HIGH TEMPERATURE ALLOYS

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1 HIGH TEMPERATURE ALLOYS for Aerospace Structures Significant advances in material production and fabrication of real structures with γ-tial and ODS alloys have begun to influence the aerospace community to consider their utility in more applications. Robert LeHolm* Brian Norris* Andrew Gurney BFGoodrich Aerospace/Aerostructures Group Chula Vista, California The demanding environments of new and existing aerospace applications require the continued development of lightweight, high-strength and -stiffness, hightemperature materials for structural components. Recently, gamma titanium aluminide (γ-tial) and oxide dispersion strengthened (ODS) alloys have become available in material forms, sizes, and quality amenable to fabrication of advanced lightweight structures such as thermal protection system (TPS) panels for space, aircraft turbine engine and nozzle components, fire barriers, and missile parts. These alloys have significant property advantages over certain temperature ranges, thus making them the preferred materials selection. This article discusses potential applications, material properties, and fabrication techniques for these materials. Potential applications Many aerospace programs will be able to improve vehicle performance by incorporating γ-tial and ODS components (Fig. 1). However, barriers to implementation must be surmounted before this *Member of ASM International Fig. 1 Potential demonstrations and applications for advanced alloys include space, military, and commercial vehicles. This is an artist s rendition of the Boeing X- 37 Flying Testbed. Image courtesy Boeing. practice becomes widely accepted. A typical transition in the application of materials is cost reduction with higher volume production. High risk, high payoff applications, such as those typically associated with space programs, allow more costly upfront development of exotic, high-priced materials. Subsequently, as the material itself becomes more available and the properties more reliable, additional applications in military, then commercial fields, enable increased production rates and lower material costs. Currently, γ-tial and ODS alloys are being considered for space, military, and commercial applications, although in limited quantities. A major production volume breakthrough could be reached for γ-tial when the next generation Reusable Launch Vehicle is built, because of the significant portion of γ-tial that may be necessary for its thermal protection system (Fig. 2). The exceptional specific ADVANCED MATERIALS & PROCESSES/MAY

2 Fig. 2 Thermal protection systems for space vehicles must be of the lightest weight possible to meet mass fractions and increase payload capacity. Metallic panels for the Second Generation Launch Vehicle concepts such as Lockheed Martin VentureStar will be constructed of advanced titanium and oxide-dispersionstrengthened alloys. Fig. 3 These are several 0.04 in. thick gamma titanium aluminide sheet sections that were hot-pack-rolled at Plansee. properties and temperature capabilities of γ-tial significantly reduce the overall vehicle weight, allowing it to achieve mass fractions required for reusable launch vehicles, and to carry increased payloads. Significant cost reductions are already being realized due to the on-going work to increase production volume and reproducibility of the material in sheet and foil forms. ODS alloys are currently utilized by Rolls Royce for high-temperature turbine engine seals and were also a key part of the Lockheed Martin X-33 thermal protection system. Both γ-tial and ODS alloys may also be used for heat shields, turbine engine nozzles and plugs, hypersonic vehicle missile fins, control surfaces, and hot support structures. Gamma titanium aluminide Gamma titanium aluminide (γ-tial) alloys are a group of very promising, low-density intermetallic materials that offer many attractive properties for high-temperature aerospace environments. These alloys are in competition with nickel and iron-based superalloys in the 600 C to 900 C (1112 F to 1652 F) range. Historically, the superalloys have been selected for high-temperature aerospace components because of their excellent creep strength, toughness, oxidation resistance, and general long-term stability at temperature. Replacing superalloys with the lighter intermetallic family members in hightemperature aerospace applications is very attractive for spacecraft, as well as commercial aviation, to achieve mass ratio or weight reduction. γ-tial alloys exhibit many outstanding properties, including high melting-point, low density, high elastic modulus, high resistance to oxygen absorption, and very good specific strength, creep strength, burn resistance, and oxidation resistance. However, γ-tial alloys have limitations, even with the recent improvements achieved for second-generation alloys. The most frequently cited weaknesses include low room-temperature (RT) ductility, low fracture toughness, unknown random fatigue properties, and relatively high crack-growth rates. In γ-tial alloys, tensile ductility is inversely proportional to fracture toughness properties. γ-tial oxidation resistance above 750 C (1380 F) is a concern, as is the possible need for a protective coating over long time intervals at high temperature. Manufacture of g-tial thin product Until recently, γ-tial sheet and foil having consistent microstructure was not available in reasonable product sizes because sheet technology was not sufficiently developed. Now some γ-tial alloys, including Plansee s PM γ-met (as a powder metallurgy alloy), are sufficiently developed to be manufactured into useful thin product, and to be qualified as a new material system for various high temperature aerospace structures. The γ-met alloy is manufactured from PM γ-tial preforms and subsequently rolled by conventional hot rolling equipment. Plansee is currently developing PM γ-met foil less than 300 µm (0.012 mils) thick, using production equipment and a proprietary rolling process. Examples of γ-met sheet are shown in Fig. 3. Chemical/phase compositions of g-met The nominal chemical composition of γ-met alloy, in atomic percent, is Ti 46.5Al 4(Cr, Nb, Ta, B). This γ-tial alloy has exhibited reasonable isotropic behavior and possesses many outstanding properties. The chromium is added to increase material ductility, while the niobium and tantalum increase creep and oxidation resistance. Boron hinders grain growth during heat treatments in the alpha field. γ-met, as well as most γ-tial alloys, generally consists of two major phases: the γ phase and the α 2 phase, with the possibility of some minor β (B2) phase and borides. γ-met thin product is usually received in the primary annealed (PA) heat treat condition, which consists of a 1000 C (1830 F) anneal for two hours. The fully lamellar (FL) condition consists of heat treating γ-tial material above 28 ADVANCED MATERIALS & PROCESSES/MAY2001

3 Fig. 4 Microstructure of primary annealed (PA) gamma titanium aluminide. the α-transus temperature (above ~1320 C/2408 F) to produce a specific or designed γ + α 2 lamellar microstructure. The resulting FL colony size in the microstructure depends on annealing time and temperature. Microstructure of g-met sheet Cross-sectional specimens of various γ-met products have been studied in both the PA and FL heat treat conditions. A typical PA microstructure of as-received γ-met sheet is shown in Fig. 4.A matrix of fine grained, equiaxed or globular γ grains is observed, with some small amounts of α 2 and other phases (e.g., β phase) distributed at grain boundaries and triple points. Fig. 5 Microstructure of fully lamellar (FL) gamma titanium aluminide. Nominal composition of ODS alloys, wt% Fig. 6 Microstructure of fully lamellar gamma titanium aluminide, shown at reduced magnification from Fig. 5. Alloy Fe Ni Cr Al Ti Yttria PM2000 Base MA 956 Base PM 1000 Base MA Base When heat treated above the α-transus temperature, the PA microstructure is transformed into a FL microstructure, consisting of random oriented lamellar colonies (made of α 2 and γ lamellae) (Fig. 5) The average lamellar colony size here is 200 µm (0.008 in.). This lamellar structure is the result of phase transformations and ordering reactions as the material cools from annealing in the single Circle 18 or visit 29

4 were subsequently chemically milled and brazed between 1 mm (0.039 mm)γ-met sheet skins to form three 180 mm 20 mm 400 mm ( in.) truss core structures. An example of one of these SPF truss core structures is shown in Fig. 7. They were hot formed in standard production facilities, and the resulting structures exhibited excellent form and braze footprint. These structures were tested at NASA-Glenn and exhibited strengths much greater than expected. Fig. 7 Truss core structure of gamma TiAl was manufactured by Plansee of α-met sheet via a powder metallurgy process. Truss core structures were tested at NASA- Glenn and exhibited strengths much greater than expected. Fig. 8 Examples of hot-formed PM -MET sheet hat sections shaped by superplastic forming. α phase field, and through the non-ordered, two phase α + γfield, then to the ordered, two-phase α 2 + γ field. For a complete picture of the γ-met sheet microstructure, through the entire thickness, Fig. 5 was reduced in magnification until the entire sheet cross section became visible in one micrograph. This low magnification micrograph is shown in Fig. 6. Hot-forming trials Various hot-forming trials have been conducted in a superplastic forming (SPF) press on γ-met sheet (in the PA condition) and, for comparison, on IM alloy Ti-48Al-2Cr sheet (also in the PA condition). These trials proved to be very successful in establishing a baseline hot forming database for γ-met sheet. Elongated hat sections (Fig.8) were formed by metal superplastic forming (SPF) in a press at temperatures that varied between 900 C (1650 F) and 1100 C (2010 F). Cross sectional metallographic evaluations (of the as-hot formed γ-met hat sections) revealed no increase in the microporosity already in the as-received sheet. More recently, a special program was jointly pursued by NASA-Glenn Research Center, in which 1 mm thick γ-met sheet sections were hot formed into truss core hat sections by a matched tooling die set in a standard press. These truss core sections Oxide dispersion strengthened alloys ODS alloys have numerous advantages over competing alloys above 980 C (1800 F). For example, columbium-base alloys have good hightemperature strength, but they oxidize rapidly and require protective coating systems. The coatings not only add weight to lightweight, thin-gage structures, but also are a source of potential failure due to coating breakdown. On the other hand, the ODS alloys generate their own protective coating of alumina or chromia, and hence this problem does not develop. Other materials such as coated carboncarbon or ceramic-ceramic composites are also suitable for this temperature range, but structures tend to be difficult to design and are very expensive. Oxide dispersion strengthened alloys have the highest unprotected tensile strength and creep resistance of all commercial alloys in the 980 to 1230 C (1800 to 2250 F) range. Unlike conventional gammaprime strengthened superalloys, the oxide particles in the ODS alloys do not readily go into solution at high temperatures, and hence the dispersoid-created strength is relatively stable. The two main families of commercial ODS alloys include the iron-based family (MA 956 and PM 2000) and the nickel based family (MA 754 and PM 1000). A tenacious, self-healing alumina film protects the 956/2000 iron-based alloys. The 754/1000 nickel based alloys have a less protective chromia film; however, they have a superior high temperature creep strength. A representative microstructure of PM 1000 is shown in Fig. 9. ODS structures development Several types of ODS structures, including honeycomb sandwich and isogrids, are being developed for various applications such as thermal protection systems on reusable launch vehicles. Honeycomb sandwich structures offer significant stiffness-to-weight advantages over competing designs such as sheet and stringer, hat stiffened, foamfilled, truss core, and other conventional panels. In fact, honeycomb offers the highest strength and stiffness-to-weight ratios of any other structural platform available. Honeycomb core is usually fabricated by various methods ranging from semi-manual to fully automatic. As with many advanced alloys, one of the main challenges of producing honeycomb core is the availability (production) of the thin gage foil (~ <75 micron / in.) needed before the core can be corrugated. The requirement for thin gage ribbon necessitates hybrid ODS honeycomb sandwich panels (PM 2000 core and PM 1000 face 30 ADVANCED MATERIALS & PROCESSES/MAY 2001

5 sheets). For the short time at temperature encountered in the re-entry environment, the 754/1000 series of alloys would be advantageous for both face skins and core due to their high temperature creep strength in the coarse-grained condition. Hot rolling generates this condition, but unfortunately it is difficult to acquire hot rolled continuous foil thinner than 0.25 to 0.50 mm (10 to 20 mil). Cold rolling below this thickness produces a fine-grain structure with resulting lower creep strength. In response to these challenges, hybrid panels have been fabricated with PM 1000 face sheets and PM 2000 core. The 956/2000 series of alloys is inherently brittle at room temperature for gages above 0.25 mm (0.010 in.). This brittleness is due to a high brittle- to-ductile transition temperature for the thicker gages. Fortunately, this problem does not manifest itself in the thinner gages used in TPS. However another problem, braze embrittlement of thin-gage foils, presented an initial challenge. This challenge was finally overcome through the development of a ductile brazing system. Isogrids are rib- or stringer-reinforced sheet structures fabricated by machining the reinforcements directly from a thick sheet of parent material. This construction offers advantages over other types of structures for complex designs and areas of high curvature. Several panels on the X-33 near the nose of the vehicle were ODS isogrid constructions. As the aerospace community continues to push the envelope of performance beyond the capabilities of conventional materials, development of new, advanced materials and structural concepts must also continue. For many of these 21 st Century applications, γ-tial and ODS alloys will not only be required because of their outstanding high temperature performance, but also will be considered enabling technologies. With the associated decrease in cost due to increased production quantities, these materials will become widely specified in the commercial aerospace industry as well as other marketplaces. For more information: Robert LeHolm, Brian Norris, Andrew Gurney, BFGoodrich Aerospace/Aerostructures Group, 850 Lagoon Drive, Chula Vista, CA 91910; e- mail: agurney@aerostructures.bfg.com. How useful did you find the information presented in this article? Very useful, Circle 288 Of general interest, Circle 289 Not useful, Circle 290 Fig. 9 Microstructure of coarse-grained PM 1000 (original photo 100X). Circle 19 or visit 31