In situ evaluation of the microstructure evolution during rapid hardening of an Al–2.5Cu–1.5Mg (wt.%) alloy Original Research Article

The kinetics of the microstructural evolution during ageing at 200 °C of an Al–2.5%Cu–1.5%Mg (wt.%) alloy is evaluated using nuclear magnetic resonance and in situ small-angle X-ray scattering. This alloy is known to exhibit “rapid hardening” upon artificial ageing, where 50–70% of the total age hardening increment is reached within minutes at elevated temperatures. It is shown that formation of small solute-rich entities which correspond to Cu–Mg clusters or GPB zones occurs within seconds at 200 °C. These entities have radius ∼0.5 nm, volume fraction ∼2% and Cu concentration ∼30 at.%. Within the stage of constant yield strength (rapid hardening plateau) at this temperature, they are shown to be the dominant constituent of the precipitate microstructure and to be extremely stable in terms of size and volume fraction. The S phase is observed to nucleate within the hardening plateau, but remains a minor constituent until the end of this stage. Based on the quantitative measurement of their size and volume fraction, the obstacle strength of these solute-rich clusters is calculated to be about one-tenth of the Orowan strength. The energy required to shear these clusters (∼0.5 eV) is shown to be compatible with existing atomistic calculations of similar objects.