The kinetics of natural ageing in AlMgSi alloys （自然老化的动力学AlMgSi合金）.pdf

The kinetics of natural ageing in Al-Mg-Si alloys studied by positron annihilation lifetime spectroscopy 1,2 3 1 3 J. Banhart , M.D.H. Lay , C.S.T. Chang , A.J. Hill 1 Helmholtz Centre Berlin for Materials and Energy, Hahn-Meitner-Platz, 14109 Berlin, Germany 2 Technische Universität Berlin, Materials Science and Technology, Hardenbergstr. 36, 10623 Berlin, Germany 2 CSIRO Materials Science and Engineering, Clayton, Victoria 3169, Australia The process of natural ageing in pure ternary Al-Mg-Si alloys was studied by positron annihilation lifetime spectroscopy in real time in order to clarify the sequence and kinetics of clustering and precipitation. It was found that natural ageing take place in at least 5 stages in these alloys, four of which were directly observed. This is interpreted as the result of complex interactions between vacancies and solute atoms or clusters. One of the early stages of positron lifetime evolution coincides with a clustering process observed by differential scanning calorimetry (DSC) and involves the formation of a positron trap with around 0.200 ns lifetime. In later stages, a positron trap with a higher lifetime develops in coincidence with the DSC signal of a second clustering reaction. Mg governs both the kinetics and the lifetime change in this stage. Within the first 10 minutes after quenching, a period of nearly constant positron lifetime was found for those Mg-rich alloys that later show an insufficient hardness response to artificial ageing, the so-called ‘negative effect’. The various processes observed could be described by two effective activation energies that were f