Microstructure changes in a 2618 aluminium alloy during ageing and creep

Microstructure changes and precipitation processes in the 2618 aluminium alloy were investigated. Phase analysis of the initial state (T6/peak condition) confirmed that the majority of intermetallic phases belong to the phase Al9FeNi which improves significantly the mechanical properties of the alloy. The main strengthening phase Al2CuMg (S-phase) was distributed uniformly in the form of tiny plate-like precipitates (GPB II zones) of the size of several tens of nanometers in grain interiors. During the creep test simulating the conditions of the application of this alloy in devices for the exhaustion of hot gasses generated during the fire in closed or heavily accessible areas extensive precipitation occurred. The coherent S-phase transformed into numerous partially coherent needle-like S′ precipitates. The stable S-phase was formed mainly along grain boundaries and exists in two morphologies as plate-like precipitates, and/or irregular precipitates. Denuded precipitation-free zones exist along grain boundaries due to reduced concentration of alloying elements (Cu and Mg) and strong vacancy concentration gradient in the vicinity of grain boundaries. These zones have a detrimental effect on mechanical properties of the alloy.