The enhanced theta-prime (θ′) precipitation in an Al-Cu alloy with trace Au additions

Linking the atomic level kinetic precipitation pathways induced by elemental additions to the resulting microstructure is fundamentally desirable for the design of new classes of light alloys. Aberration-corrected scanning transmission electron microscopy (AC-STEM) and first principles calculations were used to investigate the influence of trace Au (200 ppm) additions on precipitation in an Al-Cu-Au alloy. These Au additions resulted in a significant enhancement of the low-temperature age hardening, which was demonstrated to be associated with accelerated precipitate nucleation and growth. Atomic-resolution annular dark field (ADF) imaging showed the clearly reduced critical length and thickness of θ′ precipitates with Au additions, therefore accelerating the nucleation of θ′. Agglomerated Au clusters were observed in θ′ precipitates, which were demonstrated to correspond to a localised energetically favourable state. These observations have been explained through first-principles calculations and relevant thermodynamic modelling. This work provides a potential way to refine the alloy microstructure for improving the mechanical behaviour of light alloys.