Mechanical properties enhanced by deformation-modified precipitation of θ′-phase approximants in an Al-Cu alloy

The influence of large pre-deformation on the age hardening behaviors, tensile properties and precipitate characteristics in an Al-Cu alloy during artificial ageing was investigated using mechanical property measurements and electron microscopy. Room-temperature rolling of Al-Cu alloy with a thickness reduction of 80% plus a subsequent artificial ageing can result in a strength increase of 35% without sacrificing ductility compared with an alloy that is peak-aged using a conventional heat treatment process (T6). A large quantity of dislocations that accumulated during cold-rolling was retained in the alloy after post-ageing at 150 °C. Although dense dislocation walls and dislocation tangles that exist in cold-rolled Al-Cu alloys were expected to cause micro-structural heterogeneity, the plate-shaped precipitates on the {001}Al planes exhibited an exceptionally uniform distribution. Atomic-scale angular dark-field scanning transmission electron microscopy (ADF-STEM) revealed these precipitates are θ′-approximants with a crystal structure similar with the well-known tetragonal θ′ phase but with the body-centered position not fully occupied by Cu. In addition, the precipitate sizes were considerably refined as compared to the θ′ precipitates that were formed in the T6 peak-aged alloys. The microstructure-property relationships in the alloy are discussed to explain the observations. Our results suggest the deformation-modified precipitation may play a vital role in achieving better strength-ductility synergy.