Materials Science and Engineering A 475 (2008) 241–248 Thermal stability of Al 3 (Sc x ,Zr 1-x )-dispersoids in extruded aluminium alloys B. Forbord a,∗ , H. Hallem b,c , J. Røyset d , K. Marthinsen b a SINTEF Materials and Chemistry, N-7465 Trondheim, Norway b Norwegian University of Science and Technology, N-7491 Trondheim, Norway c Raufoss Technology & Industrial Management, N-2831 Raufoss, Norway d Hydro Aluminium R&D Sunndal, N-6600 Sunndalsøra, Norway Received 26 April 2006; received in revised form 11 April 2007; accepted 12 April 2007 Abstract In aluminium alloys the highest resistance against recrystallisation is obtained through combined additions of Sc and Zr. Upon annealing these elements form a high number density of homogeneously distributed and coherent Al 3 (Sc x Zr 1-x )-L1 2 -dispersoids, and the coarsening of these phases subsequently control the softening processes of recovery and recrystallisation. In this work the kinetics of Al 3 (Sc x Zr 1-x )-coarsening has been investigated during annealing of two precipitation annealed and extruded Al–(Mn)–Fe–Si–Sc–Zr-alloys at temperatures between 500 ◦ C and 600 ◦ C. A simple analysis based on widely used theories for volume diffusion controlled dispersoid coarsening (R 3 (t) - R 0 3 ∝ t) was used to identify the dominant mechanisms. It was found that coarsening in both alloys is controlled by volume diffusion of Zr, as activation energies, Q, of 285 ± 31 kJ/mol and 250 ± 43 kJ/mol were obtained, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Aluminium alloys; Extrusion; Zirconium; Scandium; Dispersoids; Recrystallisation 1. Introduction Second phase particles formed in aluminium during high temperature annealing (e.g. homogenisation/precipitation annealing) are usually referred to as dispersoids. One of the most important abilities of dispersoids is to stabilise the sub- grain structure by exerting a drag force (Zener-drag), P Z , on their boundaries [1,2], thus preventing recrystallisation and rapid strength loss during thermomechanical processing (e.g. extrusion) and subsequent annealing. The dispersoids are most efficient when they are coherent with the matrix, present at high volume fractions, f, and number densities, N, as well as being homogeneously distributed [3]. In materials intended for high temperature applications, it is also important that the dispersoids coarsen slowly, i.e. are thermally stable, in order to maintain the drag force on subgrain boundaries dur- ing heat exposure. If the volume fraction of dispersoids is high enough, subgrain growth may even stagnate completely [3], ∗ Corresponding author. Tel.: +47 98230463; fax: +47 73597040. E-mail address: borge.forbord@sintef.no (B. Forbord). and in such cases alloy softening is controlled by dispersoid coarsening. In aluminium alloys the most promising results have been obtained through additions of Zr and Sc. Since the solid sol- ubility of these elements decreases rapidly with temperature [4,5], a large supersaturation can be achieved if the cooling rate after casting is sufficiently high. The driving force for pre- cipitation of Sc- and Zr-containing dispersoids may in such cases be substantial even at high temperatures. A single addi- tion of Zr often results in the formation of metastable, coherent and cubic L1 2 –Al 3 Zr-dispersoids, and the low diffusivity of Zr ensures that these nm-sized particles coarsen relatively slowly. Due to microsegregation after casting, however, they are unfortu- nately found to be heterogeneously distributed after precipitation annealing of most aluminium alloys [6–11]. The structural sta- bility of alloys containing only Zr as a dispersoid-forming element is therefore limited, as regions with a low number den- sity will be prone to recrystallisation. Additions of only Sc, on the other hand, tend to lead to homogeneous distributions of stable L1 2 –Al 3 Sc-dispersoids as microsegregation of Sc is smaller due to similarities in the liquid (C l ) and maximum solid solubility (C max ), i.e. the partition coefficient of Al–Sc-alloys 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.04.054