Solute clustering behavior between 293K and 373K in a 6082 Aluminum alloy

We present a systematic study of the effect of pre-ageing between 293 K and 373 K in a 6082 Aluminum alloy using mainly differential scanning calorimetry in combination with Vickers hardness and transmission electron microscopy. The pre-ageing effects are characterized through the modifications of the β″ precipitation DSC signal. Pre-ageing at 293 K leads to a progressive increase of the peak temperature, until it reaches a plateau at long aging times (≥250min), while pre-ageing at 373 K leads to a continuous decrease of the peak temperature. The results indicate the formation of two different types of clusters, named 1 and 2, below and above a pre-ageing transition temperature, determined to be around 343 K. After pre-ageing below 343 K, the dissolution of clusters 1 superimposed with the β″ precipitation occurs, and the enthalpy change remains almost independent of pre-ageing time. Instead, pre-ageing above 343 K leads to the formation of type 2 clusters that transform on heating into β″ phase precipitates, with a decreasing enthalpy change as pre-ageing time increases. This result indicates that clusters 2 evolve during pre-ageing, reaching a structure/composition with formation enthalpy similar to that of β″ phase. Vickers hardness reveals the different evolution of both types of clusters with aging time. Transmission electron microscopy observations on samples subsequently aged at 453 K, show that type 2 clusters favour the nucleation of β″ precipitates, while type 1 clusters hamper such nucleation process.