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ALUMINUM alloys provide a significant weight advantage over steels, which render them desirable for aerospace, armor, and transportation applications where weight has a measurable effect on performance and efficiency. Traditional Al alloys have limited use due to their moderate yield strength.[1,2] Strengthening of Al alloys will expand their applicability and increase their use as light-weight substitutes for steel or possibly enable thinner components to minimize weight.[3,4] It is well established that solid solution strengthening from Mg contributes to the strength of Al-Mg alloys. Mg atoms substitute for Al, creating a spherical strain field around the solute atoms. Moving dislocations interact with the strain fields resulting in the elastic interaction energy, which is a result of size and modulus differences between the Al matrix and the Mg solute atoms. Mg

TAMMY J. HARRELL, Recent Graduate with M.S., TAO HU, Postdoctoral Researcher, JULIE M. SCHOENUNG, Professor, and ENRIQUE J. LAVERNIA, Distinguished Professor, are with the, Department of Chemical Engineering and Materials Science, University of California, Davis, Davis CA. Contact e-mail: lavernia@ ucdavis.edu TROY D. TOPPING, Assistant Professor, is with the Department of Mechanical Engineering, California State University, Sacramento, Sacramento, and also Assistant Adjunct Professor with the Department of Chemical Engineering and Materials Science, University of California. HAIMING WEN, Postdoctoral Researcher, formerly with the Department of Chemical Engineering and Materials Science, University of California, Davis, is now with the Idaho National Lab, Idaho Falls, ID. Manuscript submitted October 9, 2013. Article published online September 30, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A

concentrations up to about 5 wt pct in Al alloys have been found to remain in solid solution and effectively increase strength and work hardening while resisting susceptibility to stress-corrosion.[5,6] Higher concentrations of Mg in Al alloys results in stress-corrosion and precipitation of the Al3Mg2 b phase, which has no significant strengthening effect and can degrade corrosion response.[5] Sc additions to Al and Al-Mg alloys are known to increase the strength in coarse-grained (CG) and ultrafine-grained (UFG) materials by forming a high density of fine, coherent Al3Sc precipitates with an ordered L12 structure.[3,7,8] Despite the low solid solubility of Sc in Al [0.35 wt pct at the eutectic temperature of 938 K (665 C)], heat treatments of supersaturated Al-Sc and Al-Mg-Sc alloys yield nanometer-sized Al3Sc precipitates at grain interiors. The precipitates facilitate strengthening by Orowan and precipitate-cutting mechanisms, making Sc the most potent strengthener in Al on a per atom basis.[7,9–14] Prior studies report an increase in yield strength of up to 200 MPa for Sc additions ranging from 0.1 to 0.6 wt pct.[3] In addition to the dispersion strengthening effect provided by Sc-containing precipitates and Mg atoms in the lattice, grain boundaries (GBs) can provide