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-/theta' precipitates on dislocations. Two early-stage pre-theta' phases (pre-theta'-1 and pre-theta'-2) are frequently observed by atomic scale imaging. Pre-/theta' consists of three Cu layers with each separated by two {200}(Al) planes, while pre-theta'-2 has a regular structure in between two Cu layers with a distance of 2a(Al) (similar to 1.5c(theta')). Pre-existing dislocations have a profound impact on the phase selection during aging, resulting in the unusual occurrence of pre-theta'-1, pre-theta'-2 and 1.5 c(theta), thick theta'. The 1.5a(Al) (similar to 1c(theta)) thick hitherto unknown pre-theta' 3 and the theta'-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 theta'; (ii) formation of pre-theta'-1 and 2 c(theta), thick theta'; (iii) formation of pre-theta'-2 and 1.5 c(theta), thick There are striking structural similarities between pre-theta' phase and theta' 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.