High Temperature Aluminum Alloys

From Northwestern University Center for Atom-Probe Tomography
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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.

Precipitation Strengthened Al-Sc Alloys

Marsha van Dalen and Richard Karnesky, co-advised by David Dunand

Two-beam bright field transmission electron micrograph of an Al(Sc) alloy after creep deformation. Dislocation loops are evident around the Al3Sc precipitates.

The Al-Sc system can be compared to the well-studied nickel-based γ-γ’ system; in both systems, the matrix is fcc and the precipitates have the L12 structure. The thermodynamically stable Al3Sc has a low overall lattice mismatch with Al and consequently the coarsening rate of nanoscale spherical L12 Al3Sc has been found to be slow, in keeping with the expected lower interfacial energy. These Al3Sc precipitates remain coherent to a diameter of 20 to 30 nm, since the lattice parameter mismatch is 1.25 % at room temperature.

However, the solubility of Sc in Al is quite small (0.21 at.%), which limits the attainable volume fraction of Al3Sc in these alloys. However, with only 0.75 vol.% of nano-size precipitates in a cast, heat-treated binary alloy, remarkably high creep resistance is attained. Furthermore, these Al3Sc precipitates are stable against long-term coarsening up to 350 °C.

Current research by Marsha involves ternary additions of Ti and other transition metals in solid solution within the Al3Sc precipitate, identifying how this affects their shearability and coarsening behavior. Through three-dimensional atom probe we also hope to identify (i) the partitioning of solute atoms between the matrix and the precipitates; (ii) the relative Gibbsian interfacial excess of solute atoms at the matrix/precipitate interfaces; (iii) the mechanism of precipitate formation; and (iv) the temporal evolution of the precipitates.

Marsha and Rick are both also studying the effects of adding various rare earth elements (Er, Dy, Er, Gd, Y, and Yb) to Al-Sc alloys.

Precipitation Strengthened Al-Zr Alloys

Keith Knipling, co-advised by David Dunand and Fine

Crystal structures of the L12, D022, and D023 trialuminide compounds.
Impurity diffusion of transition metals in Al.

Numerous transition elements form binary trialuminide compounds with Al, but very few crystallize in the L12 form. The group IV-A (Ti, Zr, Hf) and group V-A (V, Nb, Ta) elements form binary trialuminide compounds that crystallize with the tetragonal D022 or D023 structures. Additionally, Sc, Y, and the rare earth elements Er, Yb, Ho, Tm, Lu, and U form trialuminide compounds but of these, only Sc, Er, Yb, and U trialuminides crystallize in the L12 structure.

What is fortunate, however, is that these tetragonal crystal structures are closely related to the L12 structure and can be transformed to the high-symmetry cubic L12 crystal by alloying with fourth-period transition elements such as Cr, Mn, Fe, Co, Ni, Cu, and Zn. Furthermore, the intermetallic Al3Zr precipitates as a coherent metastable L12 form. Partially substituting Ti for Zr reduces the lattice mismatch of the L12 precipitate with the Al matrix, thereby reducing the barrier to nucleation, increasing the stability of the L12 phase, and very substantially delaying the transformation to the tetragonal phase. Finally, Zr is a much more sluggish diffuser in Al than Sc which should offer enhanced coarsening resistance since the kinetics of Ostwald ripening are mediated by volume diffusion, as the solute is transferred through the matrix from the shrinking particles to the growing ones.

Al(Sc) Alloys with Alumina Dispersoids

Richard Karnesky, co-advised by David Dunand

Building on the experiences developed in our earlier work on the basic binary Al(Sc) alloys (where precipitation strengthening is the only operative mechanism) and the two simple ternary alloys (where solid-solution strengthening is also active), Rick's research focuses on the concept of dual particle strengthening, where Al3Sc precipitates coexist with much larger (submicron-sized) alumina (Al2O3) dispersoids. Thus strengthening is expected to occur through two different populations of chemically-distinct particles. The basic science questions we seek to answer are the effects of this dual precipitate microstructure upon the coarsening kinetics of the precipitates, interfacial segregation, and creep properties of the alloys. Chesapeake Composites Corp. dispersion casts these alloys.