dc.description.abstract
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.
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