Precipitation processes in Al-Mg-Si extending down to initial clustering revealed by the complementary techniques of positron lifetime spectroscopy and dilatometry

The design of specific material properties of aluminum alloys demands for a detailed understanding of clustering and precipitation processes occurring during heat treatments. Positron lifetime spectroscopy in combination with high-precision dilatometry measurements were taken, allowing for a comprehensive analysis of the aging mechanisms occurring on different timescales and in different temperature regimes, during artificial aging. From the results, unambiguous experimental evidence for the following three main steps of the precipitation process is obtained. In the first seconds of artificial aging, a competitive process of dissolution and growth of different cluster types occurs. Subsequently, clusters start to transform into coherent precipitates, which are mainly responsible for the hardening effect. For prolonged artificial aging, the number density of the coherent precipitates increases, while positron lifetime spectroscopy already reveals the simultaneous formation of less coherent precipitates.