Predicting the quench sensitivity of Al-Zn-Mg-Cu alloys: A model for linear cooling and strengthening

In this work the quench sensitivity of Al-Zn-Mg-Cu alloys is studied through continuous cooling at constant rates of a range of alloys using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and hardness testing. The DSC, TEM and SEM data show that the cooling reactions are dominated by a high temperature reaction (typically similar to 450 degrees C down to similar to 350 degrees C) due mostly to S-Al2CuMg phase formation, a medium temperature reaction (similar to 350 degrees C down to similar to 250 degrees C) due predominantly tO eta-Mg(Al,Cu,Zn)(2) phase formation and a lower temperature reaction (similar to 250 degrees C down to similar to 150 degrees C) due to a Zn-Cu rich thin plate phase. A new, physically-based model is constructed to predict rates of all reactions, enthalpy changes and resulting yield strength in the artificially aged condition. The model incorporates a recently derived model for diffusion-controlled reactions based on the extended volume fraction concept as well as recent findings from first principles modelling of enthalpies of the relevant phases. The model shows a near perfect correspondence with data on all 6 alloys studied extensively by cooling DSC and hardness testing, and allows prediction of the influence of the 3 major elements and 3 dispersoid forming elements on quench sensitivity. (C) 2015 Elsevier Ltd. All rights reserved.