Multiple precipitation reactions and formation of θ'-phase in a pre-deformed Al-Cu alloy

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are applied to investigate the influence of 2% pre-deformation on precipitation in an Al-Cu alloy. This pre-deformation results in a significant hardness increase and an enhanced age hardening response, which is demonstrated to be associated with the heterogeneous formation of pre-/θ' precipitates on dislocations. Two early-stage pre-θ' phases (pre-θ'−1 and pre-θ'−2) are frequently observed by atomic scale imaging. Pre-θ'− 1 consists of three Cu layers with each separated by two {200}Al planes, while pre-θ'− 2 has a regular structure in between two Cu layers with a distance of 2aAl (~ 1.5cθ'). Pre-existing dislocations have a profound impact on the phase selection during aging, resulting in the unusual occurrence of pre-θ'− 1, pre-θ'− 2 and 1.5 cθ' thick θ'. The 1.5aAl (~ 1cθ') thick hitherto unknown pre-θ'− 3 and the θ'-approximant phase reported before in aged Al-Cu alloy with large pre-deformation also exist but with rare occurrence. The unveiled precipitation scenario leads us to sort out different precipitation reactions at dislocations: (i) direct formation of thick θ'; (ii) formation of pre-θ'− 1 and 2 cθ' thick θ'; (iii) formation of pre-θ'− 2 and 1.5 cθ' thick θ'. There are striking structural similarities between pre-θ' phase and θ' with the same thickness, while the only difference is the atom rearrangement inside the two stable interfacial Cu layers on {002}Al planes. This work provides not only new insights into the effect of pre-deformation on the precipitation behaviors of Al-Cu alloys, but also a guide to tailor the microstructure to achieve desirable mechanical behaviors.