Modelling of strengthening in Al-Cu-Mg alloys during isothermal and non-isothermal treatments

Abstract

This present work consists of developing and testing a model for the prediction of
precipitation kinetics and strengthening in Al-Cu-Mg alloys with composition in the
. + S phase field. The model is applied to a range of conditions including isothermal
and non-isothermal treatments. The non-isothermal treatments include controlled slow
heating and cooling cycles and rapid heating and cooling cycles as experienced during
fusion welding. In these Al-Cu-Mg alloys the Cu:Mg ratio is close to 1 and the Cu:Mg
co-clusters and the S phase precipitates are the dominant strengthening phases. The
model consists of two integrated modules, one for the prediction of the
microstructural evolution of the Cu:Mg co-clusters and the S phase precipitates and
the other for the prediction of yield strength or hardness.
The modelling of precipitation kinetics of S phase is based on the Kampmann and
Wagner (KW) numerical model. The major predictions of the microstructural model
are the volume fraction and average radius of the S phase precipitates and the volume
fraction of the Cu:Mg co-clusters evolving during the isothermal and non-isothermal
treatments. The modelling of the thermal profile representing fusion welding is based
on the Rosenthal’s thin plate solution for two dimensional heat flow.
In the strength model the total critical resolved shear stress (CRSS) of the grains is
evaluated by including contributions from the precipitates, solid solution, dislocations
and the aluminium matrix. The strengthening due to the Cu:Mg co-clusters is based
on the modulus strengthening mechanism and the strengthening due to the S phase
precipitates is modelled using the Orowan looping mechanism. The predicted CRSS is
then converted to yield strength and hardness data in order to compare with the
experimental results.
The testing of the model predictions is carried out by experimental data on 2024
T351 aluminium alloys. Some of the experimental data has been taken from other
published works. The model is tested not only by the strength and hardness data but
also by heat flow measured by the calorimetry experiments and the S phase average
size measured from the transmission electron microscopy (TEM) micrographs.
The predictions of the model correspond well with the experimental results for all
the three models (one isothermal and two non-isothermal).

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