On the precipitation kinetics of a wrought 6061 alloy during continuous heating

The precipitation sequence during continuous heating in a Mg-rich AlMgSi alloy containing minor additions of Cu is investigated. Differential scanning calorimetry (DSC) curves were acquired for wrought Al1.01 Mg0.61Si0.25Cu (wt%) specimens and the resulting peaks were studied using scanning transmission electron microscopy (STEM), atom probe tomography (APT) and hardness measurements to clarify the sequence of metastable phase precipitation. APT results indicated that the first exothermic reaction is dominated by the formation of Mg-Si co-clusters enriched in Mg. High-resolution TEM revealed that the initial Mg-Si precipitates are Guinier-Preston (GP) zones, with the peak of the first exotherm corresponding to an increase in their number density. STEM lattice imaging confirmed that these GP zones exhibit a single β ′′-eye structure. Analysis of the second exotherm showed that its onset belongs predominantly to precipitation of L phases while its peak is related to formation of β Cu ′ precipitates. Measurements showed that the continuous heating of the alloy from an as-quenched state to the peak of the first exotherm leads to a hardness increase from 61 HV to 98 HV, making the GP zones responsible for the hardening of the alloy under these tempering conditions. • Non-isothermal precipitation sequence in a Mg-rich 6061 alloy was clarified using DSC, STEM, and APT, identifying distinct exothermic stages linked to specific metastable phases. • APT analysis revealed that the first exothermic peak corresponds to Mg-Si co-clusters enriched in Mg, marking the early stage of GP zone formation. • High-resolution TEM and STEM confirmed that the initial precipitates are GP zones with single β'′-eye structures, serving as precursors to β'′ precipitates. • Hardness increased from 61 HV to 98 HV during continuous heating up to the first exothermic peak, indicating that GP zones are primarily responsible for hardening under these conditions. • The second exotherm was attributed to the precipitation of L and β Cu ′ phases, revealing a clear correlation between nanoscale precipitation and thermal-mechanical response in 6061 alloys.