Kinetics of natural aging in Al-Mg-Si alloys studied by positron annihilation lifetime spectroscopy

摘要

The process of natural aging in pure ternary Al-Mg-Si alloys was studied bypositron annihilation lifetime spectroscopy in real time in order to clarifythe sequence and kinetics of clustering and precipitation. It was found thatnatural aging takes place in at least five stages in these alloys, four ofwhich were directly observed. This is interpreted as the result of complexinteractions between vacancies and solute atoms or clusters. One of the earlystages of positron lifetime evolution coincides with a clustering processobserved by differential scanning calorimetry (DSC) and involves the formationof a positron trap with \sim 0.200 ns lifetime. In later stages, a positrontrap with a higher lifetime develops in coincidence with the DSC signal of asecond clustering reaction. Mg governs both the kinetics and the lifetimechange in this stage. Within the first 10 min after quenching, a period ofnearly constant positron lifetime was found for those Mg-rich alloys that latershow an insufficient hardness response to artificial aging, the so-called"negative effect." The various processes observed could be described by twoeffective activation energies that were found by varying the aging temperaturefrom 10 to 37\degree C.