Subtle atomistic processes of S-phase formation in Al-Cu-Mg alloys

Fundamental understanding of S-precipitate formation has received extensive interest owing to its significant importance in strengthening Al-Cu-Mg alloys. One of the most poorly understood phenomena in S-precipitate formation is its transient early stages. Here we employ analytic transmission electron microscopy and first-principles calculations to address this problem. Our study identifies several crucial intermediate phases in two main evolution paths of S. The thickening of S follows the similar evolution path as the one-unit cell-thick S-phase formation. Further study reveals the vacancy-rich interfacial Al region between S and matrix during S precipitate formation. These observations provide more subtle detail regarding the evolution of strengthening S precipitates of Al-Cu-Mg alloys in response to thermal aging, and have important implications for understanding the atomistic formation process of nanoprecipitates in Al-Cu-Mg alloys.