Structural Uses for Ductile Ordered Alloys (1984)

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POTENTIAL DEPARTMENT OF DEFENSE APPLICATIONS Inasmuch as current development programs are concentrating on alloys of nickel, titanium, or iron with aluminum, applications for these types of materials will be emphasized. The potential advantages of such alloys are their relatively low density coupled with good stiffness, strength, and oxidation characteristics, and uses that exploit such features must be sought. Components in powerplants and in certain airplanes are obvious candidates, especially for parts required to operate at high temperatures. It is recognized that additional projects are seeking to identify alloys with other attributes (e.g., wear resistance, magnetic properties, radiation resistance). Although space constraints prevent detailed consideration, much of the following discussion should apply equally well to these alloys. To date, no ordered alloy has reached the stage of development at which sufficient manufacturing and design data have been generated to permit incorporation in defense systems. Consequently, it is not possible to attempt a correlation between specific alloys and specific applications. Many factors, however, influence such alloy selections, and identification of these factors will be useful not only in seeking applications but also in orienting development efforts to address such factors. Even when engineering data are at hand, there is a series of contractual and administrative considerations that may impede the timely introduction of a new material into production. These include specifications from which approval for deviation would be required, sole source conditions to which many contractors might justifiably object, production schedules so tight that time for usage validation is not available before final commitment to full scale production is required, and poor part performance, which encourages conservatism on the contractors part. Component selection for early ductile ordered alloy application should attempt to minimize these factors.

50 APPLICATIONS OF NEW ALLOYS It is sometimes assumed that the existence of a new alloy that exhibits properties better than those currently available will be sufficient to justify its application since it will improve system performance. However, the factors that must be considered are many and complex, and include performance specifications and various elements of cost. The following paragraphs deal with some of these considerations. Performance and Cost Specifications Most vehicles or equipment that could utilize ordered alloys will be designed to meet a specific set of comprehensive requirements (e.g., weight, thrust, speed). With these specifications established, the designer must not only meet these requirements but also strive to ensure that the system can be built on schedule and within cost. Thus, improved performance over and above the goal is not necessarily an advantage even in military aircraft where performance is so important. There is no incentive to exceed the requirements unless it can be done with no increase in cost and no compromise in reliability. To an extent, materials of construction are set by the specification; given a choice, a designer will select a proven cheap alloy. However, cost-weight trade-offs will have to be made for each system so a new material can buy its way in, assuming the risk factors are deemed acceptable. Risk is made up of several considerations ranging from fear of the unknown to the fact that only a limited amount of characterization has been performed and only a small amount of mechanical property data may be available. This last point can serve to introduce another important factor: In the development of new alloy systems, tensile strength is often the only property determined. Increases in strength may be useful but it represents only one potential limiting characteristic. Limit Conditions or Failure Modes Estimates of the structural advantage of a new material are frequently made by relating specific strength (strength:density) to component size. Using such a method, an alloy that is twice as strong will result in a part half as thick or an alloy half as dense will cut the weight in half. This will be true only if all the key properties of the alloy do not result in a change in failure mode. In typical aircraft structures, a variety of limit conditions may be encountered (e.g., overload, buckling, fatigue). Each can serve to size the part. It is clear, therefore, that weight reduction made possible by an improvement in one property can be restricted to a point where another failure mode becomes critical. This is illustrated in Figure 12 (Ekvall et al. 1982) which shows the required structural thicknesses to sustain the various failure limits in an aircraft structure built of two competitive materials. The original alloy (shown on the left), limited by strength, is compared with a new alloy (shown on the right) for which the limit is set by damage tolerance. In the development of ordered alloys it will be important to consider the balance of properties that can be achieved. Improvement in one property may not yield a commensurate component benefit should other properties change. Evaluation of the resistance of new alloys to a spectrum of test conditions will be required to permit the determination of application potential. This type of evaluation unfortunately should not be relegated to the later stages of a development program.

51 Q) "8 1 •o 2 cr Strength Crack Propagation Strength Aerodynamic Loads Buckling Buckling Aerodynamic Loads Basic material Hypothetical new material that is stronger and stiffer, but no better in crack propagation resistance FIGURE 12 Potential weight savings considering additional failure modes (based on Ekvall 1982).

52 Timing Constraints and Application Windows A review of the Department of Defense planning documents about two years ago yielded a list of projected major new systems scheduled for production between 1983 and 2000 v Reams 1982). Details are given in Table 7; the numbers although subject to change are of the right order of magnitude and would seem to offer many opportunities for new materials. However, timing constraints alter this assessment because of the qualification requirements, and these constraints vary widely depending on whether the material is to be included: in the original design, during the design phase, to correct a test deficiency, or to alleviate a service problem. The original concept of high-performance systems usually precedes first production by at least 10 years. Many of those listed in Table 7 have already passed this conceptual stage and, therefore, application windows have apparently been closed. However, high-performance vehicles often experience unacceptable weight increases during prototype development, and component failures also are not uncommon at this stage. Such occurrences offer additional opportunities for the application of new materials. Problems in service due to unanticipated operational conditions often arise and these, depending on the magnitude of the problem, could result in materials substitutions. In Figure 13, the potential application opportunities in airframes and engines are indicated by arrows at the appropriate times. TABLE 7 New Systems in the Department of Defense Plans Year A/Cl M!> LV£ si Other! 1983 2 1 1 1 - - - 1984 1986 - 2 - - - 1987 2 1 6 4 1 1988 3 3 1 1 - 1989 4 2 1 3 - 1990 2 1 - 1 1 1991 1 2 - 5 - 1992 1 3 - - - 1993 1 2 2 1 - 1994 2 - 3 - 1 1996 1 - - - - 2000 2 ^ ^ ~~ ™ —Fixed-wing aircraft and helicopter. ^Missiles. —Land vehicles. liShips and other seaborn vehicles. f-Guns, etc. SOURCE: Keams (1982).

53 AIRCRAFT ENG1NES T CF ADV DEV PROTOTYPE DEV PRODUCT1ON t t t ft ORIG1NAL DES1GN WE1GHT TEST 1N•SERV1CE ENV1RONMENTAL GROWTH FA1LURES FA1LURES TROUBLES TECH DEMO DEMO ENGINE PROTOTYPE DEV PRODUCT1ON 1 t OR1GINAL TEST DES1GN FA1LURES TECH DEMO MOD DEV PRODUCT1ON EXPER1MENTAL RA1SE PERFORMANCE OR TECH DEMO MOD DEV PRODUCT1ON EXPER1MENTAL 1NCREASE TBO 15 10 T1ME (years) 10 FIGURE 13 New material application windows (Ekvall et al. 1982).

54 QUALIFICATION AND INCORPORATION PLANNING FOR NEW MATERIALS Having indicated some of the problems that will be encountered in gaining acceptance of a new material and getting it incorporated into systems, it may be informative to examine how this has occurred in practice. If the materials used in the various systems and equipment operated by the armed forces are surveyed, it becomes clear that new and upgraded materials are incorporated on a regular basis. Some of the changes are obviously evolutionary, but it is the more revolutionary changes that are of interest here. There are a series of steps through which a (successful) new material must pass on the way to qualification. These are represented in Figure 14, which shows not only the timing of these stages but also the approximate cost. Based on the first inception of a new idea, a series of laboratory tests usually is performed to confirm the promise of the system and define basic capabilities; this is a relatively inexpensive stage. The development stage is much more costly and involves processing studies (e.g., large-scale melting, conversion to mill products), development of preliminary design property data, fabrication trials, etc. Assuming success at this stage, prototype hardware would next be produced and subjected to rig, engine, and, in some cases, service tests. In parallel, qualification of alternate production sources would occur and for incorporation approval would be sought from the various control agencies. Finally, after 10 or more years and the expenditure of about $15 million, components made from the new material would be incorporated into production hardware. It is obvious that such a complex development sequence involves many participants—the customer, the manufacturer, vendors, technical laboratories, etc. Even these descriptive titles are in some ways misleading in that parties are concerned with many aspects of the activities. For example, government agencies often fund much of the initial screening program through to manufacturing technology demonstrations before the material is incorporated into a product acquired by DOD. In many cases close coordination of all stages aids greatly in providing periodic feedback and program continuity. The other important, and often less tangible, factor that is created by such a team is an advocate (or advocates) of the technology in the key manufacturer and customer areas. Without such advocates, a technology can wither away and die in the current competitive atmosphere. Another key element for any major thrust is frequent reviews of the payoff for the technology. If this is high, it obviously will serve to maintain momentum especially in times of technical tribulation. Such cost-benefit studies need to be repeated at regular intervals, especially in the time period between the euphoria of laboratory demonstration and the bleak realism of the development program, because the baseline assumptions often are modified as new information becomes available. It is well to remember that the first thing you ever hear about a new material is the best. The situation can be summarized, in relation to the above scenario, for the new alloys that have been covered in presentations to the committee: 1. Titanium aluminides—Development path is well under way. If basic engineering concerns can be overcome, limited engine utilization could occur as early as 1987.

55 10 w z o 1ncorporation ^•^B QUAL1F1CAT1ON'" Engine Test F Rig Test Specifications Processing-Secondary DEVELOPMENT r Design Data Processing•Primary Cost Benefit Study T Laboratory. ' Demonstration I 5 YEARS 10 FIGURE 14 Typical development sequence for a new material being incorporated into a defense system (courtesy of United Technologies, Inc., Pratt and Whitney Aircraft).

56 2. Iron aluminides—Preliminary laboratory demonstration is nearing completion. If systems pass cost-benefit tests, they should proceed to development stage within three years with qualification possible by 1990. 3. Nickel aluminides—Laboratory demonstration has been initiated and results have been promising. Some preliminary component identification has been performed, but more is needed. Clearer picture of payoffs will be required in the future. 4. Iron-base ordered alloys—Laboratory demonstrations have shown some attractive properties. Lack of potential user interest and specific payoffs seem to be major drawbacks at present. SOME POTENTIAL APPLICATIONS Based on the known characteristics of the alloys that have been covered in this review, the most obvious applications are in gas turbine engines. The intermediate- and high-temperature strength characteristics, low density, etc., all point to such applications. Material temperatures encountered in propulsion systems of this type are characteristically high. Alloys with good high-temperature strength and oxidation resistance and low density would seem especially advantageous in rotating parts in which centrifugal loads may account for about 75 percent of the total loading. Figure 15 illustrates a typical large transport aircraft turbofan engine and Table 8 lists property requirements and failure modes possible for the various components. Potential applications for the various classes of ordered alloys, especially the aluminide systems, are covered in more detail below with emphasis on rotating components. The illustrations chosen also include brief descriptions of the extremes in operating conditions together with the materials and processes used to make the components. Requirements for fan and low pressure compressor parts, shown in Figure 16, are low density but high stiffness and strength. These characteristics seem best fitted by the titanium aluminides. The front end of an engine operates at low temperatures and is subject to the ingestion of foreign objects. Thus the basic damage tolerance of any aluminide system would have to be shown to be adequate. Temperatures rise rapidly as one proceeds through the high pressure compressor, shown in Figure 17, and several of the ordered alloy systems could be considered for these components. Weight, durability, and the fire danger will dictate the specific alloy selected. It is probable that a combination of systems would be the best compromise. It should be noted that the drum rotor design requires good welding and repair characteristics. The burner, Figure 18, and high pressure turbine, Figure 19, of gas turbine engines represent the most challenging environment for materials. Stresses and temperatures are high, but maximum values usually do not occur simultaneously, leading to complex durability (thermo-mechanical fatigue) requirements. Nickel aluminides are the only current candidates that have any prospect of meeting the mechanical and corrosion properties needed in

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59 FIGURE 16 Fan assembly of a CF6-80 engine (courtesy of General Electric Company).

60 FIGURE 17 Compressor rotor of a CF6-6 engine (courtesy of General Electric Company).

61 FIGURE 18 Combustion chamber of a JT9D engine (courtesy of United Technologies Corporation, Pratt and Whitney Aircraft).

62 FIGURE 19 High pressure turbine assembly of a CF6-50 engine (courtesy of General Electric Corporation).

63 turbine disks, blades, and vanes. It is possible that the lower stresses that prevail in the burner could permit the exploitation of the excellent oxidation resistance of the iron aluminides. Components are presently made by a variety of methods and the ability to create these complex shapes from these new systems will also be a key factor. An example are the complex hollow blade and vane castings shown in Figure 20. The temperatures and, to a lesser extent, stresses are lower in the low pressure turbine (Figure 21), and the less demanding conditions open up several potential applications. Both the titanium and nickel aluminides appear candidates for disks and blades. It is obvious from Figure 15 and the other illustrations that there are many other components in a turbine engine. The payoffs, compatibility with design needs, etc., would have to be assessed individually to define applicability. It is hoped that this rather brief treatment of some of the challenges for new material set by gas turbines will stimulate more in-depth studies. A role for ordered alloys in rocket propulsion systems also can be foreseen if improvements over existing nickel-base superalloys are achieved. Of particular interest would be a thermal-fatigue and high-cycle-fatigue resistance superior to that of the MAR-M246 currently used for turbine blades on the Space Shuttle main engine, for example. Figures 22 and 23 show typical cross-sections of advanced rocket engine turbomachinery with potential applications being turbine blades, disks, and combustion chamber liners. Other potential aerospace hardware applications for ordered alloys are in advanced metallic thermal protection systems and hot gas ducting for lift augmentation devices. In these last two cases, low density is of paramount importance, making the titanium aluminides of great interest. Another possible application for advanced ductile ordered alloys is in nuclear space power systems. Several alternate system types are currently under study, and the maximum temperature to which structural materials will be exposed varies from one design to another. However, maximum temperatures would typically be in the range of 830 to 1230°C. Since these are flight systems, weight is an important parameter and materials that offer strength-to-weight advantages are attractive. The ductile ordered alloys currently being developed will not be adequate for use in the higher temperature systems being studied (i.e., maximum temperature of 1230°C); however, other alloy systems, such as the CoAl-base alloys, may be useful in the higher temperature ranges. The space power systems based on the use of in-core thermionic conversion expose structural materials to lower maximum temperatures (i.e., 830°C). In these systems, the ductile aluminides, for example, could play a useful role in components such as the radiator, piping, pumps, manifolds, and nozzles. To be applied in space power systems, materials must be compatible with the heat transfer fluids (typically molten metals such as lithium, sodium, or sodium/potassium). Limited experiments have shown good compatibility for

64 VANE BLADE FIGURE 20 Turbine blades and vanes of a PW2037 engine (courtesy of United Technologies Corporation, Pratt and Whitney Aircraft).

65 FIGURE 21 Low pressure turbine assembly of a CF6-6 engine (courtesy of General Electric Corporation).

66 FIGURE 22 High pressure fuel turbopump (courtesy of Rockwell International, Rocketdyne Division).

67 • OX1D1ZER PREBURNfR H1GH PRESSURE HYDROGEN TURBOPUMP MA1N COMBUST1ON CHAMBER H1GH PRESSURE OX1D1ZER TURBOPUMP FIGURE 23 Space shuttle main engine powerhead component arrangement (courtesy of Rockwell International, Rocketdyne Division).

68 some ordered alloys with such molten alloys. The high nickel content of some of the higher strength ductile ordered alloys may be a disadvantage in this respect since many alloys with a high nickel content exhibit high corrosion rates in liquid metals of the types indicated. Good resistance to irradiation damage will be important for applications in shielded locations, and the boron content of some of the high-strength ductile ordered alloys may be a disadvantage in this regard. In addition, since space power systems are typically fabricated from formed and joined sheet, pipe, etc., good fabricability and weldability will be required. Overall, however, the weight saving potential of some of the ductile ordered alloys suggests that they should be evaluated for space power. REFERENCES Ekvall, J. C., J. E. Rhodes, and G. G. Wald. 1982. Methodology for Evaluating Weight Savings From Basic Material Properties. Special Technical Publication No. 761. Philadelphia, Pennsylvania: American Society for Testing and Materials. Kearns, T. F. 1982. An Assessment of Rapid Solidification Technology (RST) and Its DOD Applications. Paper P-1627 (limited distribution). Alexandria, Virginia: Institute for Defense Analyses.