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

Abstract

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.

Microscopic Aspects of Artificial Ageing in Al-Mg-Si Alloys

“…The increase observed is much more pronounced in this study than in other work [22,23]. Only Resch et al [43] find a comparable increase to 231 ps after 2 weeks of AA at 180 °C. Over-ageing for 1 week at 180 °C lets precipitates coarsen and hardness correspondingly decreases but the value for 𝜏 1𝐶 remains high.…”

“…Here, 84% of all positrons are trapped by clusters or vacancies compared to less than 50% in alloy 4-4. In the literature, a similar value of ∼90% is given, however after even slower heating [43]. Therefore, alloy 6-8 contains more vacancies and clusters than alloy 4-4 after a similar short AA treatment.…”

“…Studies in the literature on similar alloys show a comparable behaviour [22,43]. Altogether, these ageing stages are not very interesting from the viewpoint of positron annihilation since the lifetime values obtained are rather insensitive to the microscopic configuration.…”

“…In accordance with Refs. [23,43], we assume that additional open volume associated with the partial incoherency of the precipitates formed at this stage causes the increase of 𝜏 2+3 . As the precipitation sequence involves a transition from early GP zones to β" and/or β', the degree of lattice mismatch and lack of coherency increases in this order.…”

Development of Precipitation Hardening Parameters for High Strength Alloy AA 7068

“…In addition to DSC, the quench-induced precipitation was analysed by DIL, which basically allows detection of precipitation reaction in Al alloys [17][18][19], although their volume effect is very small. Beyond that it enables an extension of the accessible cooling rates [9].…”