Quench sensitivity of Al-Mg-Si and Al-Zn-Mg-Cu alloys. Part 2: predictive modelling

Starink, Marco J, Milkereit, Benjamin, Zhang, Yong and Rometsch, Paul A (2016) Quench sensitivity of Al-Mg-Si and Al-Zn-Mg-Cu alloys. Part 2: predictive modelling. Thermec 2016: International Conference on Processing & Manufacturing of Advanced Materials, Graz, Austria. 28 May - 02 Jun 2016.

Abstract

During quenching of medium to high strength heat-treatable Al based alloys precipitation on defects such as grain boundaries is in practice nearly unavoidable, and the quench sensitivity generally increases with increasing content of main alloying elements. Also the minor alloying elements Zr, Mn and Cr, which form intermetallic particles of sizes typically in the range of 10-100 nm (termed ‘dispersoids’), strongly influence the quench sensitivity as those particles act as nucleation sites for the quench-induced phase. The latter particularly holds for incoherent dispersoids, whereby the dispersoids typically lose their coherence due to (local) recrystallization. A new model for quench-induced precipitation and the resulting yield strength in age hardened condition for Al-Mg-Si (6xxx) and Al-Zn-Mg (7xxx) alloys has been derived. The new model particularly incorporates recent improved models for diffusion-controlled reactions, the advances in modelling of the thermodynamics of complex alloy systems, as well as computationally efficient schemes for integrating these components. For the validation of the model, a range of experimental data is used which covers the microstructure on a range of length scales, the thermodynamics of the reactions and the resulting mechanical properties. This includes transmission electron microscopy (TEM), high resolution TEM (HRTEM), scanning electron microscopy, and high resolution fast and slow differential scanning calorimetry. The model shows a near perfect correspondence with data on all (>10) alloys studied extensively, and allows prediction of the influence of the major alloying elements and 3 dispersoid forming elements on quench sensitivity.

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