Temporal evolution of the microstructures of aluminum(scandium, zirconium) alloys and their influences on mechanical properties

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 Al₃Sc_1−xZr_x (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 Al₃Sc_1−xZr_x precipitates, and Zr to segregate near the Al/Al₃Sc_1−xZr_x 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 Al₃Sc_1−xZr _x precipitates is found to improve the alloy strength, by pinning subgrain and grain boundaries, as shown by hardness, tensile, and fatigue measurements.