Ageing evolution process of the θ′-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 θ′ 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 θP′ and θP1′ precursor, experimental observations suggest that the θP′ 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 θP′ phase rearrange to form the θP1′ phase. Subsequently, the θP2′ precursor phase is formed locally in the θP1′ phase as the Cu atoms gradually enter the θP1′ phase. Besides, it has been observed experimentally that the θP2′ precursor phase can be directly transformed into the θ′ 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→θP′→θP1′→θP2′→θ′.