Acta Materialia 51 (2003) 4803–4814 www.actamat-journals.com Mechanical properties of Al(Sc,Zr) alloys at ambient and elevated temperatures Christian B. Fuller, David N. Seidman, David C. Dunand * Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, IL 60208-3108, USA Received 27 May 2003; received in revised form 27 May 2003; accepted 5 June 2003 Abstract This study investigates the mechanical properties of ternary Al(Sc,Zr) alloys containing 0.27–0.77 vol.% of Al 3 (Sc,Zr) precipitates with an average radius r = 2-24 nm. Microhardness values at ambient temperature follow predictions of the Orowan dislocation bypass mechanism, with a transition to the precipitate shearing mechanism predicted for r larger than 2 nm. Addition of Zr to binary Al(Sc) alloys delays the onset and kinetics of over-aging at 350 and 375 °C, but has little influence on the magnitude of the peak microhardness. Creep deformation at 300 °C is charac- terized by a threshold stress, which increases with r in the range 2–9 nm, in agreement with prior results for binary Al(Sc) alloys and a recently developed general climb model considering elastic interactions between dislocations and coherent, misfitting precipitates. At constant r and precipitate volume fraction, Zr additions do not significantly improve the creep resistance of Al(Sc) alloys.  2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Al–Sc–Zr alloys; Mechanical properties; Creep; Deformation mechanisms 1. Introduction Precipitation-strengthened binary Al(Sc) alloys are usable up to 300 °C due to the presence of elastically hard and coherent Al 3 Sc precipitates that form with an L1 2 -type crystal structure, a small lattice parameter misfit with Al (1.34% at 24 °C), a high melting temperature of 1320 °C, and a maximum solid solubility of 0.23 at.% Sc at the * Corresponding author. Tel.: +1-847-491-5370; fax: +1- 847-467-6573. E-mail addresses: cfuller@rwsc.com (C.B. Fuller); d-seid- man@northwestern.edu (D.N. Seidman); dunand@northwes- tern.edu (D.C. Dunand). 1359-6454/$30.00  2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1359-6454(03)00320-3 eutectic temperature of 660 °C [1–3]. The presence of Al 3 Sc precipitates has been shown to increase the creep resistance of coarse-grained binary Al(Sc) alloys [4–7]. In particular, Marquis et al. [5] examined the ambient- and elevated-tempera- ture strengthening mechanisms of these alloys. Among possible ternary alloying elements, Zr is known to increase the strength as well as the recrystallization resistance of Al(Sc) alloys by sub- stitution of Sc for Zr to form Al 3 (Sc 1-x Zr x ) precipi- tates with decreased coarsening kinetics (in com- parison to the coarsening kinetics of Al 3 Sc precipitates) [8–11]. Zirconium decreases the lattice parameter of Al 3 (Sc 1-x Zr x ), as observed experimentally by Refs.