Low-Temperature Differential Scanning Calorimetry of an Al-Mg-Si Alloy

The clustering behavior at room temperature of a pure ternary Al-0.59 wt pct Mg-0.82 wt pct Si alloy was investigated by low-temperature differential scanning calorimetry (DSC). We find three clustering reactions that take place in two stages. The first two reactions are linked to each other and are completed after 1 hour. The third reaction starts around 1 hour after quenching and is completed after 2 weeks. Only the latter reaction exhibits a strong shift of the peak position of the thermal signal, indicating a change in the activation energy during aging at room temperature caused by changing solute supersaturation or increasing trapping of vacancies. The first two stages are closely linked to the known adverse effect of room-temperature preaging on the ensuing age-hardening step, since 60 to 80 pct of cluster formation is sufficient to establish the full negative effect.