse grain size aids high-temperature creep and rupture properties, while fine grains increase lower t,emperature tensile and fatigue properties generally. The third factor, the presence of non-dissolved particles. has the greatest effect on properties, and will be discussed in more detail. These non-dissolved particles, or "secondary" phases, actually have a greater volume than the matrix itself in some of the highly precipitation-hardened alloys. They will be referred to simply as "phases." The phases found in the high-temperature alloys may be divided into three groups: carbides-nitridesborides, strong precipitation hardening intermetallics, and complex intermetallics. The phases in each group and their behavior are given below.
leading to fabrication or ductility problems. When Cb or Ta are present, they tend to displace T i from the carbo-nitride. In vacnum-melted alloys (as the majority of nickel-base types are), very little nitrogen is available, and nearly pure carbides are formed. C r G , Md&, MsC, etc. (where M i.s mainly Cr, Mo, W or combinations). Many of the more complex metal carbides, varying widely, are formed in these alloys. Their solution temperatures vary from 1500°F t o over 2100°F, depending on the composition. The greater the amount of Mo (or W) in the carbide, the higher is the solution temperature. Some of the carbides, like M&, tend to precipitate in the grain boundaries. While this may aid creep strength somewhat, it also may lead to emhrittlement if too much of the carbide is present. Other carbides, like M6C, are formed in a coarser more distributed manner and do not affect ductility. I n Ni-base alloys, over 6% Mo favors the formation of MsC over M& (3). Both of these form at the expense of the T i c type of carbide (3). MsRz (where M is M o or UTand N i , Co, etc.), and other borziles. The borides generally form in grain boundaries. They are considered beneficial because they apparently prevent embrittling carbide formation (4). However, they have low melting points, and lead to a weakness at higher temperatures (over 2000°F), which is nndesirable with respect t o both properties and hot workability. A proper balance of boride content must be obtained. Precipitation-Hardening Phases
N i J T i . This is an effective precipitation-strengthening phase for temperatures up to 1400°F. Solution formation takes lace from near 1600°F t o 2000°F. depending on the relative nickel-iron-cobalt content of the matrix. At temperatures of 1400°F and over, this phase tends to agglomerate in an acicnlar fashion andprovidesverylittlesubsequent strengthening. N i 3 (Al, T i ) . The Ni,AI intermetallic phase can dissolve large amounts of other metallic elements, notably Ti as a replarement for Al. The resulting
Corbides-Nitrides-Borides
MC, M N , or M ( C , N ) (where M is T i , Cb, Ta, or combinations of the three). These carho-nitrides are high solution temperature phases (over 2250°F), and are usually formed during solidification. They are relatively coarse particles, usually dispersed: they provide some strengthening. In wrought alloys, clust,ers or stringers of Ti(C,N) are occasionally found,
Toble 2.
Group
Chemical Composition by Weight of Typical High-Temperature Alloys
Alloy
High iron with nickel and Incoloy T chromium N-155 Nickel base
Fe
Ni
Cr
Co
46.15 32.0 20.5 32.5 20.0 21.0 2 0 0
Mo
W
Al
...... 310
2.5
...
Ti
Ch
1.0
...
i.0
C
Other
Forms
0.07 W 0.15 N '0.15 w Mn 1 . 5
Hast. X Incanel Ineonel X Inco 702 Waspalloy Rene' 41 GMR 235 Hast. R235 u 500 U 700 Nicrotung Inco 713 SEL
Cobalt base
-
b
W = wrought; C = cast. Mast nickel-base alloys contain same B and Zr,usually totaling 0.005% nor more. Volume 39, Number 3, M o r h 1962
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phase has ideal characteristics for precipitation hardening. The solution temperatures range from 1925"F, to 2050°F, with the higher Al content leading to the higher solution temperature. Precipitation is in the form of fine, dispersed, spherical or cubical particles. At higher precipitation temperatures, the stable particle size becomes larger, but still is not coarse enough to eliminate the strengthening effect. The particle size change is nearly completely reversible, so that at a given temperature the Ni,(Al, Ti) shows its characteristic stable size regardless of the previous thermal cycling performed. This means that ooer-heating does not necessarily cause permanent damage due to ( z averaging." In nickel-base alloys the precipitation rate is usually very rapid although with higher Ti content the rate is somewhat slowed. Higher Ti content also increases strength at lower temperatures. If the Al content is too high relative to the available Ni, N i (a phase richer in Al) will form. This phase is deleterious to high-temperature properties and is avoided in all the commercial alloys listed. Complex Intermetallics
The complex intermetallics include Laves phase [which is M1 (Ti, Mo, W), where M may be Fe, Cr, Co, etc.], Sigma phase (complex Fe-Cr-Mo compounds), Mu phase (Co7WBor Co7Moa),and some other less common phases. With the possible exception of the Sigma phase, none of these provide much strengthening. They usually coarsen at,relatively low temperatures and are harmless unless they form a continuous grain boundary network which lowers ductility. High-Iron, Nickel-Chromium Alloys
The alloys included in this group contain more than 25% iron with sufhient chromium (over 12%) and nickel (over 20%) to provide oxidation resistance. Cobalt, tungsten, molybdenum, and columbium may be present to aid high-temperature strength by solution-hardening and carbide formation. Only small amounts, if any, of the precipitation-hardening elements are included. Alloys such as A286 or Inco 901, which contain appreciable amounts of those elements contributing to precipitation-hardening (more than 2y0 titanium with aluminum and columbium), are usually not used a t high temperatures. Even though their low temperature properties are considerably better, the hardening phases [NisTi and Ni,(Al, Ti)] are usually dissolved by 1600°F, and offer very little help toward high-temperature strength. Furthermore, the melting, processing, and fabrication of these alloys are more difficult and costly; such factors restrict their use to the region where they have markedly superior properties. A phase analysis showing the behavior of alloy N-155, among others, can be found in reference (t). The high-iron, nickel-chromium alloys have low strength properties similar to the non-precipitationhardened nickel-base alloys. However, they do retain some strength to the highest temperatures. Their cost is relatively low and their fabricability into complex shapes is good. Joining by common welding and brazing processes is not difficult. In high-temperature components, these alloys find their widest use as combustion liners, turbine shrouds or bands, and after160
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burner parts--applications where the stress requirements are not high. The combination of thermal fatigue and shock properties with mechanical and physical properties is satisfactory, for these alloys perform better than some of the stronger nickel and cobalt-base alloys. Nickel-Base Alloys
Nickel-base alloys generally contain more than 40% nickel. Chromium is always present, 10-2001,, to aid in oxidation resistance; the amount required is less than in high-iron alloys. As in the high-iron alloys, molybdenum, tungsten, cobalt, or columbium are frequently used for solution-strengthening. Except for a few alloys (notably Inconel and some of the Hastelloys), most nickel-base alloys depend upon aluminum and titanium for precipitation-hardening phases. Along with the other elements listed in Table 2, most of the nickel-base alloys contain small but significant amounts of boron. All the metallurgical phases described previously may be found in nickel-base alloys. The behavior of some of the important ones have been described (34), and complete studies on some alloys are available, such as Inconel X (7,s) and Udimet 500 (7-1 0). Mechanical properties of the nickel-base alloys vary from low values similar to the high-iron, nickelchromium alloys up to the highest value in commercially suitable materials. Properties of alloys available in both wrought and cast forms will not be the same in each form. The cast condition will provide higher stress-rupture values and high-temperature tensile strength, but poorer low- and medium-temperature tensile strength, ductilities, and impact strength. Because of the great hardening ability and related properties a t elevated temperatures, the high aluminum and titanium alloys are very diicnlt to produce in the wrought form; their workability is limited because of the difficulty of attaining the low hardnesses necessary to permit forming, bending, or shearing. Similarly, machining, welding, and brazing become increasingly difficult as the aluminum and titanium content go up. There are two major reasons for this: the aluminum and titanium oxides that form upon heating interfere with melting and flowing; and rapid precipitation processes can set up high enough stresses to cause cracking. Welding must be done under a protective atmosphere and vacuum brazing is usually necessary. The higher titanium and aluminum-containing alloys find their major application as turbine buckets and turbine nozzles where their higher strengths are required. The cast alloys such as Udiiet 700, Nicrotung and Inco 713 are the strongest commercially available materials suitable for the normal metal temperatures encountered in the first stages of gas turbines (15001700°F). Mechanical fatigue properties of the agehardened alloys are not substantial a t low temperatures, but they are retained fairly well to high temperatures. Thermal fatigue and shock properties vary, but oxidation resistance of alloys is good. The non-age-hardening nickel-base alloys are relatively easy to fabricate and form. Their application a t high temperature is limited to the same type of components as are the highiron nickel-chromium alloys.
Cobdf-Base Alloys
Cobalt-base alloys always contain appreciable amounts of nickel and chromium (a minimum of 2225%) for oxidation resistance and strengthening. Most have some molybdenum or tungsten for further strengthening. The majority of cobalt alloys have sufficient carbon present to form a fair amount of carbides. A few of them can be precipitation-hardened with Ni3Ti. For example, the list in Table 2 includes one of this type (J-1570). A study of this particular alloy can be in reference (11). The phases present in other cobalt-base alloys and their reaction to heat-treatments, including X40 and S816, are described in reference (12). Tensile and stress-rupture properties of cobalt-base alloys have values in between those of the high-iron and nonhardening nickel-base alloys and those of the strongest nickel-base alloys. At temperatures over 1800°F, where the effectiveness of the hardening precipitate in nickel-base alloys starts to decrease, there ceases to be a great difference in long-time properties. A long-time exposure to elevated temperatures may cause continued formation of carbides or other phases in the cobalt-base alloys, with possible loss in ductility. Normally, they are quite ductile and workable in the wrought form. Welding and brazing is generally easy to accomplish because the problems caused by aluminum and titanium in the nickel-base alloys have been avoided. Another advantage of cast cobalt alloys is that they can be cast satisfactorily in air instead of in the vacuum necessary for the hardenable nickel-base alloys. Again, the absence of aluminum and titanium makes this possible. Air casting represents a considerable economy compared to vacuum casting. Although the cobalt-base alloys do not exhibit the maximum strength at high temperature that the hardened nickel-base alloys do, they have a considerable field of application. Every type of component listed in Table 1 is being made from cobalt-base alloys. Strength limitations hold cobalt-base turbine buckets to maximum temperatures just over 1500°F, but in lower-stressed areas such as turbine nozzles and shrouds and liners, these alloys are widely used up t o temperatures of 2000°F. As air castings, they are economical, and in wrought form (particularly sheet) they have good working and joining characteristics. Oxidation resistance at the higher temperatures is not as good as some of the nickel-base alloys, but is adequate for normally encountered exposure. Future High-Temperature Materials
The commercial alloys available at this time for turbine-bucket stress levels of 15,000-25,000 psi are limited to 1800°F metal temperature. Low-stress applications permit maximum metal temperatures of about 2000°F. Development of present alloys should raise the limit for bucket applications by 5G100 degrees F. The limit in temperatures for iron, nickel, or cobalt bases will have just about been reached, for the melting temperatures of some of the phases that are formed are near 2250°F. By reducing the alloying elements to avoid these phases, higher melting temperatures can be attained but strength will be lost. A limit of about 2300°F can be expected, and this with quite low strength.
Two methods can be employed to raise the maximumtemperature regularly reached by alloys. One is t o use present alloy bases strengthened by phases with higher melting temperatures, the other is to use new metals with higher melting temperatures for the bases. Much work is being done on the former, with high melting oxides or carbides as the dispersed phases. Success with this method using aluminum (SAP) is well known. Problems encountered in this method using metals with higher melting temperatures than aluminum for the base are greater. However, promising initial results have been obtained by various investigators using copper, cobalt, and nickel as base metals. Thus fal. the major problem has been in obtaining a fine enough particle size uniformly dispersed in the matrix. Both internal oxidation and mechanical mixing methods are being investigated. The use of metals with higher melting temperatures is an obvious way of raising maximum capabilities. The next step above nickel and cobalt alloys would employ chromium as a base. This would offer a melting temperature 600-700°F higher. Oxidation resistance is good and present technology is just about suficient for manufacturing the materials. The drawback has been the low-temperature brittleness of the chromium alloys. If, by alloying or control of contaminating elements during melting, the brittleness problem can be overcome, a probable increase of 300-400°F could be realized. Another possibility is to overcome the lowtemperature-brittleness problem by proper design or special handling and operating treatment. Use of the still higher melting metals-columbium, tantalum. molybdenum, and tungsten-is severely handicapped by their excessive rates of oxidation at elevated temperatures Alloying to reduce oxidation (and reduce embrittlement by oxygen diiusion) has been partially successful thus far in the case of columbium only. Major improvements will still have to be made before application is practical. Because of unreliable application and poor resistance to thermal cycling, protection from oxidation by external coatings has had limited success. I n small gas turbines, the thickness of coating is an important factor. When components are thin, the per cent of weight increase due to coating thickness is greater than in the case of larger components, and much of the advantage of the strength at the higher temperatures is lost. At present, coatings for molyhdenum alloys have been developed which give good static oxidation up to 2500°F and some thermal cycling resistance up to 2200°F. Development of coatings for other base materials has not been as extensive. A review of the mechanical and oxidation properties to be expected of the L'refractory" metals has been made by Stanford Research Institute (IS). Literature Cited (1) SIMMONS, W. F., AND KRIVOBOK, V. N., Am. Soc. T&ng Matwials Spec. Tech. Publ. No. 170-A,1957. (2) HAGEL,W. C., AND BEATTIE, H. J., JR., Trans. Am. Soc. Metals, 49, 97&97 (1957). (3) BEATTIB,H. J., JR. A N D VER~NYDER, F.F., Trani. Am. SOC. Metals, 49, 883-95 (1957). R. F., AND FREEMAN, J. W., NACA TN 4286, (4) DECKER, Aug. 1958. (5) GUARD, R W., AND WESTBROOK, J. H., T~an.8.AZME, 215, 807-14 (1959).
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(6) WILLIAMS, R. O., Tmm. AIME, 215, 1026-32 (1959). (7) C u a ~ C. , C., AND IWANSKI, J. S., Tmm. AIME, 215,64865 (1959). (8) HAGEL,W. C., AND BEATTIE,H. J., JR., Trans. AlME, 215, 967-75 (1959). (9) DECKER,R. F., ET AL.,NACA T N 4329, July 1958. (10) KAUFMAN,M.,AND PALTY,A. E., Tram. AIME, 218, 107116 (1960).
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(11) Guhno, R. W.,AND PRATOR,T. A., Trans. Am. Soe. Melals, 49, 842-56 (1957). (12) WEETON,J. W., AND SIGNORELLI, R. A,, NACA T N 3109, March 1954. (13) TIETZ,T. E., WILCOX,B. A,, AND WILSON,J. W., Final Report Stanford Research Institute Project SU-2436, Prepared far U.S. Dept. of the Navy, Bur. Aer., Jan. 30, 1959.
