Mechanical Properties and Microstructure of Al-Sc with Rare-Earth Element or Al2O3 Additions

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Aluminum alloys strengthened with coherent (L1<sub>2</sub>), nanosize Al<sub>3</sub>Sc precipitates may be used as structural materials at elevated temperatures. They are creep resistant at 300°C and exhibit a threshold stress, below which creep is not measurable. Introducing ternary alloying additions, such as rare-earth elements (RE=Y, Dy, Er), that segregate within Al<sub>3</sub>Sc precipitates improves this creep resistance by increasing the lattice parameter misfit of precipitates with Al. In this thesis, Al-600 Sc-200 RE and Al-900 Sc-300 Er (at. ppm) are studied. These elements are an order of magnitude less expensive than Sc, so reduce alloy costs. As an alternative or supplement to ternary additions, submicron (incoherent) Al<sub>2</sub>O<sub>3</sub> dispersoids impart additional strengthening. The dispersion-strengthened cast alloys, DSC-Al-1100 Sc and DSC-Al-800 Sc-300 Zr, studied in this thesis contain 30 vol.% Al<sub>2</sub>O<sub>3</sub>. The temporal evolution of Al-Sc-RE and DSC-Al-Sc(-Zr) alloys are measured using local-electrode atom-probe tomography, conventional transmission electron microscopy, and electrical conductivity. These techniques measure the changes in precipitate number density, size, volume fraction, chemical composition, and interprecipitate distance and are compared to models. They are also employed to measure the diffusivity and maximum solubility of Er in α-Al in Al-300 Er, Al-450 Er, and Al-600 Er. The mechanical behavior (microhardness, yield, and creep) of the alloys is studied at 25, 300, and 350°C. The effect of Al<sub>3</sub>(Sc<sub>1-x</sub>Er<sub>x</sub>) precipitate size and interprecipitate distance is studied by varying aging treatments. Various models and simulations are compared to experimental data. These include using experimentally-determined microstructures in a recent dislocation dynamics simulation and a novel model to explain the strengthening that is measured from both populations of strengthening phases in DSC-Al-Sc(-Zr).

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  • 08/27/2018
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