Long term ageing of alloy 2618A
The aluminium alloy 2618A is an Al-Cu-Mg alloy with additions of Fe and Ni, which is designed for long-term operation at elevated temperature in transportation and aerospace industries. The alloy degrades due to coarsening and dissolution of GPB-zones and coarsening of S-Phase precipitates (Al2CuMg). This work investigates the overaging process at the application relevant temperatures of 190 °C for durations ranging from 10 min up to 25000 h. The nm-sized precipitates were selectively imaged along the <001>a direction of the Al matrix using dark-field transmission electron microscopy. The resulting images were then evaluated and allow the statistically relevant determination of radii distributions and the mean radius of the precipitates dependent on temperature and heat treatment duration. Ostwald ripening of the S-phase does not describe the ageing process adequately. Instead, the evolution of GBP zones during the initial ageing regime needs to be considered in addition to the coarsening process of the S-phase. DSC measurement were performed to determine the presence of GPB zones in different ageing states. This will allow a quantitative description of the ageing process and a comparison to samples aged under external load.