Creep of Dispersion-Strengthened Light Alloys

There are many practical examples where materials are required to survive for long periods under load at high temperatures - a classic example is in the turbine blades of jet engines. In the interest of improved efficiency and performance it is desirable to maximize the operating temperatures while minimizing overall weight. An aluminum alloy with high temperature capability, therefore, is an attractive alternative to the titanium-based components currently used in the frontal sections of jet engines.

Based on the behavior of nickel-base superalloys, which resist degradation of mechanical properties to approximately 75% of their absolute melting temperature, it is conceivable that aluminum-based alloys could be similarly developed which would be useful to 400 °C. As is true for γ’ in the nickel-base systems, a high-temperature aluminum alloy must contain a large volume fraction of a suitable dispersed phase, which must be thermodynamically stable at the intended service temperature.

The creep resistance of nickel-base superalloys is achieved by the presence of ordered Ni3(Al,Ti) precipitates. These precipitates, usually termed γ’, have the cubic L12 structure and are therefore isomorphous with the fcc Ni-alloy matrix (called the γ phase). The low mismatch in the lattice parameter between the γ matrix and the γ’ precipitates confer particle stabilities well beyond the levels possible with precipitates having a high particle/matrix interfacial energy, and hence these stable precipitates are effective barriers to dislocations at elevated temperatures.

An effective high temperature aluminum alloy should exhibit a similar structural constitution. Trialuminide intermetallic compounds (Al3X) have many attractive characteristics, such as low density, high specific strength, good heat resistance and excellent oxidation resistance. Therefore, they are excellent candidates for use as dispersoids or precipitates in the design of high strength Al alloys for high temperature applications. We investigate microstructure and creep properties of binary and ternary Al-Sc-X, Al-Ti-X and Al-Zr-X alloys with nanoscale, coherent, coarsening-resistant precipitates. Additions of submicron alumina dispersoids are also investigated.