Precipitate evolution and related strengthening during long term ageing of 2618A aluminium alloy

The 2618A alloy is used for aerospace applications where high resistance to long-term thermal ageing is required. Being able to predict the evolution of mechanical properties during long-term ageing is therefore crucial to determine the end-of-life of these products. It requires an in-depth understanding of the evolution of hardening precipitates during extended ageing, which happens along a complex sequence of metastable and stable phases. Here we used a high-throughput experimental methodology to gather systematic microstructure and related mechanical properties data resolved in time and temperature, using samples aged within a temperature gradient up to 10,000 h. Hardness maps were used to monitor the evolution of the mechanical properties along samples aged in this temperature gradient. Local transmission electron microscopy (TEM) with scanning precession electron diffraction (SPED) and atom probe tomography (APT) were used to identify the nano-precipitates for selected conditions, while systematic small angle X-ray scattering (SAXS) was used to evaluate the evolution of sizes and volume fractions of the precipitates along the graded samples. Our results evidence for the first time the important contribution of Si-containing L-phase to the initial strength of the T851. In the first stages of long-term ageing up to 5,000h at 200°C the strength loss is controlled by the partial dissolution of this L phase together with coarsening of S precipitates. Subsequently, the Q phase appeared to form, which destabilised partly the S phase and results in an accelerated loss of mechanical properties.