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Karnesky, Richard A. |
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Mechanical Properties and Microstructure of Al–Sc with Rare-Earth Element or Al[sub:2]O[sub:3] Additions |
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2007 |
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258 |
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Al-Sc |
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Aluminum alloys strengthened with coherent (L12), nanosize Al3Sc precipitates are structural materials that have outstanding strength at ambient and 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 Al3Sc 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) Al2O3 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.% Al2O3.
In this thesis, the temporal evolution of Al–Sc–RE and DSC–Al–Sc(–Zr) alloys are measured using Local-Electrode Atom-Probe (LEAP) 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 solid solubility of Er in 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 Al3(Sc1-xErx) precipitate size and interprecipitate distance is studied by varying isochronal and isothermal aging treatments. Various models and simulations are compared to experimental data. At ambient temperatures, very small Al3(Sc1-xMx) precipitates contribute to order strengthening and larger (unshearable) precipitates are bypassed by dislocations through Orowan bowing. Dislocation dynamics simulations allow both processes to operate in a glide plane, where precipitate distributions may be gathered directly or be informed by LEAP tomography data. At elevated temperatures, the lattice parameter and modulus mismatches of Al3(Sc1-xMx) oppose both dislocation climb over Al3(Sc1-xMx) and dislocation detachment from Al2O3. |
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Northwestern University |
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Ph.D. thesis |
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Evanston, IL |
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