AAWSAP DIRD, Materials for Advanced Aerospace Platforms, January 2010

Therefore, instabilities that cause time-dependent reductions in toughness must be understood before these materials can be considered candidates for future TPS applications. Titanium Aluminides Titanium aluminides are intermetallic compounds that form between Ti and Al. There are three such compounds—Ti3Al, TiAl and Al3Ti—but the one of principal interest is TiAl, often referred to as gamma titanium aluminide or γ-TiAl. This compound contains about 35 weight % Al, but ternary and quaternary alloys based on TiAl typically contain somewhat less than this amount. γ-TiAl is interesting for intermediate-temperature applications for several reasons. These include a lower density, at least a 125 °Celsius higher temperature capability, and better surface stability (oxidation resistance) compared with conventional Ti alloys. The surface scale that forms during oxidation of γ-TiAl is Al rich and is more protective than the TiO2 scale that forms on conventional Ti alloys. Like most other intermetallic compounds, γ-TiAl has very limited ductility at temperatures up to about 600 °Celsius but is not brittle in the classical sense; that is, it has the capability to deform plastically before fracturing. The ductility increases as temperatures increase and is quite good at normal service temperatures. γ-TiAl alloys' limited ductility poses manufacturing problems for any wrought products. These issues are manageable but reduce product yields, which in turn impacts cost. Recent work in Austria has demonstrated that some γ-TiAl alloys can be made into sheet. This is particularly relevant to applications such as metallic TPS. Other work has demonstrated that some γ-TiAl alloys can be conventionally forged, albeit with care. Should one or more of the efforts currently under way to produce low-cost, prealloyed Ti powder prove successful, the availability of affordable γ-TiAl alloy powder could be a major cost breakthrough. The development and maturation of γ-TiAl alloys has taken more than 30 years. This is in part due to the lack of actual production applications because of designer's concerns regarding the limited low-temperature ductility of all the γ-TiAl alloys. Today, finally, there are several applications of alloys based on γ-TiAl, the most significant of which are two stages of low-pressure turbine blades in the engine General Electric is providing for the Boeing 787 and the growth 747 (called the 747-8). These blades are cast to near net shape and will operate at temperatures up to about 750 °Celsius. Another application is a cast γ-TiAl turbocharger rotor Mitsubishi uses in a production auto because it has half the mass of an equivalent Ni-base alloy component. A potentially ideal, high-volume application of γ-TiAl alloys is exhaust valves for autos. If the low-cost powder mentioned earlier becomes a reality, this factor, coupled with the current pressure to improve fuel consumption in American cars, could make this prospect a reality. The point of this short discussion of non-space-related applications of γ-TiAl alloys is that realization of any or all of these applications will create a stronger production base for this class of alloy, which in turn will make their introduction into future space platforms easier and more cost effective. 20 UNCLASSIFIED//FOR OFFICIAL USE ONLY

UNCLASSIFIED//FOR OFFICIAL USE ONLY PROPULSION SYSTEMS Among propulsion systems, only reusable rocket engines are considered here because they hold potential for significant progress that could reduce the cost of placing payloads into orbit. The basic concept of the space shuttle main engine is still viable, but the durability of the materials used to make the hardware has been a major expense and source of concern for NASA. Much of the concern is related to the effects of hydrogen on the Ni-base alloys used in the turbo pumps. The turbo pumps are the heart of a liquid-fueled hydrogen-oxygen rocket engine because they deliver the fuel and oxidizer to the thrust chamber. The turbo pumps rotate at a very high speed\u2014up to 35,000 rpm. Such speeds create enormous centrifugal stresses in the rotating components, particularly the disks that hold the air foils used to extract work from the hot gas stream. One side of the turbo pump rotor operates in the hot gas stream created by the combustion of the oxygen-hydrogen mixture; the other end is a cryopump that operates at cryogenic temperatures, either in liquid oxygen (90 K) or in liquid hydrogen (20 K). The hot side, a turbine, resembles the turbine rotor in a gas turbine engine and is made of many of the same Ni-base alloys. The cryopump is made of Ti alloys. The turbine essentially operates in a hydrogen-rich supercritical steam environment at a maximum temperature of about 1,050 \u00b0Celsius. The turbine disk is a forged Ni-base alloy, and the air foils are single-crystal investment castings. Neither material is well suited to operate in a hydrogen-rich environment; however, no other material class can withstand the operating temperatures and has better hydrogen tolerance\u2014a situation that still prevails today. The mechanism of hydrogen-induced cracking is much better understood today as the result of extensive research over the past 25 years. Therefore, it would be very useful to use this improved understanding to design a Ni-base alloy that has improved hydrogen tolerance. If successful, a turbine that has improved resistance to hydrogen cracking would greatly reduce the intermission refurbishment time and cost. Traditionally, the cryopump rotating parts have been made from forgings of the near-\u03b1 phase Ti alloy Ti-5Al-2.5Sn (Ti-5-2.5). This alloy has been chosen for its superior notched tensile strength compared with Ti-6-4 when tested at cryogenic temperatures. Hindsight suggests it is unclear that notched tensile strength is the best criterion for selecting a cryogenic rotor material. The use of notched tensile strength originated with steels. Here, the hydrostatic stress state at the notch root could trigger the onset of brittle fracture of the type seen in smooth tensile tests below the ductile brittle transition temperature (DBTT). Ti alloys do not exhibit a DBTT; therefore, true fracture toughness measured at the relevant temperature is a more accurate indication of the fracture resistance of the rotor. An examination of the limited available fracture toughness data for the two alloys shows no clear advantage in using Ti-5-2.5. Because Ti-5-2.5 is more difficult to produce than Ti-6-4, using Ti-5-2.5 adds cost to the cryopump. Other, newer, higher strength Ti alloys\u2014for example, several of the newer \u03b2-Ti alloys\u2014may be even better suited than Ti-6-4 is for this application. The critical question of how much toughness is really required cannot be answered at present because the notched tensile ratio has been the deciding criterion. Therefore, a new 21 UNCLASSIFIED//FOR OFFICIAL USE ONLY

design of a cryopump potentially can benefit from these new materials and from abandoning the use of notched strength as a design paradigm . The combustion chamber of a rocket engine is made from a copper alloy and is cooled by passing liquid hydrogen through channels in the outer wall. The space shuttle main engine combustion chamber is made of NARloy-Z, a Cu-3%Ag-0.5%Zr alloy that has excellent thermal conductivity and better strength than the commercially available Cu-0.15%Zr alloy known as AMZIRC. While a lighter combustion chamber would be welcome, developing hydrogen-resistant turbine materials would be a much better use of available rocket engine alloy development resources. SUMMARY AND RECOMMENDATIONS This document has attempted to review a range of materials that may have promise for all aspects of aerospace platforms. Because of the breadth of this topic, discussion has focused on opportunities and associated risks, but with little technical detail. Practical issues such as manufacturing capabilities and costs and the availability of materials also have been addressed where appropriate. Another recurring issue is the ability to achieve better design efficiency through a design synthesis process that concurrently treats form, fit and function, manufacturing capability, and materials capability as equally important design constraints. Although this approach has yet to be successfully used for a major high-performance structure project, engineering is nearing a state of development where it may now be feasible. If the space shuttle is used as a benchmark, it is clear that numerous opportunities will exist to improve structural efficiency. This possibility reflects the progress made since the shuttle and its engine were designed. Improved durability and reduced operating cost also are possible. The issue of long lead time holding back these improved design methods is most often one of timely availability of new, attractive materials that are mature enough to be used in a system without increasing the risk of failure. The materials community is actively developing and using computational models and simulation methods to address this concern. Despite some progress, further advances are required to bring computational materials engineering to the desired level of maturity. 1. Hatch, J. ed. Aluminum, Properties and Physical Metallurgy. American Society for Metals, 1984. 2. Chawla K. Composite Materials, Science and Engineering 2nd edition. Springer, 1998. 3. Baker, A., Dutton, S. and Kelly, D. Composite Materials for Aircraft Structures 2nd edition. AIAA, 2002. 4. Lutjering, G and Williams, J. Titanium, 2nd edition. Springer, 2007. 5. Sims, C. and Hagel, W. The Superalloys. Wiley, 1972. 6. Mishra, R. and Ma, Z. Friction Stir Welding and Processing, Materials Science and Engineering. v.50, pp. 1-78, 2005. 22 UNCLASSIFIED/ [REDACTED] Note: The header and footer read "UNCLASSIFIED//FOR OFFICIAL USE ONLY" with the "FOR OFFICIAL USE ONLY" portion struck through.