Effect of pre-ageing on dynamic strain ageing in Al-Mg-Si alloys

The effect of pre-ageing (PA) on dynamic strain ageing (DSA) in Al-Mg-Si alloys with different Mg/Si ratios has been studied by using hardness measurements, tensile testing, three dimensional atom probe (3DAP), and scanning electron microscopy (SEM). The results show that a pre-ageing step prior to one week of natural ageing promotes DSA and gives rise to a reduced strain rate sensitivity (SRS) of the flow stress. Mechanisms for enhanced DSA are discussed in terms of the 3DAP results. It is concluded that the most likely mechanism for the observed decrease in the SRS is associated with the presence of excess vacancies promoted by pre-ageing. The excess vacancies may reside in vacancy-solute complexes (e.g. after pre-ageing at 100 °C for 5 min) or be trapped in clusters (e.g. after pre-ageing at 100 °C for 2 h).