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Fuller, Christian B. |
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Temporal Evolution of the Microstructures of Al(Sc,Zr) Alloys and Their Influences on Mechanical Properties |
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2003 |
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179 |
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Al-Sc |
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Al(Sc) alloys represent a new class of potential alloys for aerospace and automotive applications. These alloys have superior mechanical properties due to the presence of fine, coherent, unshearable Al3Sc precipitates, which form upon the decomposition of an supersaturated Al(Sc) solid-solution. Additions of Zr to Al(Sc) are found to improve alloy strength and coarsening resistance, but the operating mechanisms are not well understood.
In this thesis, the relationships between the mechanical and microstructural properties of Al(Sc,Zr) alloy are presented. Three-dimensional atom probe microscopy (3DAP) and conventional and high-resolution transmission electron microscopies (CTEM and HREM) are utilized to study the temporal evolution of Al3Sc1-xZrx (L12 structure) precipitates in dilute Al(Sc,Zr) alloys (precipitate volume fractions < 1%) aged between 300 and 375°C.
Concentration profiles, obtained with 3DAP, show Sc and Zr to partition to Al3Sc1-xZrx precipitates, and Zr to segregate near the Al/Al3Sc1-xZrx interface. CTEM and 3DAP are utilized to determine the temporal evolution of Al(Sc,Zr) alloys, which is discussed employing diffusion-limited coarsening theories. Zirconium additions are found to retard the precipitate coarsening kinetics and stabilize precipitate morphologies.
Mechanical properties of Al(Sc,Zr) alloys are investigated utilizing Vicker’s microhardness and creep. Deformation at ambient-temperature is explained by classic precipitation-strengthening mechanisms, where a transition between
precipitate shearing and Orowan looping is calculated to occur at an average precipitate radius, <r>, of 2-3 nm. Al(Sc,Zr) alloys deformed by creep at 300°C are found to exhibit a climb controlled threshold stress, which is shown to increase with <r>, in agreement with previous results in Al(Sc) alloys and a previous general climb model considering the interaction between dislocations and coherent misfitting precipitates. Finally, the effect of various heat-treatments upon the microstructure and mechanical properties of a rolled 5754 aluminum alloy modified with 0.23 wt.% Sc and 0.22 wt. % Zr are investigated. The presence of the Al3Sc1-xZrx precipitates is found to improve the alloy strength, by pinning subgrain and grain boundaries, as shown by hardness, tensile, and fatigue measurements. |
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Northwestern University |
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Ph.D. thesis |
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NU @ karnesky @ |
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147 |
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Krug, Matthew E. |
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Microstructural Evolution and Mechanical Properties in Al-Sc Alloys With Li and Rare Earth Additions |
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2011 |
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372 |
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Al-Sc, Al-Li, Al-Li-Sc, rare earth, LEAP, atom probe |
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Aluminum-scandium alloys have excellent mechanical properties at ambient and elevated temperatures due to the presence of coherent, nano-scale, L12-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 Al3(Sc1-x-yLixYby) 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, α′–Al3(Li,Sc,Yb) precipitates, and δ′–Al3Li precipitates. |
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Northwestern University |
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Ph.D. thesis |
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NU @ m-krug @ |
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11430 |
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Harada, Y.; Dunand, D.C. |
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Microstructure and Creep Properties of Al[sub:3]Sc with Ternary Transition Metal Additions |
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Book Chapter |
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2001 |
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The 7th International Conference on Creep and Fatigue at Elevated Temperature (Creep VII) |
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219-226 |
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Al-Sc |
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Japan Society of Mechanical Engineers |
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NU @ karnesky @ |
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1904 |
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