Sheet Gamma TiAl Status and Opportunities

Sheet Gamma TiAl:

Status and Opportunities

Gopal Das, H. Kestler, H. Clemens, and P.A. Bartolotta

Gamma TiAl alloys have attractive properties such as low density, high-temperature strength, and high modulus, oxidation, and burn resistance. As a result, these alloys have the potential to replace heavier superalloys in aircraft engine components. Gamma TiAl alloys were investigated in the 1950s but were too brittle for thermo-mechanical processing. However, interest in this

class of material rekindled with several U.S. aerospace programs: the National Aerospace Plane, the Integrated High Performance Turbine Engine Technol-ogy, and Enabling Propulsion Materials/High Speed Civil Transport, as well as German hypersonic technology pro-grams. Intense metallurgical and metal processing research during the last two decades led to signi? cant progress in this area. As a result, gamma TiAl alloys are now available in all conventional product forms: ingots, forgings, extru-sions, and sheets. This article reviews the current status of sheet gamma TiAl technology and its future opportunities.

INTRODUCTION

In 1989, Texas Instruments success-fully cold rolled γ-TiAl thin foils from an ingot on a laboratory scale.1 Likewise, Plansee, an Austrian company, demon-strated hot rolling of γ-TiAl sheets using both powder metallurgy (P/M) and ingot metallurgy (I/M) approaches in the early 1990s.2–4 The γ-TiAl sheet received a tremendous boost when it was selected for fabrication of the divergent ? ap of the nozzle for the high-speed civil transport (HSCT) engine. This led to the development of several new and chal-lenging technologies3,4 including produc-tion of large sheets, hot-die forming of sheets, and joining of sheets by brazing,

diffusion bonding, electron beam welding, and superplastic forming/diffusion bonding (SPF/DB), as well as joining by rivets. Also included was the non-destructive evaluation of braze quality. Since the termination of the U.S. enabling propulsion materials (EPM)/HSCT program, use of sheet γ-TiAl has been explored for several other aerospace applications. Critical technologies are being developed to support their needs, including nozzle tiles for gas turbine engines, nozzles for helicopters, back structures for scramjets, and thermal protection systems for reusable launch vehicles (RLV). Additional technologies such as waterjet machining, laser joining, and drilling are being developed in order to support these activities.

GAMMA-TiAl SHEET

PRODUCTION In Plansee's use of the P/M process, argon gas atomized powders are canned, evacuated at elevated temperatures, sealed, and then hot isostatically pressed (HIPed) to a billet at 1,300°C for 2 h to attain complete densi? cation. This is followed by rolling in the (α+γ) tempera-ture range close to the desired thickness. Subsequently, the sheets are creep annealed to minimize bending and residual stresses at 1,000°C for 2 h in vacuum. The ?

at sheet is then ground

Figure 3. A hot-formed double corrugation for sub-element braz-ing. Size: 300 mm × 150 mm × 83 mm.

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from both surfaces in order to achieve the ? nal thickness. The resultant sheet material is called primary annealed (PA) material. The yield is high; however, the P/M sheet exhibits thermally induced porosity at elevated temperature due to inert argon gas entrapped in powder particles. This limits its superplastic forming capability.5,6

For the I/M process, as-cast γ-TiAl ingots serve as the starting material. These ingots are subjected to HIPing to close the shrinkage porosity as well as to minimize segregation effects. The HIPed ingots are then cut into desired sizes and isothermally forged by either single or multiple steps at 1,200°C to pancakes. Rectangular plates are sliced from the pancakes by electrical discharge machin-ing (EDM). Both EDM surfaces and forged surfaces are ground prior to canning for rolling, as described previously. The yield is low in the I/M process because a signi? cant part of the pancake cannot be utilized.

Instead of forged materials, hot-extruded ingots of rectangular shape can be used for rolling. Typical dimensions of extruded bars are 120 mm × 30 mm × 400 mm.7 The bars are cut to size, surface treated, encapsulated, and then rolled to size. However, the I/M sheets are amenable to superplastic forming, as they do not suffer from thermally induced porosity.5,6 Occasionally, the presence of a banded microstructure resulting from segregation of alloying elements is observed in the I/M sheets. These banded regions are thought to cause a loss of ductility in γ-TiAl, especially at low

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temperatures.8

A third approach has been used to produce thin γ-TiAl sheets at a lower cost.9 In this approach, cast γ-TiAl is directly rolled into thin sheets, thereby eliminating costly and wasteful interme-diate steps. Direct rolling of cast plates into thin sheets has been demonstrated for a number of γ-TiAl alloys. Limited microstructural and mechanical property evaluations on these sheets have produced encouraging results.10 It has been estimated that the direct rolling may lead to a cost reduction of ~35% over the conventional P/M and I/M routes.

For the EPM program, TiAl having the composition Ti-46.5Al-4 (Cr,Nb,Ta,B) (at.%) was developed and the PA P/M γ-TiAl sheets were used for the fabrication of the divergent ? ap. The microstructure of these sheets consists of elongated ? ne-grained γ-TiAl (10–15 µm) with α2-Ti3Al particles at triple points and at the grain boundaries of γ-TiAl (Figure 1). A few elongated (Ti,Ta)-boride particles are also present. With suitable heat treatments, the primary annealed microstructure can be converted to a duplex or a fully lamellar microstructure.11

Figure 2 summarizes the tensile properties of P/M Ti-46.5Al-4 (Cr,Nb,Ta,B) sheet material with the PA microstructure. The yield stress decreases as a function of test tempera-ture while the ultimate tensile strength (UTS) achieves a peak value around 600°C and then decreases rapidly with increasing temperature. The increase in UTS around 600°C may be explained in terms of increasing dislocation activity leading to work hardening. At room temperature, the plastic strain to failure is greater than 1% and the ductility increases above the brittle-to-ductile transition temperature (BDTT), which

is around 750°C. Although Young’s modulus is not shown in Figure 2, it is ~168 GPa at room temperature and gradually decreases with temperature. The γ-TiAl is characterized by high specific stiffness (modulus/density), which is valuable whenever clearances are concerned such as frames, seal supports, cases, and linings. Also, higher speci? c stiffness shifts acoustic vibra-tions toward higher frequencies, which is bene? cial for structural components. Low-cycle fatigue, creep, and fatigue crack growth properties of P/M Ti-46.5Al-4 (Cr,Nb,Ta,B) sheets are available in the literature.12–14

In recent years, high niobium-containing γ-TiAl alloys, the so-called TNB alloys, have been developed at GKSS in Germany to meet the increasing demand for higher strength and higher-temperature capable γ-TiAl alloys.15,16 One alloy in this family is called γ-MET PX, which has the following composi-tion: Ti-45 Al-(5–10) at.% Nb –X (B,C). Under certain conditions, the yield stress of this alloy can exceed nickel-based alloys up to temperatures of ~800°C with a plastic elongation at fracture at room temperature of ~ 2%. Sheet sizes of 1,000 mm × 450 mm × 1 mm and 750 mm × 350 mm × 0.25 mm are available for both alloys. Recently, Plansee successfully produced γ-TiAl foils as thin as 75 µm for use in honeycomb structures.

HOT FORMING OF SHEETS Sheets have been successfully hot formed into corrugations using dies.3,4,6 Hot forming runs were conducted at temperatures ranging from 982°C to 1,232°C in vacuum at a displacement rate of 0.127 mm/min. An example of a large double corrugation, measuring 300 mm × 150 mm × 83 mm, is shown in Figure 3. It was hot formed at 1,093°C in

Figure 5. A brazed nozzle sub-element: 838 mm long × 152 mm wide with a cor-rugation depth of 83

mm.

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vacuum. A few cavities were formed in the tensile side of the hat section of corrugations in P/M γ-TiAl sheets at 1,093°C and above.4 Tensile strain above the neutral axis coupled with high forming temperature may help agglom-erate argon gas into cavities. However, no such cavitations were found in I/M γ-TiAl sheets hot formed under similar conditions. Boron nitride (BN), com-monly used to coat sheets and dies prior to hot forming, did not contaminate sheets hot formed below 1,093°C.

SHEET JOINING DEVELOPMENT

To integrate sheets into aerospace structures, sound joining is of great importance. Among the joining methods under development, brazing has by far received the most attention and is most developed. This method is simple and does not require elaborate tooling. An example of a brazed joint is shown in Figure 4. The brazing was conducted in vacuum at 1,010°C at 30 min. by sandwiching a piece of commercially available TiCuNi 70 ? ller foil between two thin P/M γ-TiAl sheets under a small load.3,4 The microstructure of the brazed area is inhomogeneous and contains hard and brittle intermetallic compounds including α2-Ti3Al3,4,6 as indicated by the size of the microhard-ness indentations. The brazed strength was determined to be 280 MPa at room temperature and 406 MPa at 704°C. Figure 5 shows brazed γ-TiAl sheet modules of the divergent ? ap for the HSCT engine—one of the largest

44

components ever built with sheet _γ-TiAl (see References 3 and 4 for details). Also, transient liquid phase bonding, an extension of brazing, was used to

17

bond γ-TiAl.

Diffusion bonding of γ-TiAl has been successful at 975–1,200°C.3,4,18,19 A typical microstructure of an optimized diffusion bond is shown in Figure 6. The P/M γ-TiAl sheets were bonded at 1,038°C at 21 MPa for 4 h in vacuum. The bond was sound, with no evidence of residual porosity or other defects being present. The bond strength was measured to be greater than the parent γ-TiAl material, indicating the success of the diffusion bond process.

Fusion welding methods such as gas tungsten arc welding and electron beam welding have successfully joined γ-TiAl alloys with preheating and in a controlled atmosphere. Proper thermal manage-ment is required to prevent solidi? cation cracking due to the limited ductility of γ-TiAl alloys.3 Similarly, laser welding of γ-TiAl sheets was demonstrated.4 More work is needed to optimize the fusion welding methods in terms of preheating and post-joining stress-relief temperatures. In addition, mechanical fasteners such as rivets have been successfully used to fasten sheet γ-TiAl together.3,4

GAMMA TiAl SHEETS: FORMING, FABRICATING,

AND MACHINING SPF/DB Technology

Under the EPM/HSCT program, the process parameters for SPF and DB have

been developed for I/M γ-TiAl sheets.4 An example of a successful SPF/DB process involving a two-sheet approach is shown in Figure 7. With this process, two γ-TiAl sheets were diffusion bonded at the edge in a die at 1,093°C/21 MPa/3 h followed by argon gas in? ation. It appears that complicated con? gurations can be made with this approach.Honeycomb and

“Gator Hide” Structures

Efforts are underway to fabricate honeycomb structures involving γ-TiAl sheets to be used as thermal protection systems for future RLVs. This applica-tion requires lightweight, stiff, durable, and easy-to-assemble components for the RLV’s outer skin in areas where, during reentry, the temperatures reach up to 850°C.20 A prototype honeycomb panel, fabricated entirely from _γ-TiAl sheet and foil, is shown in Figure 8. Honeycomb cores were assembled from thin foil corrugations ~75 µm thick by laser-spot welding followed by high-temperature brazing of core and face sheets ~125 µm thick to make the honeycomb sandwich panels.21

As a low-cost alternative to the honeycomb structure, the “gator hide” structure was developed.4 It involved hot-die forming of γ-TiAl sheets into gator hide cores followed by brazing of these cores with face sheets to mimic a honeycomb structure, as shown in Figure 9a. Nondestructive evaluation tech-niques such as x-ray radiography, ultrasonic scans, and thermography were successfully developed to evaluate the quality of the brazed bond. Among these,

Figure 7. A γ-TiAl product involv-ing two-sheet SPF/DB process. Part size: 152 mm × 64 mm × 16 mm.

Figure 8. An all γ-TiAl honeycomb core and sandwich panel. Size: 30 cm × 20 cm × 15 mm high.

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ultrasonic scans and thermography techniques were determined to be effective, as shown in Figure 9b where unbrazed areas were readily observed.Machining

Machining poses no serious dif? culty for γ-TiAl, as demonstrated in Figure 10, which shows tile machined from a rolled γ-TiAl plate for potential applica-tion in a nozzle. These tiles may require ? ne holes for cooling passages, and advanced laser drilling techniques show promise to completely eliminate the recast layer. Initial trials have produced holes with ~10 µm recast layers. This is much smaller than the 25 µm commonly associated with EDM machining. Conventional drilling can also be used to produce holes in γ-TiAl, which may leave some ? ne chips around the hole at the exit end. In addition, waterjet machining has been successfully used to make similar tiles as shown in Figure 10.

CONCLUSION

Due to its high-temperature capabili-ties, lower density, and higher speci? c stiffness, design engineers are beginning to consider γ-TiAl for high-temperature aerospace applications where superal-loys have been predominantly the material of choice. Technology programs such as EPM and Trailblazer (X-33) have created an optimistic future for γ-TiAl in new applications. Programs such as NASA’s Next Generation Launch

Figure 10. γ-TiAl tiles for nozzle application in aerospace engines. Part size: 274 mm × 147 mm × 3.8 mm. Weight: 180 g.

Technology (NGLT) consider γ-TiAl sheet as an enabling technology material system for next-generation RLVs to replace the space shuttle. Likewise, the Propulsion Research & Technology project of NGLT has developed a new backstructure concept utilizing γ-TiAl sheet for scramjet combustor walls. To truly capture the full potential of this class of material, current state-of-the art technologies as described in this paper need to be improved. Research areas such as low-cost material production, robust joining methods, incorporation of brittle matrix design philosophies, and a basic understanding of the structure and properties of γ-TiAl sheet need to be expanded before widespread use of this material system can be realized.ACKNOWLEDGEMENTS The authors would like to thank the dedicated researchers of the HSCT exhaust nozzle program. The work was

performed through Contract No. NAS3-26385 and Contract No. C-70682-K.

References

1. Aviation Week & Space Technology (September 2, 1991), p. 65.

2. H. Clemens et al., Gamma Titanium Aluminides, ed. Y.-W. Kim, R. Wagner, and M. Yamaguchi (Warrendale, PA: TMS, 1995), p. 555.

3. P.A. Bartolotta and D.L. Krause, Gamma Titanium Aluminides 1999, ed. Y.-W. Kim, D. Dimiduk, and M.H. Loretto (Warrendale, PA: TMS, 1999), p. 3.

4. G. Das et al., Structural Intermetallics 2001, ed. K.J. Hemker et al. (Warrendale, PA: TMS, 2001), p. 121.5. H. Clemens et al., Mater. Res. Soc. Symp. Proc., 460 (1997), p. 29.

6. G. Das and H. Clemens, in Ref. 3, p. 281.

7. H. Clemens and H. Kestler, Adv. Eng. Mater., 2 (2000), p. 551.

8. H. Clemens, Z. Metallkd., 96 (1995), p. 814.9. G. Das, patent pending (2004).

10. G. Das et al., (Paper presented at the Aeromat 2001 Conference, 11–14 June 2001, Long Beach, CA).11. H. Clemens, Structural Intermetallics, ed. R. Darolia et al. (Warrendale, PA: TMS, 1993), p. 205.12. A. Chatterjee et al., in Ref. 3, p. 401.

13. A. Chatterjee et al., Z. Metallkd., 92 (2001), p. 1,000.14. R. Pippan et al., Intermetallics, 9 (2001), p. 89.15. F. Appel et al., Advanced Engineering Materials, 2 (11) (2000), p. 699.

16. F. Appel et al., Gamma Titanium Aluminide 2003, ed. Y.-W. Kim, H. Clemens, and A.H. Rosenberger (Warrendale, PA: TMS, 2003), p. 139.

17. D.A. Butts and W.F. Gale, Gamma Titanium Aluminide 2003, ed. Y.-W. Kim, H. Clemens, and A.H. Rosenberger (Warrendale, PA: TMS, 2003), p. 605.18. W. Glatz and H. Clemens, Intermetallics, 5 (1997), p. 415.

19. G. Cam et al., Z. Metallkd., 90 (4) (1999), p. 284.20. R. Leholm, H. Clemens, and H. Kestler, in Ref. 3, p. 25.

21. H. Clemens et al., in Ref. 3, p. 209.

Gopal Das is a materials technologist with Pratt & Whitney in East Hartford, CT. H. Kestler is with Plansee Aktiengesellschaft in Reutte/Tirol, Austria. H. Clemens is with the Department of Physical Metallurgy and Materials Testing at Montan University in Leoben, Austria. P.A. Bartolotta is with NASA GRC in Cleveland, OH.

For more information, contact Gopal Das, Pratt & Whitney, 400 Main Street, E. Hartford, CT 06108 USA; (860) 557-1413; e-mail gopal.das@http://wendang.chazidian.com.

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