| Aluminum-scandium alloys have excellent mechanical properties at ambient and elevated temperatures due to the presence of coherent, nano-scale, L1 2-ordered Al3Sc precipitates. In this thesis, a variety of Al-Sc alloys with additions of Li and RE elements, primarily Yb, are studied. An addition of ytterbium reduces the cost of Al-Sc alloys by replacing some of the more-expensive Sc. Lithium is a unique alloying addition to Al-Sc alloys, because it has significant solubility in both the matrix and precipitate phases. Lithium also provides solid solution strengthening, and a large strengthening increment on aging through the formation of Al3Li precipitates. The effects of these alloying additions on Al-Sc alloys are investigated in detail, and discussed in the context of physical models linking the microstructure to measured mechanical properties.;The alloys undergo a variety of aging treatments between 170--450 °C, producing a range of precipitate distributions. Their aging response is assessed using Vickers microhardness to monitor ambient-temperature strength, and electrical conductivity to monitor the progress of the precipitation reaction. The alloys are creep-tested in compression at 300 °C, and exhibit threshold stresses, below which no measurable creep occurs. Detailed microstructural investigations rely primarily on local electrode atom probe tomography, as well as transmission electron microscopy. The volume fractions, number densities, and chemical compositions of precipitates are measured at the nano-scale, and their size and spatial distributions are quantitatively determined.;Compared to binary Al-Sc alloys, Al-Li-Sc and Al-Li-Sc-Yb alloys contain a finer distribution of alpha'-Al3(Sc1-x-yLixYb y) precipitates at a greater number density and volume fraction, as well as solid-solution strengthening in the Al(Li) matrix, all of which lead to a greater peak strength at ambient-temperature. Because partitioning of Li to the precipitates results in a smaller lattice parameter mismatch with the matrix, a Li addition is detrimental to the elevated temperature strength of Al-Sc alloys, but this effect is mitigated if additions of both Li and Yb are made. A model for threshold stresses at elevated temperature semi-quantitatively captures experimentally-observed trends in threshold stress data in Al-Sc-X alloys. Dislocation dynamics simulations on directly-measured precipitate arrangements lead to a rule for superposition of strength contributions from dissolved solutes, alpha'-Al3(Li,Sc,Yb) precipitates, and delta'-Al 3Li precipitates. |
