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E. Povoden-Karadeniz, P. Warczok, P. Lang, A. Falahati, M. Ahmadi, E. Kozeschnik:
"Thermodynamic modeling of metastable phases in Al-Cu-Mg-Si and applications to precipitation kinetics simulations";
Talk: Calphad XL, Rio de Janeiro; 2011-05-22 - 2011-05-27.

Due to their good hardenability response by nano-scale metastable precipitates, Al-alloys of the Al-Cu-Fe-Mg-Mn-Si system, such as AA2024 (Al-Cu), AA6016 (Al-Mg-Si), AA6082 (Al-Mg-Si-Fe-Mn), are widely used as high-strength materials in many industrial applications. Whereas the stable phases of the Al-Cu-Fe-Mg-Mn-Si system, such as the AlFeSi-phases [1] and the Q-phase [2] have been assessed recently, a systematic CALPHAD assessment of metastable phases is not yet available. In this paper, we present thermodynamic models of metastable Mg-Si co-clusters, GP-zones, ´´-, ´-, ´-, -, ´´-, and ´-phases, which are successfully tested in thermo-kinetic precipitation simulations. Particular focus is put on the model development for disordered co-clusters and coherency strain-induced ordered particles (GP-zones) at the very early stages of precipitation (low temperatures from room temperature up to about 180°C). Specific properties of these early structures, such as their affinity to vacancies that form during quenching after solution treatment, obviously influence the precipitation of hardening phases like ´´ at higher temperatures. MgSi co-clusters and GP-zones are almost fully coherent with the fcc Al-Matrix. Hence, in a first approximation these structures are associated with the model description used for the fcc Al-Matrix: Mg-Si co-clusters are simply described as highly metastable Mg-Si solid solution (Mg,Si)(Va), and GP-zones are treated as an fcc-based ordered phase by using a split-model with 4 substitutional sublattices. For Al-Mg-Si GP-zones, the preferred sublattice occupancy then reads (Al)(Al)(Mg)(Si)(Va), analogous to the L10 structure in the case of chemical ordering. The crystallographic Mg-Si layering proposed by Matsuda et al. [3] can be reproduce. The thermodynamic model parameters of Mg-Si co-clusters are based on new energy data determined by first-principles and optimized with experimental differential scanning analysis data. Subsequent thermo-kinetic test-simulations aim at giving best reproduction of experimental particle sizes and number densities determined after various heat treatments [4].
[1] J. Lacaze, L. Eleno, B. Sundman, Metal Mater Trans A 2010;41:2208.
[2] K. Chang, S. Liu, D. Zhao, Y. Du, L. Zhou, L. Chen, Thermochim Acta 2011;512:258.
[3] K. Matsuda, H. Gamada, K. Fujii, Y, Uetani, T. Sato, A. Kamio, S. Ikeno, Metall Mater Trans A 1998;29:1161.
[4] A. Falahati, E. Povoden-Karadeniz, P. Lang, P. Warcok, E. Kozeschnik, Int J Mater Res 2010;101:1089.