Thermodynamics-Based Computational Design of Al-Mg-Sc-Zr Alloys G.N. HAIDEMENOPOULOS, A.I. KATSAMAS, and H. KAMOUTSI Alloying additions of Sc and Zr raise the yield strength of Al-Mg alloys significantly. We have studied the effects of Sc and Zr on the grain refinement and recrystallization resistance of Al-Mg alloys with the aid of computational alloy thermodynamics. The grain refinement potential has been assessed by Scheil–Gulliver simulations of solidification paths, while the recrystallization resistance (Zener drag) has been assessed by calculation of the precipitation driving forces of the Al 3 Sc and Al 3 Zr intermetallics. Microstructural performance indices have been derived, used to rank several alloy composition variants, and finally select the variant with the best combination of grain refinement and recrystallization resistance. The method can be used, with certain limitations, for a thermodynamics-based design of Al-Mg and other alloy compositions. DOI: 10.1007/s11661-009-0168-8 Ó The Minerals, Metals & Materials Society and ASM International 2010 I. INTRODUCTION WHILE it has been experimentally documented [1–3] that additions of Sc or combinations of Sc and Zr can raise significantly the yield strength of Al-Mg alloys, a sound thermodynamic analysis that would allow the design of these alloys is not available. This article presents a computational thermodynamics-based meth- od for assessing the effects of combined Sc and Zr additions on the grain refinement and recrystallization resistance of Al-Mg alloys. The method can be used for the selection of the optimum alloy variants, in an effort to reduce the time and costs related to the traditional empirical alloy development methodologies. Non-heat-treatable aluminum alloys are utilized in all of the major industrial markets for aluminum flat-rolled products. Transportation, packaging, and the building/ construction sectors represent the largest usage of non- heat-treatable sheet. Among them, wrought non-heat- treatable Al-Mg alloys are used, rather widely, as a structural material due to their good weldability, excel- lent corrosion resistance, and ductility. Recently there has been an increasing interest in Al-Mg alloys in the form of hot-rolled thick plates (8 to 15 mm) for marine applications. However, even alloys containing 5 to 6 pct Mg do not show adequate strength. These alloys are strengthened mainly by solid-solution strengthening from the Mg atoms and work-hardening during cold rolling. However, in hot-rolled products in which the work-hardening contribution is negligible, additional alloying is needed in order to improve the mechanical strength of the alloys. As already mentioned, there has been sound experi- mental evidence that Sc and Zr additions raise the yield strength of Al-Mg alloys. The strengthening mecha- nisms have been discussed [4,5] and it is clear that the strength improvements obtained are mainly due to the following: (1) grain refinement of the as-cast structure (modification effect) [2] and (2) inhibition of recrystalli- zation during hot working. [6] Precipitation strengthen- ing [4] has also been considered; however, its contribution is not as strong as the previous two mechanisms. All these effects are activated by the formation of fine dispersions of Al 3 Sc (in the case of alloying with Sc) and Al 3 Sc/Al 3 Zr or Al 3 (Sc x Zr 1–x ) (in the case of alloying with Sc and Zr) intermetallics. In the Al-Mg-Sc-Zr system under consideration, it has been found that the Al 3 Sc phase dissolves approximately 12 at. pct Zr, which corresponds to the substitution of almost 50 pct of the Sc lattice sites. [7] The grain-refining effect of Sc is attributed to the formation of primary Al 3 Sc during solidification. The reason for this effect is that the Al 3 Sc intermetallic, having a L1 2 structure with a lattice parameter of 0.4104 nm, very close to the lattice parameter of the Al-rich solid-solution matrix (a = 0.404 nm), can act as a heterogeneous nucleation site for the matrix Al-rich phase. [5] In Al-Sc alloys, the formation of Al 3 Sc takes place at Sc contents higher than 0.6 mass pct, which is higher than the eutectic composition. Similar conditions hold for the combined Sc/Zr alloying, with the addi- tional argument that Al 3 Zr precipitation occurs first from the melt and is then followed by Al 3 Sc precipita- tion on the Al 3 Zr particles. It is the combined Al 3 Sc/ Al 3 Zr particles that have the necessary critical size to act as nucleation sites for the solidification of the matrix phase. [2] G.N. HAIDEMENOPOULOS, Professor, and H. KAMOUTSI, Research Associate, are with the Laboratory of Materials, Department of Mechanical Engineering, University of Thessaly, Volos, Greece. Contact e-mail: hgreg@mie.uth.gr A.I. KATSAMAS, formerly Research Associate, Laboratory of Materials, Department of Mechan- ical Engineering, University of Thessaly, is with the Directorate of Environment and Land Planning, Region of Thessaly, Larissa, Greece. Manuscript submitted October 1, 2009. Article published online February 5, 2010 888—VOLUME 41A, APRIL 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A