The feasibility of using superplastic gamma titanium aluminum for the construction of lighter aerospace vehicles and more energy-efficient machines for power production has been demonstrated experimentally for the first time, according to researchers at the University of California, Davis.
Their findings will be presented Friday, Dec. 4, at the 1992 Materials Research Society meeting in Boston.
Superplasticity refers to an extreme malleability of specially treated fine-grained crystalline materials (like metals and ceramics) that allows them to be shaped into new components. Especially good for fabricating complex shapes, superplastic materials can be deformed at an elevated temperature until they are able to endure massive strain and then are shaped inside a mold by using gas pressure. The superplastic process offers a significant savings in materials costs and labor intensive machining costs compared with other conventional ways of tooling parts, such as cutting and drilling.
"Superplasticity is being investigated nowadays both for its scientific merit in the context of fundamental flow and failure mechanisms, as well as for its technological significance in forming operations," says Amiya Mukherjee, a professor of materials science at UC Davis. "Titanium aluminum is particularly promising because of its ability to withstand high temperatures, its strength, low density and resistance to oxidation."
Driving this research is the search for strong, light, heat-resistant materials for the proposed hypersonic National Aerospace Plane, anticipated to exceed 15 times the speed of sound in flight. The new materials can be used for the skin of the plane, which, at that speed, will literally glow with the heat produced by the aerodynamic friction. New materials may also make possible more energy-efficient airborne propulsion systems and terrestrial power plants.
Major leaps in the efficiency of power-producing engines have been blocked by the limited ability of metal parts to withstand the very high temperatures necessary for much more efficient combustion. Researchers hope some day to develop ceramic materials for engine parts. Unfortunately, while ceramics can withstand the highest temperatures of combustion, they are also so brittle that as engine parts they would be as dysfunctional as building a bicycle tire out of glass.
In the meantime, between metals and ceramics is a class of materials known as intermetallic compounds, which are more heat tolerant than metals and less fragile than ceramics. Gamma titanium aluminum is one the the strongest and lightest of the intermetallic compounds and can withstand sustained temperatures up to 1,000 degrees centigrade (significantly higher than what the materials now used in gas turbine engines can tolerate).
Making gamma titanium aluminum superplastic involves rendering the material malleable by means of a special thermomechanical treatment that produces a microstructural state so that the material can be shaped into its intended design form. Then, the material must be returned to its intrinsically strong molecular state before being put into service, so that it will not continue to deform under the stress of use.
The hard part, according to Mukherjee and research associates Henry Yang and Mikhail Zelin, was finding a way to return the crafted part to its original microstructural state after deformation.
Mukherjee will discuss the mechanisms of the deformation phenomenon, with an interpretation of the behavior at the microstructural level, on Friday, Dec. 4, at 2:40 p.m. at the superplasticity symposium.