Aircraft manufacturers are focusing on new engine styles that burn fuel more economically. These new lean-burning engines operate at temperatures that significantly exceed the safe operating levels of today’s nickel-based superalloy engine components. As a result, new materials such as titanium aluminide (TiAl) are emerging, which have higher thermal stability or specific creep properties, lower density (3.9 to 4.1 g/cm3) and higher specific strength than previous materials.
These alloys also have high specific yield strength (yield strength/density), high specific stiffness (modulus of elasticity/density), good oxidation resistance, resistance against titanium fire and good fatigue properties at high temperatures. For example, TiAl has strength equivalent to superalloys up to 760° C (1,400° F), yet the density of TiAl (4.0 g/cm3) is less than half of 718 Inconel’s density (8.2 g/cm3).
Replacing superalloy blades with lighter TiAl blades in the low-pressure turbine section of aerospace engines reduces weight and increases efficiency. The reduction in blade weight also allows the use of a smaller, nickel alloy supporting disc, which further reduces weight. TiAl alloys can provide weight reductions of up to 50% in low-pressure turbine stages, improving thrust-to-weight ratios, reducing fuel consumption and lowering exhaust emissions.
The high specific modulus or stiffness is valuable for components and assemblies with tight clearances, such as seal supports and linings. The high specific modulus also shifts acoustic vibrations to higher frequencies, which reduces fretting and fatigue in other structural areas. Conventional titanium alloys rubbing against other components at high temperatures (greater than 400° C or 752° F) can ignite in jet engines, causing titanium fires. TiAl is almost as resistant to titanium fires as superalloys, so superalloy barriers to stop titanium fires could be replaced with TiAl. After the success of TiAl in low-pressure (LP) turbine blades, aerospace engineers are also implementing TiAl in high-pressure (HP) compressor blades, vanes and blade dumpers.
However, the same properties that make TiAl desirable for future aircraft engines also make it difficult to work with. TiAl is an intermetallic compound with mixed metallic and covalent bonding. Intermetallic materials such as TiAl exhibit a combination of metallic and ceramic properties. They have the elevated temperature strength desirable for high-temperature applications, but they typically have low room temperature ductility, toughness and poor manufacturing characteristics. The good news: by studying these materials and redefining existing manufacturing processes, a cost-effective solution is within reach.
Read more: Grinding Solutions for Aerospace
