Ageing evolution process of the θ′ observations and first-principles calculations-phase in Al-Si-Cu-Mg alloys: Atomic-scale observations and first-principles calculations

This work characterizes the high temperature (above 200 C) ageing process of the theta ' phase in Al-Si-Cu-Mg alloys by using atomic-resolution HAADF-STEM and first-principles calculations. By characterizing the morphology and coarsening growth process of the theta '(P) and theta '(P1) precursor, experimental observations suggest that the theta ' P phase is based on GPI (Guinier-Preston I) growth expansion. With the ageing time increases and the atomic concentration changes, the Cu atoms on the outside of the theta ' (P) phase rearrange to form the theta '(P1) phase. Subsequently, the theta '(P2 )precursor phase is formed locally in the theta '(P1 )phase as the Cu atoms gradually enter the theta '(P1) phase. Besides, it has been observed experimentally that the theta '(P2) precursor phase can be directly transformed into the theta ' phase without changing the morphology, structure and orientation relationships of the phase. Based on the experimental characterization and theoretical calculations of this work, we have proposed a novel ageing precipitation sequence: SSSS (Super Saturated Solid Solution)-> GPI ->theta '(P)->theta '(P1)->theta '(P2)->theta '